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go/src/arena/arena.go

@@ -0,0 +1,108 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build goexperiment.arenas
+
+/*
+The arena package provides the ability to allocate memory for a collection
+of Go values and free that space manually all at once, safely. The purpose
+of this functionality is to improve efficiency: manually freeing memory
+before a garbage collection delays that cycle. Less frequent cycles means
+the CPU cost of the garbage collector is incurred less frequently.
+
+This functionality in this package is mostly captured in the Arena type.
+Arenas allocate large chunks of memory for Go values, so they're likely to
+be inefficient for allocating only small amounts of small Go values. They're
+best used in bulk, on the order of MiB of memory allocated on each use.
+
+Note that by allowing for this limited form of manual memory allocation
+that use-after-free bugs are possible with regular Go values. This package
+limits the impact of these use-after-free bugs by preventing reuse of freed
+memory regions until the garbage collector is able to determine that it is
+safe. Typically, a use-after-free bug will result in a fault and a helpful
+error message, but this package reserves the right to not force a fault on
+freed memory. That means a valid implementation of this package is to just
+allocate all memory the way the runtime normally would, and in fact, it
+reserves the right to occasionally do so for some Go values.
+*/
+package arena
+
+import (
+	"internal/reflectlite"
+	"unsafe"
+)
+
+// Arena represents a collection of Go values allocated and freed together.
+// Arenas are useful for improving efficiency as they may be freed back to
+// the runtime manually, though any memory obtained from freed arenas must
+// not be accessed once that happens. An Arena is automatically freed once
+// it is no longer referenced, so it must be kept alive (see runtime.KeepAlive)
+// until any memory allocated from it is no longer needed.
+//
+// An Arena must never be used concurrently by multiple goroutines.
+type Arena struct {
+	a unsafe.Pointer
+}
+
+// NewArena allocates a new arena.
+func NewArena() *Arena {
+	return &Arena{a: runtime_arena_newArena()}
+}
+
+// Free frees the arena (and all objects allocated from the arena) so that
+// memory backing the arena can be reused fairly quickly without garbage
+// collection overhead. Applications must not call any method on this
+// arena after it has been freed.
+func (a *Arena) Free() {
+	runtime_arena_arena_Free(a.a)
+	a.a = nil
+}
+
+// New creates a new *T in the provided arena. The *T must not be used after
+// the arena is freed. Accessing the value after free may result in a fault,
+// but this fault is also not guaranteed.
+func New[T any](a *Arena) *T {
+	return runtime_arena_arena_New(a.a, reflectlite.TypeOf((*T)(nil))).(*T)
+}
+
+// MakeSlice creates a new []T with the provided capacity and length. The []T must
+// not be used after the arena is freed. Accessing the underlying storage of the
+// slice after free may result in a fault, but this fault is also not guaranteed.
+func MakeSlice[T any](a *Arena, len, cap int) []T {
+	var sl []T
+	runtime_arena_arena_Slice(a.a, &sl, cap)
+	return sl[:len]
+}
+
+// Clone makes a shallow copy of the input value that is no longer bound to any
+// arena it may have been allocated from, returning the copy. If it was not
+// allocated from an arena, it is returned untouched. This function is useful
+// to more easily let an arena-allocated value out-live its arena.
+// T must be a pointer, a slice, or a string, otherwise this function will panic.
+func Clone[T any](s T) T {
+	return runtime_arena_heapify(s).(T)
+}
+
+//go:linkname reflect_arena_New reflect.arena_New
+func reflect_arena_New(a *Arena, typ any) any {
+	return runtime_arena_arena_New(a.a, typ)
+}
+
+//go:linkname runtime_arena_newArena
+func runtime_arena_newArena() unsafe.Pointer
+
+//go:linkname runtime_arena_arena_New
+func runtime_arena_arena_New(arena unsafe.Pointer, typ any) any
+
+// Mark as noescape to avoid escaping the slice header.
+//
+//go:noescape
+//go:linkname runtime_arena_arena_Slice
+func runtime_arena_arena_Slice(arena unsafe.Pointer, slice any, cap int)
+
+//go:linkname runtime_arena_arena_Free
+func runtime_arena_arena_Free(arena unsafe.Pointer)
+
+//go:linkname runtime_arena_heapify
+func runtime_arena_heapify(any) any

go/src/arena/arena_test.go

@@ -0,0 +1,42 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build goexperiment.arenas
+
+package arena_test
+
+import (
+	"arena"
+	"testing"
+)
+
+type T1 struct {
+	n int
+}
+type T2 [1 << 20]byte // 1MiB
+
+func TestSmoke(t *testing.T) {
+	a := arena.NewArena()
+	defer a.Free()
+
+	tt := arena.New[T1](a)
+	tt.n = 1
+
+	ts := arena.MakeSlice[T1](a, 99, 100)
+	if len(ts) != 99 {
+		t.Errorf("Slice() len = %d, want 99", len(ts))
+	}
+	if cap(ts) != 100 {
+		t.Errorf("Slice() cap = %d, want 100", cap(ts))
+	}
+	ts[1].n = 42
+}
+
+func TestSmokeLarge(t *testing.T) {
+	a := arena.NewArena()
+	defer a.Free()
+	for i := 0; i < 10*64; i++ {
+		_ = arena.New[T2](a)
+	}
+}

go/src/bufio/bufio.go

@@ -0,0 +1,842 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package bufio implements buffered I/O. It wraps an io.Reader or io.Writer
+// object, creating another object (Reader or Writer) that also implements
+// the interface but provides buffering and some help for textual I/O.
+package bufio
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"unicode/utf8"
+)
+
+const (
+	defaultBufSize = 4096
+)
+
+var (
+	ErrInvalidUnreadByte = errors.New("bufio: invalid use of UnreadByte")
+	ErrInvalidUnreadRune = errors.New("bufio: invalid use of UnreadRune")
+	ErrBufferFull        = errors.New("bufio: buffer full")
+	ErrNegativeCount     = errors.New("bufio: negative count")
+)
+
+// Buffered input.
+
+// Reader implements buffering for an io.Reader object.
+// A new Reader is created by calling [NewReader] or [NewReaderSize];
+// alternatively the zero value of a Reader may be used after calling [Reset]
+// on it.
+type Reader struct {
+	buf          []byte
+	rd           io.Reader // reader provided by the client
+	r, w         int       // buf read and write positions
+	err          error
+	lastByte     int // last byte read for UnreadByte; -1 means invalid
+	lastRuneSize int // size of last rune read for UnreadRune; -1 means invalid
+}
+
+const minReadBufferSize = 16
+const maxConsecutiveEmptyReads = 100
+
+// NewReaderSize returns a new [Reader] whose buffer has at least the specified
+// size. If the argument io.Reader is already a [Reader] with large enough
+// size, it returns the underlying [Reader].
+func NewReaderSize(rd io.Reader, size int) *Reader {
+	// Is it already a Reader?
+	b, ok := rd.(*Reader)
+	if ok && len(b.buf) >= size {
+		return b
+	}
+	r := new(Reader)
+	r.reset(make([]byte, max(size, minReadBufferSize)), rd)
+	return r
+}
+
+// NewReader returns a new [Reader] whose buffer has the default size.
+func NewReader(rd io.Reader) *Reader {
+	return NewReaderSize(rd, defaultBufSize)
+}
+
+// Size returns the size of the underlying buffer in bytes.
+func (b *Reader) Size() int { return len(b.buf) }
+
+// Reset discards any buffered data, resets all state, and switches
+// the buffered reader to read from r.
+// Calling Reset on the zero value of [Reader] initializes the internal buffer
+// to the default size.
+// Calling b.Reset(b) (that is, resetting a [Reader] to itself) does nothing.
+func (b *Reader) Reset(r io.Reader) {
+	// If a Reader r is passed to NewReader, NewReader will return r.
+	// Different layers of code may do that, and then later pass r
+	// to Reset. Avoid infinite recursion in that case.
+	if b == r {
+		return
+	}
+	if b.buf == nil {
+		b.buf = make([]byte, defaultBufSize)
+	}
+	b.reset(b.buf, r)
+}
+
+func (b *Reader) reset(buf []byte, r io.Reader) {
+	*b = Reader{
+		buf:          buf,
+		rd:           r,
+		lastByte:     -1,
+		lastRuneSize: -1,
+	}
+}
+
+var errNegativeRead = errors.New("bufio: reader returned negative count from Read")
+
+// fill reads a new chunk into the buffer.
+func (b *Reader) fill() {
+	// Slide existing data to beginning.
+	if b.r > 0 {
+		copy(b.buf, b.buf[b.r:b.w])
+		b.w -= b.r
+		b.r = 0
+	}
+
+	if b.w >= len(b.buf) {
+		panic("bufio: tried to fill full buffer")
+	}
+
+	// Read new data: try a limited number of times.
+	for i := maxConsecutiveEmptyReads; i > 0; i-- {
+		n, err := b.rd.Read(b.buf[b.w:])
+		if n < 0 {
+			panic(errNegativeRead)
+		}
+		b.w += n
+		if err != nil {
+			b.err = err
+			return
+		}
+		if n > 0 {
+			return
+		}
+	}
+	b.err = io.ErrNoProgress
+}
+
+func (b *Reader) readErr() error {
+	err := b.err
+	b.err = nil
+	return err
+}
+
+// Peek returns the next n bytes without advancing the reader. The bytes stop
+// being valid at the next read call. If necessary, Peek will read more bytes
+// into the buffer in order to make n bytes available. If Peek returns fewer
+// than n bytes, it also returns an error explaining why the read is short.
+// The error is [ErrBufferFull] if n is larger than b's buffer size.
+//
+// Calling Peek prevents a [Reader.UnreadByte] or [Reader.UnreadRune] call from succeeding
+// until the next read operation.
+func (b *Reader) Peek(n int) ([]byte, error) {
+	if n < 0 {
+		return nil, ErrNegativeCount
+	}
+
+	b.lastByte = -1
+	b.lastRuneSize = -1
+
+	for b.w-b.r < n && b.w-b.r < len(b.buf) && b.err == nil {
+		b.fill() // b.w-b.r < len(b.buf) => buffer is not full
+	}
+
+	if n > len(b.buf) {
+		return b.buf[b.r:b.w], ErrBufferFull
+	}
+
+	// 0 <= n <= len(b.buf)
+	var err error
+	if avail := b.w - b.r; avail < n {
+		// not enough data in buffer
+		n = avail
+		err = b.readErr()
+		if err == nil {
+			err = ErrBufferFull
+		}
+	}
+	return b.buf[b.r : b.r+n], err
+}
+
+// Discard skips the next n bytes, returning the number of bytes discarded.
+//
+// If Discard skips fewer than n bytes, it also returns an error.
+// If 0 <= n <= b.Buffered(), Discard is guaranteed to succeed without
+// reading from the underlying io.Reader.
+func (b *Reader) Discard(n int) (discarded int, err error) {
+	if n < 0 {
+		return 0, ErrNegativeCount
+	}
+	if n == 0 {
+		return
+	}
+
+	b.lastByte = -1
+	b.lastRuneSize = -1
+
+	remain := n
+	for {
+		skip := b.Buffered()
+		if skip == 0 {
+			b.fill()
+			skip = b.Buffered()
+		}
+		if skip > remain {
+			skip = remain
+		}
+		b.r += skip
+		remain -= skip
+		if remain == 0 {
+			return n, nil
+		}
+		if b.err != nil {
+			return n - remain, b.readErr()
+		}
+	}
+}
+
+// Read reads data into p.
+// It returns the number of bytes read into p.
+// The bytes are taken from at most one Read on the underlying [Reader],
+// hence n may be less than len(p).
+// To read exactly len(p) bytes, use io.ReadFull(b, p).
+// If the underlying [Reader] can return a non-zero count with io.EOF,
+// then this Read method can do so as well; see the [io.Reader] docs.
+func (b *Reader) Read(p []byte) (n int, err error) {
+	n = len(p)
+	if n == 0 {
+		if b.Buffered() > 0 {
+			return 0, nil
+		}
+		return 0, b.readErr()
+	}
+	if b.r == b.w {
+		if b.err != nil {
+			return 0, b.readErr()
+		}
+		if len(p) >= len(b.buf) {
+			// Large read, empty buffer.
+			// Read directly into p to avoid copy.
+			n, b.err = b.rd.Read(p)
+			if n < 0 {
+				panic(errNegativeRead)
+			}
+			if n > 0 {
+				b.lastByte = int(p[n-1])
+				b.lastRuneSize = -1
+			}
+			return n, b.readErr()
+		}
+		// One read.
+		// Do not use b.fill, which will loop.
+		b.r = 0
+		b.w = 0
+		n, b.err = b.rd.Read(b.buf)
+		if n < 0 {
+			panic(errNegativeRead)
+		}
+		if n == 0 {
+			return 0, b.readErr()
+		}
+		b.w += n
+	}
+
+	// copy as much as we can
+	// Note: if the slice panics here, it is probably because
+	// the underlying reader returned a bad count. See issue 49795.
+	n = copy(p, b.buf[b.r:b.w])
+	b.r += n
+	b.lastByte = int(b.buf[b.r-1])
+	b.lastRuneSize = -1
+	return n, nil
+}
+
+// ReadByte reads and returns a single byte.
+// If no byte is available, returns an error.
+func (b *Reader) ReadByte() (byte, error) {
+	b.lastRuneSize = -1
+	for b.r == b.w {
+		if b.err != nil {
+			return 0, b.readErr()
+		}
+		b.fill() // buffer is empty
+	}
+	c := b.buf[b.r]
+	b.r++
+	b.lastByte = int(c)
+	return c, nil
+}
+
+// UnreadByte unreads the last byte. Only the most recently read byte can be unread.
+//
+// UnreadByte returns an error if the most recent method called on the
+// [Reader] was not a read operation. Notably, [Reader.Peek], [Reader.Discard], and [Reader.WriteTo] are not
+// considered read operations.
+func (b *Reader) UnreadByte() error {
+	if b.lastByte < 0 || b.r == 0 && b.w > 0 {
+		return ErrInvalidUnreadByte
+	}
+	// b.r > 0 || b.w == 0
+	if b.r > 0 {
+		b.r--
+	} else {
+		// b.r == 0 && b.w == 0
+		b.w = 1
+	}
+	b.buf[b.r] = byte(b.lastByte)
+	b.lastByte = -1
+	b.lastRuneSize = -1
+	return nil
+}
+
+// ReadRune reads a single UTF-8 encoded Unicode character and returns the
+// rune and its size in bytes. If the encoded rune is invalid, it consumes one byte
+// and returns unicode.ReplacementChar (U+FFFD) with a size of 1.
+func (b *Reader) ReadRune() (r rune, size int, err error) {
+	for b.r+utf8.UTFMax > b.w && !utf8.FullRune(b.buf[b.r:b.w]) && b.err == nil && b.w-b.r < len(b.buf) {
+		b.fill() // b.w-b.r < len(buf) => buffer is not full
+	}
+	b.lastRuneSize = -1
+	if b.r == b.w {
+		return 0, 0, b.readErr()
+	}
+	r, size = utf8.DecodeRune(b.buf[b.r:b.w])
+	b.r += size
+	b.lastByte = int(b.buf[b.r-1])
+	b.lastRuneSize = size
+	return r, size, nil
+}
+
+// UnreadRune unreads the last rune. If the most recent method called on
+// the [Reader] was not a [Reader.ReadRune], [Reader.UnreadRune] returns an error. (In this
+// regard it is stricter than [Reader.UnreadByte], which will unread the last byte
+// from any read operation.)
+func (b *Reader) UnreadRune() error {
+	if b.lastRuneSize < 0 || b.r < b.lastRuneSize {
+		return ErrInvalidUnreadRune
+	}
+	b.r -= b.lastRuneSize
+	b.lastByte = -1
+	b.lastRuneSize = -1
+	return nil
+}
+
+// Buffered returns the number of bytes that can be read from the current buffer.
+func (b *Reader) Buffered() int { return b.w - b.r }
+
+// ReadSlice reads until the first occurrence of delim in the input,
+// returning a slice pointing at the bytes in the buffer.
+// The bytes stop being valid at the next read.
+// If ReadSlice encounters an error before finding a delimiter,
+// it returns all the data in the buffer and the error itself (often io.EOF).
+// ReadSlice fails with error [ErrBufferFull] if the buffer fills without a delim.
+// Because the data returned from ReadSlice will be overwritten
+// by the next I/O operation, most clients should use
+// [Reader.ReadBytes] or ReadString instead.
+// ReadSlice returns err != nil if and only if line does not end in delim.
+func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
+	s := 0 // search start index
+	for {
+		// Search buffer.
+		if i := bytes.IndexByte(b.buf[b.r+s:b.w], delim); i >= 0 {
+			i += s
+			line = b.buf[b.r : b.r+i+1]
+			b.r += i + 1
+			break
+		}
+
+		// Pending error?
+		if b.err != nil {
+			line = b.buf[b.r:b.w]
+			b.r = b.w
+			err = b.readErr()
+			break
+		}
+
+		// Buffer full?
+		if b.Buffered() >= len(b.buf) {
+			b.r = b.w
+			line = b.buf
+			err = ErrBufferFull
+			break
+		}
+
+		s = b.w - b.r // do not rescan area we scanned before
+
+		b.fill() // buffer is not full
+	}
+
+	// Handle last byte, if any.
+	if i := len(line) - 1; i >= 0 {
+		b.lastByte = int(line[i])
+		b.lastRuneSize = -1
+	}
+
+	return
+}
+
+// ReadLine is a low-level line-reading primitive. Most callers should use
+// [Reader.ReadBytes]('\n') or [Reader.ReadString]('\n') instead or use a [Scanner].
+//
+// ReadLine tries to return a single line, not including the end-of-line bytes.
+// If the line was too long for the buffer then isPrefix is set and the
+// beginning of the line is returned. The rest of the line will be returned
+// from future calls. isPrefix will be false when returning the last fragment
+// of the line. The returned buffer is only valid until the next call to
+// ReadLine. ReadLine either returns a non-nil line or it returns an error,
+// never both.
+//
+// The text returned from ReadLine does not include the line end ("\r\n" or "\n").
+// No indication or error is given if the input ends without a final line end.
+// Calling [Reader.UnreadByte] after ReadLine will always unread the last byte read
+// (possibly a character belonging to the line end) even if that byte is not
+// part of the line returned by ReadLine.
+func (b *Reader) ReadLine() (line []byte, isPrefix bool, err error) {
+	line, err = b.ReadSlice('\n')
+	if err == ErrBufferFull {
+		// Handle the case where "\r\n" straddles the buffer.
+		if len(line) > 0 && line[len(line)-1] == '\r' {
+			// Put the '\r' back on buf and drop it from line.
+			// Let the next call to ReadLine check for "\r\n".
+			if b.r == 0 {
+				// should be unreachable
+				panic("bufio: tried to rewind past start of buffer")
+			}
+			b.r--
+			line = line[:len(line)-1]
+		}
+		return line, true, nil
+	}
+
+	if len(line) == 0 {
+		if err != nil {
+			line = nil
+		}
+		return
+	}
+	err = nil
+
+	if line[len(line)-1] == '\n' {
+		drop := 1
+		if len(line) > 1 && line[len(line)-2] == '\r' {
+			drop = 2
+		}
+		line = line[:len(line)-drop]
+	}
+	return
+}
+
+// collectFragments reads until the first occurrence of delim in the input. It
+// returns (slice of full buffers, remaining bytes before delim, total number
+// of bytes in the combined first two elements, error).
+// The complete result is equal to
+// `bytes.Join(append(fullBuffers, finalFragment), nil)`, which has a
+// length of `totalLen`. The result is structured in this way to allow callers
+// to minimize allocations and copies.
+func (b *Reader) collectFragments(delim byte) (fullBuffers [][]byte, finalFragment []byte, totalLen int, err error) {
+	var frag []byte
+	// Use ReadSlice to look for delim, accumulating full buffers.
+	for {
+		var e error
+		frag, e = b.ReadSlice(delim)
+		if e == nil { // got final fragment
+			break
+		}
+		if e != ErrBufferFull { // unexpected error
+			err = e
+			break
+		}
+
+		// Make a copy of the buffer.
+		buf := bytes.Clone(frag)
+		fullBuffers = append(fullBuffers, buf)
+		totalLen += len(buf)
+	}
+
+	totalLen += len(frag)
+	return fullBuffers, frag, totalLen, err
+}
+
+// ReadBytes reads until the first occurrence of delim in the input,
+// returning a slice containing the data up to and including the delimiter.
+// If ReadBytes encounters an error before finding a delimiter,
+// it returns the data read before the error and the error itself (often io.EOF).
+// ReadBytes returns err != nil if and only if the returned data does not end in
+// delim.
+// For simple uses, a Scanner may be more convenient.
+func (b *Reader) ReadBytes(delim byte) ([]byte, error) {
+	full, frag, n, err := b.collectFragments(delim)
+	// Allocate new buffer to hold the full pieces and the fragment.
+	buf := make([]byte, n)
+	n = 0
+	// Copy full pieces and fragment in.
+	for i := range full {
+		n += copy(buf[n:], full[i])
+	}
+	copy(buf[n:], frag)
+	return buf, err
+}
+
+// ReadString reads until the first occurrence of delim in the input,
+// returning a string containing the data up to and including the delimiter.
+// If ReadString encounters an error before finding a delimiter,
+// it returns the data read before the error and the error itself (often io.EOF).
+// ReadString returns err != nil if and only if the returned data does not end in
+// delim.
+// For simple uses, a Scanner may be more convenient.
+func (b *Reader) ReadString(delim byte) (string, error) {
+	full, frag, n, err := b.collectFragments(delim)
+	// Allocate new buffer to hold the full pieces and the fragment.
+	var buf strings.Builder
+	buf.Grow(n)
+	// Copy full pieces and fragment in.
+	for _, fb := range full {
+		buf.Write(fb)
+	}
+	buf.Write(frag)
+	return buf.String(), err
+}
+
+// WriteTo implements io.WriterTo.
+// This may make multiple calls to the [Reader.Read] method of the underlying [Reader].
+// If the underlying reader supports the [Reader.WriteTo] method,
+// this calls the underlying [Reader.WriteTo] without buffering.
+func (b *Reader) WriteTo(w io.Writer) (n int64, err error) {
+	b.lastByte = -1
+	b.lastRuneSize = -1
+
+	if b.r < b.w {
+		n, err = b.writeBuf(w)
+		if err != nil {
+			return
+		}
+	}
+
+	if r, ok := b.rd.(io.WriterTo); ok {
+		m, err := r.WriteTo(w)
+		n += m
+		return n, err
+	}
+
+	if w, ok := w.(io.ReaderFrom); ok {
+		m, err := w.ReadFrom(b.rd)
+		n += m
+		return n, err
+	}
+
+	if b.w-b.r < len(b.buf) {
+		b.fill() // buffer not full
+	}
+
+	for b.r < b.w {
+		// b.r < b.w => buffer is not empty
+		m, err := b.writeBuf(w)
+		n += m
+		if err != nil {
+			return n, err
+		}
+		b.fill() // buffer is empty
+	}
+
+	if b.err == io.EOF {
+		b.err = nil
+	}
+
+	return n, b.readErr()
+}
+
+var errNegativeWrite = errors.New("bufio: writer returned negative count from Write")
+
+// writeBuf writes the [Reader]'s buffer to the writer.
+func (b *Reader) writeBuf(w io.Writer) (int64, error) {
+	n, err := w.Write(b.buf[b.r:b.w])
+	if n < 0 {
+		panic(errNegativeWrite)
+	}
+	b.r += n
+	return int64(n), err
+}
+
+// buffered output
+
+// Writer implements buffering for an [io.Writer] object.
+// If an error occurs writing to a [Writer], no more data will be
+// accepted and all subsequent writes, and [Writer.Flush], will return the error.
+// After all data has been written, the client should call the
+// [Writer.Flush] method to guarantee all data has been forwarded to
+// the underlying [io.Writer].
+type Writer struct {
+	err error
+	buf []byte
+	n   int
+	wr  io.Writer
+}
+
+// NewWriterSize returns a new [Writer] whose buffer has at least the specified
+// size. If the argument io.Writer is already a [Writer] with large enough
+// size, it returns the underlying [Writer].
+func NewWriterSize(w io.Writer, size int) *Writer {
+	// Is it already a Writer?
+	b, ok := w.(*Writer)
+	if ok && len(b.buf) >= size {
+		return b
+	}
+	if size <= 0 {
+		size = defaultBufSize
+	}
+	return &Writer{
+		buf: make([]byte, size),
+		wr:  w,
+	}
+}
+
+// NewWriter returns a new [Writer] whose buffer has the default size.
+// If the argument io.Writer is already a [Writer] with large enough buffer size,
+// it returns the underlying [Writer].
+func NewWriter(w io.Writer) *Writer {
+	return NewWriterSize(w, defaultBufSize)
+}
+
+// Size returns the size of the underlying buffer in bytes.
+func (b *Writer) Size() int { return len(b.buf) }
+
+// Reset discards any unflushed buffered data, clears any error, and
+// resets b to write its output to w.
+// Calling Reset on the zero value of [Writer] initializes the internal buffer
+// to the default size.
+// Calling w.Reset(w) (that is, resetting a [Writer] to itself) does nothing.
+func (b *Writer) Reset(w io.Writer) {
+	// If a Writer w is passed to NewWriter, NewWriter will return w.
+	// Different layers of code may do that, and then later pass w
+	// to Reset. Avoid infinite recursion in that case.
+	if b == w {
+		return
+	}
+	if b.buf == nil {
+		b.buf = make([]byte, defaultBufSize)
+	}
+	b.err = nil
+	b.n = 0
+	b.wr = w
+}
+
+// Flush writes any buffered data to the underlying [io.Writer].
+func (b *Writer) Flush() error {
+	if b.err != nil {
+		return b.err
+	}
+	if b.n == 0 {
+		return nil
+	}
+	n, err := b.wr.Write(b.buf[0:b.n])
+	if n < b.n && err == nil {
+		err = io.ErrShortWrite
+	}
+	if err != nil {
+		if n > 0 && n < b.n {
+			copy(b.buf[0:b.n-n], b.buf[n:b.n])
+		}
+		b.n -= n
+		b.err = err
+		return err
+	}
+	b.n = 0
+	return nil
+}
+
+// Available returns how many bytes are unused in the buffer.
+func (b *Writer) Available() int { return len(b.buf) - b.n }
+
+// AvailableBuffer returns an empty buffer with b.Available() capacity.
+// This buffer is intended to be appended to and
+// passed to an immediately succeeding [Writer.Write] call.
+// The buffer is only valid until the next write operation on b.
+func (b *Writer) AvailableBuffer() []byte {
+	return b.buf[b.n:][:0]
+}
+
+// Buffered returns the number of bytes that have been written into the current buffer.
+func (b *Writer) Buffered() int { return b.n }
+
+// Write writes the contents of p into the buffer.
+// It returns the number of bytes written.
+// If nn < len(p), it also returns an error explaining
+// why the write is short.
+func (b *Writer) Write(p []byte) (nn int, err error) {
+	for len(p) > b.Available() && b.err == nil {
+		var n int
+		if b.Buffered() == 0 {
+			// Large write, empty buffer.
+			// Write directly from p to avoid copy.
+			n, b.err = b.wr.Write(p)
+		} else {
+			n = copy(b.buf[b.n:], p)
+			b.n += n
+			b.Flush()
+		}
+		nn += n
+		p = p[n:]
+	}
+	if b.err != nil {
+		return nn, b.err
+	}
+	n := copy(b.buf[b.n:], p)
+	b.n += n
+	nn += n
+	return nn, nil
+}
+
+// WriteByte writes a single byte.
+func (b *Writer) WriteByte(c byte) error {
+	if b.err != nil {
+		return b.err
+	}
+	if b.Available() <= 0 && b.Flush() != nil {
+		return b.err
+	}
+	b.buf[b.n] = c
+	b.n++
+	return nil
+}
+
+// WriteRune writes a single Unicode code point, returning
+// the number of bytes written and any error.
+func (b *Writer) WriteRune(r rune) (size int, err error) {
+	// Compare as uint32 to correctly handle negative runes.
+	if uint32(r) < utf8.RuneSelf {
+		err = b.WriteByte(byte(r))
+		if err != nil {
+			return 0, err
+		}
+		return 1, nil
+	}
+	if b.err != nil {
+		return 0, b.err
+	}
+	n := b.Available()
+	if n < utf8.UTFMax {
+		if b.Flush(); b.err != nil {
+			return 0, b.err
+		}
+		n = b.Available()
+		if n < utf8.UTFMax {
+			// Can only happen if buffer is silly small.
+			return b.WriteString(string(r))
+		}
+	}
+	size = utf8.EncodeRune(b.buf[b.n:], r)
+	b.n += size
+	return size, nil
+}
+
+// WriteString writes a string.
+// It returns the number of bytes written.
+// If the count is less than len(s), it also returns an error explaining
+// why the write is short.
+func (b *Writer) WriteString(s string) (int, error) {
+	var sw io.StringWriter
+	tryStringWriter := true
+
+	nn := 0
+	for len(s) > b.Available() && b.err == nil {
+		var n int
+		if b.Buffered() == 0 && sw == nil && tryStringWriter {
+			// Check at most once whether b.wr is a StringWriter.
+			sw, tryStringWriter = b.wr.(io.StringWriter)
+		}
+		if b.Buffered() == 0 && tryStringWriter {
+			// Large write, empty buffer, and the underlying writer supports
+			// WriteString: forward the write to the underlying StringWriter.
+			// This avoids an extra copy.
+			n, b.err = sw.WriteString(s)
+		} else {
+			n = copy(b.buf[b.n:], s)
+			b.n += n
+			b.Flush()
+		}
+		nn += n
+		s = s[n:]
+	}
+	if b.err != nil {
+		return nn, b.err
+	}
+	n := copy(b.buf[b.n:], s)
+	b.n += n
+	nn += n
+	return nn, nil
+}
+
+// ReadFrom implements [io.ReaderFrom]. If the underlying writer
+// supports the ReadFrom method, this calls the underlying ReadFrom.
+// If there is buffered data and an underlying ReadFrom, this fills
+// the buffer and writes it before calling ReadFrom.
+func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
+	if b.err != nil {
+		return 0, b.err
+	}
+	readerFrom, readerFromOK := b.wr.(io.ReaderFrom)
+	var m int
+	for {
+		if b.Available() == 0 {
+			if err1 := b.Flush(); err1 != nil {
+				return n, err1
+			}
+		}
+		if readerFromOK && b.Buffered() == 0 {
+			nn, err := readerFrom.ReadFrom(r)
+			b.err = err
+			n += nn
+			return n, err
+		}
+		nr := 0
+		for nr < maxConsecutiveEmptyReads {
+			m, err = r.Read(b.buf[b.n:])
+			if m != 0 || err != nil {
+				break
+			}
+			nr++
+		}
+		if nr == maxConsecutiveEmptyReads {
+			return n, io.ErrNoProgress
+		}
+		b.n += m
+		n += int64(m)
+		if err != nil {
+			break
+		}
+	}
+	if err == io.EOF {
+		// If we filled the buffer exactly, flush preemptively.
+		if b.Available() == 0 {
+			err = b.Flush()
+		} else {
+			err = nil
+		}
+	}
+	return n, err
+}
+
+// buffered input and output
+
+// ReadWriter stores pointers to a [Reader] and a [Writer].
+// It implements [io.ReadWriter].
+type ReadWriter struct {
+	*Reader
+	*Writer
+}
+
+// NewReadWriter allocates a new [ReadWriter] that dispatches to r and w.
+func NewReadWriter(r *Reader, w *Writer) *ReadWriter {
+	return &ReadWriter{r, w}
+}

go/src/bufio/bufio_test.go

@@ -0,0 +1,1999 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bufio_test
+
+import (
+	. "bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"internal/asan"
+	"io"
+	"math/rand"
+	"strconv"
+	"strings"
+	"testing"
+	"testing/iotest"
+	"time"
+	"unicode/utf8"
+)
+
+// Reads from a reader and rot13s the result.
+type rot13Reader struct {
+	r io.Reader
+}
+
+func newRot13Reader(r io.Reader) *rot13Reader {
+	r13 := new(rot13Reader)
+	r13.r = r
+	return r13
+}
+
+func (r13 *rot13Reader) Read(p []byte) (int, error) {
+	n, err := r13.r.Read(p)
+	for i := 0; i < n; i++ {
+		c := p[i] | 0x20 // lowercase byte
+		if 'a' <= c && c <= 'm' {
+			p[i] += 13
+		} else if 'n' <= c && c <= 'z' {
+			p[i] -= 13
+		}
+	}
+	return n, err
+}
+
+// Call ReadByte to accumulate the text of a file
+func readBytes(buf *Reader) string {
+	var b [1000]byte
+	nb := 0
+	for {
+		c, err := buf.ReadByte()
+		if err == io.EOF {
+			break
+		}
+		if err == nil {
+			b[nb] = c
+			nb++
+		} else if err != iotest.ErrTimeout {
+			panic("Data: " + err.Error())
+		}
+	}
+	return string(b[0:nb])
+}
+
+func TestReaderSimple(t *testing.T) {
+	data := "hello world"
+	b := NewReader(strings.NewReader(data))
+	if s := readBytes(b); s != "hello world" {
+		t.Errorf("simple hello world test failed: got %q", s)
+	}
+
+	b = NewReader(newRot13Reader(strings.NewReader(data)))
+	if s := readBytes(b); s != "uryyb jbeyq" {
+		t.Errorf("rot13 hello world test failed: got %q", s)
+	}
+}
+
+type readMaker struct {
+	name string
+	fn   func(io.Reader) io.Reader
+}
+
+var readMakers = []readMaker{
+	{"full", func(r io.Reader) io.Reader { return r }},
+	{"byte", iotest.OneByteReader},
+	{"half", iotest.HalfReader},
+	{"data+err", iotest.DataErrReader},
+	{"timeout", iotest.TimeoutReader},
+}
+
+// Call ReadString (which ends up calling everything else)
+// to accumulate the text of a file.
+func readLines(b *Reader) string {
+	s := ""
+	for {
+		s1, err := b.ReadString('\n')
+		if err == io.EOF {
+			break
+		}
+		if err != nil && err != iotest.ErrTimeout {
+			panic("GetLines: " + err.Error())
+		}
+		s += s1
+	}
+	return s
+}
+
+// Call Read to accumulate the text of a file
+func reads(buf *Reader, m int) string {
+	var b [1000]byte
+	nb := 0
+	for {
+		n, err := buf.Read(b[nb : nb+m])
+		nb += n
+		if err == io.EOF {
+			break
+		}
+	}
+	return string(b[0:nb])
+}
+
+type bufReader struct {
+	name string
+	fn   func(*Reader) string
+}
+
+var bufreaders = []bufReader{
+	{"1", func(b *Reader) string { return reads(b, 1) }},
+	{"2", func(b *Reader) string { return reads(b, 2) }},
+	{"3", func(b *Reader) string { return reads(b, 3) }},
+	{"4", func(b *Reader) string { return reads(b, 4) }},
+	{"5", func(b *Reader) string { return reads(b, 5) }},
+	{"7", func(b *Reader) string { return reads(b, 7) }},
+	{"bytes", readBytes},
+	{"lines", readLines},
+}
+
+const minReadBufferSize = 16
+
+var bufsizes = []int{
+	0, minReadBufferSize, 23, 32, 46, 64, 93, 128, 1024, 4096,
+}
+
+func TestReader(t *testing.T) {
+	var texts [31]string
+	str := ""
+	all := ""
+	for i := 0; i < len(texts)-1; i++ {
+		texts[i] = str + "\n"
+		all += texts[i]
+		str += string(rune(i%26 + 'a'))
+	}
+	texts[len(texts)-1] = all
+
+	for h := 0; h < len(texts); h++ {
+		text := texts[h]
+		for i := 0; i < len(readMakers); i++ {
+			for j := 0; j < len(bufreaders); j++ {
+				for k := 0; k < len(bufsizes); k++ {
+					readmaker := readMakers[i]
+					bufreader := bufreaders[j]
+					bufsize := bufsizes[k]
+					read := readmaker.fn(strings.NewReader(text))
+					buf := NewReaderSize(read, bufsize)
+					s := bufreader.fn(buf)
+					if s != text {
+						t.Errorf("reader=%s fn=%s bufsize=%d want=%q got=%q",
+							readmaker.name, bufreader.name, bufsize, text, s)
+					}
+				}
+			}
+		}
+	}
+}
+
+type zeroReader struct{}
+
+func (zeroReader) Read(p []byte) (int, error) {
+	return 0, nil
+}
+
+func TestZeroReader(t *testing.T) {
+	var z zeroReader
+	r := NewReader(z)
+
+	c := make(chan error)
+	go func() {
+		_, err := r.ReadByte()
+		c <- err
+	}()
+
+	select {
+	case err := <-c:
+		if err == nil {
+			t.Error("error expected")
+		} else if err != io.ErrNoProgress {
+			t.Error("unexpected error:", err)
+		}
+	case <-time.After(time.Second):
+		t.Error("test timed out (endless loop in ReadByte?)")
+	}
+}
+
+// A StringReader delivers its data one string segment at a time via Read.
+type StringReader struct {
+	data []string
+	step int
+}
+
+func (r *StringReader) Read(p []byte) (n int, err error) {
+	if r.step < len(r.data) {
+		s := r.data[r.step]
+		n = copy(p, s)
+		r.step++
+	} else {
+		err = io.EOF
+	}
+	return
+}
+
+func readRuneSegments(t *testing.T, segments []string) {
+	got := ""
+	want := strings.Join(segments, "")
+	r := NewReader(&StringReader{data: segments})
+	for {
+		r, _, err := r.ReadRune()
+		if err != nil {
+			if err != io.EOF {
+				return
+			}
+			break
+		}
+		got += string(r)
+	}
+	if got != want {
+		t.Errorf("segments=%v got=%s want=%s", segments, got, want)
+	}
+}
+
+var segmentList = [][]string{
+	{},
+	{""},
+	{"日", "本語"},
+	{"\u65e5", "\u672c", "\u8a9e"},
+	{"\U000065e5", "\U0000672c", "\U00008a9e"},
+	{"\xe6", "\x97\xa5\xe6", "\x9c\xac\xe8\xaa\x9e"},
+	{"Hello", ", ", "World", "!"},
+	{"Hello", ", ", "", "World", "!"},
+}
+
+func TestReadRune(t *testing.T) {
+	for _, s := range segmentList {
+		readRuneSegments(t, s)
+	}
+}
+
+func TestUnreadRune(t *testing.T) {
+	segments := []string{"Hello, world:", "日本語"}
+	r := NewReader(&StringReader{data: segments})
+	got := ""
+	want := strings.Join(segments, "")
+	// Normal execution.
+	for {
+		r1, _, err := r.ReadRune()
+		if err != nil {
+			if err != io.EOF {
+				t.Error("unexpected error on ReadRune:", err)
+			}
+			break
+		}
+		got += string(r1)
+		// Put it back and read it again.
+		if err = r.UnreadRune(); err != nil {
+			t.Fatal("unexpected error on UnreadRune:", err)
+		}
+		r2, _, err := r.ReadRune()
+		if err != nil {
+			t.Fatal("unexpected error reading after unreading:", err)
+		}
+		if r1 != r2 {
+			t.Fatalf("incorrect rune after unread: got %c, want %c", r1, r2)
+		}
+	}
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
+func TestNoUnreadRuneAfterPeek(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadRune()
+	br.Peek(1)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after Peek")
+	}
+}
+
+func TestNoUnreadByteAfterPeek(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadByte()
+	br.Peek(1)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after Peek")
+	}
+}
+
+func TestNoUnreadRuneAfterDiscard(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadRune()
+	br.Discard(1)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after Discard")
+	}
+}
+
+func TestNoUnreadByteAfterDiscard(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.ReadByte()
+	br.Discard(1)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after Discard")
+	}
+}
+
+func TestNoUnreadRuneAfterWriteTo(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.WriteTo(io.Discard)
+	if err := br.UnreadRune(); err == nil {
+		t.Error("UnreadRune didn't fail after WriteTo")
+	}
+}
+
+func TestNoUnreadByteAfterWriteTo(t *testing.T) {
+	br := NewReader(strings.NewReader("example"))
+	br.WriteTo(io.Discard)
+	if err := br.UnreadByte(); err == nil {
+		t.Error("UnreadByte didn't fail after WriteTo")
+	}
+}
+
+func TestUnreadByte(t *testing.T) {
+	segments := []string{"Hello, ", "world"}
+	r := NewReader(&StringReader{data: segments})
+	got := ""
+	want := strings.Join(segments, "")
+	// Normal execution.
+	for {
+		b1, err := r.ReadByte()
+		if err != nil {
+			if err != io.EOF {
+				t.Error("unexpected error on ReadByte:", err)
+			}
+			break
+		}
+		got += string(b1)
+		// Put it back and read it again.
+		if err = r.UnreadByte(); err != nil {
+			t.Fatal("unexpected error on UnreadByte:", err)
+		}
+		b2, err := r.ReadByte()
+		if err != nil {
+			t.Fatal("unexpected error reading after unreading:", err)
+		}
+		if b1 != b2 {
+			t.Fatalf("incorrect byte after unread: got %q, want %q", b1, b2)
+		}
+	}
+	if got != want {
+		t.Errorf("got %q, want %q", got, want)
+	}
+}
+
+func TestUnreadByteMultiple(t *testing.T) {
+	segments := []string{"Hello, ", "world"}
+	data := strings.Join(segments, "")
+	for n := 0; n <= len(data); n++ {
+		r := NewReader(&StringReader{data: segments})
+		// Read n bytes.
+		for i := 0; i < n; i++ {
+			b, err := r.ReadByte()
+			if err != nil {
+				t.Fatalf("n = %d: unexpected error on ReadByte: %v", n, err)
+			}
+			if b != data[i] {
+				t.Fatalf("n = %d: incorrect byte returned from ReadByte: got %q, want %q", n, b, data[i])
+			}
+		}
+		// Unread one byte if there is one.
+		if n > 0 {
+			if err := r.UnreadByte(); err != nil {
+				t.Errorf("n = %d: unexpected error on UnreadByte: %v", n, err)
+			}
+		}
+		// Test that we cannot unread any further.
+		if err := r.UnreadByte(); err == nil {
+			t.Errorf("n = %d: expected error on UnreadByte", n)
+		}
+	}
+}
+
+func TestUnreadByteOthers(t *testing.T) {
+	// A list of readers to use in conjunction with UnreadByte.
+	var readers = []func(*Reader, byte) ([]byte, error){
+		(*Reader).ReadBytes,
+		(*Reader).ReadSlice,
+		func(r *Reader, delim byte) ([]byte, error) {
+			data, err := r.ReadString(delim)
+			return []byte(data), err
+		},
+		// ReadLine doesn't fit the data/pattern easily
+		// so we leave it out. It should be covered via
+		// the ReadSlice test since ReadLine simply calls
+		// ReadSlice, and it's that function that handles
+		// the last byte.
+	}
+
+	// Try all readers with UnreadByte.
+	for rno, read := range readers {
+		// Some input data that is longer than the minimum reader buffer size.
+		const n = 10
+		var buf bytes.Buffer
+		for i := 0; i < n; i++ {
+			buf.WriteString("abcdefg")
+		}
+
+		r := NewReaderSize(&buf, minReadBufferSize)
+		readTo := func(delim byte, want string) {
+			data, err := read(r, delim)
+			if err != nil {
+				t.Fatalf("#%d: unexpected error reading to %c: %v", rno, delim, err)
+			}
+			if got := string(data); got != want {
+				t.Fatalf("#%d: got %q, want %q", rno, got, want)
+			}
+		}
+
+		// Read the data with occasional UnreadByte calls.
+		for i := 0; i < n; i++ {
+			readTo('d', "abcd")
+			for j := 0; j < 3; j++ {
+				if err := r.UnreadByte(); err != nil {
+					t.Fatalf("#%d: unexpected error on UnreadByte: %v", rno, err)
+				}
+				readTo('d', "d")
+			}
+			readTo('g', "efg")
+		}
+
+		// All data should have been read.
+		_, err := r.ReadByte()
+		if err != io.EOF {
+			t.Errorf("#%d: got error %v; want EOF", rno, err)
+		}
+	}
+}
+
+// Test that UnreadRune fails if the preceding operation was not a ReadRune.
+func TestUnreadRuneError(t *testing.T) {
+	buf := make([]byte, 3) // All runes in this test are 3 bytes long
+	r := NewReader(&StringReader{data: []string{"日本語日本語日本語"}})
+	if r.UnreadRune() == nil {
+		t.Error("expected error on UnreadRune from fresh buffer")
+	}
+	_, _, err := r.ReadRune()
+	if err != nil {
+		t.Error("unexpected error on ReadRune (1):", err)
+	}
+	if err = r.UnreadRune(); err != nil {
+		t.Error("unexpected error on UnreadRune (1):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after UnreadRune (1)")
+	}
+	// Test error after Read.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (2):", err)
+	}
+	_, err = r.Read(buf)
+	if err != nil {
+		t.Error("unexpected error on Read (2):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after Read (2)")
+	}
+	// Test error after ReadByte.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (2):", err)
+	}
+	for range buf {
+		_, err = r.ReadByte()
+		if err != nil {
+			t.Error("unexpected error on ReadByte (2):", err)
+		}
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after ReadByte")
+	}
+	// Test error after UnreadByte.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (3):", err)
+	}
+	_, err = r.ReadByte()
+	if err != nil {
+		t.Error("unexpected error on ReadByte (3):", err)
+	}
+	err = r.UnreadByte()
+	if err != nil {
+		t.Error("unexpected error on UnreadByte (3):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after UnreadByte (3)")
+	}
+	// Test error after ReadSlice.
+	_, _, err = r.ReadRune() // reset state
+	if err != nil {
+		t.Error("unexpected error on ReadRune (4):", err)
+	}
+	_, err = r.ReadSlice(0)
+	if err != io.EOF {
+		t.Error("unexpected error on ReadSlice (4):", err)
+	}
+	if r.UnreadRune() == nil {
+		t.Error("expected error after ReadSlice (4)")
+	}
+}
+
+func TestUnreadRuneAtEOF(t *testing.T) {
+	// UnreadRune/ReadRune should error at EOF (was a bug; used to panic)
+	r := NewReader(strings.NewReader("x"))
+	r.ReadRune()
+	r.ReadRune()
+	r.UnreadRune()
+	_, _, err := r.ReadRune()
+	if err == nil {
+		t.Error("expected error at EOF")
+	} else if err != io.EOF {
+		t.Error("expected EOF; got", err)
+	}
+}
+
+func TestReadWriteRune(t *testing.T) {
+	const NRune = 1000
+	byteBuf := new(bytes.Buffer)
+	w := NewWriter(byteBuf)
+	// Write the runes out using WriteRune
+	buf := make([]byte, utf8.UTFMax)
+	for r := rune(0); r < NRune; r++ {
+		size := utf8.EncodeRune(buf, r)
+		nbytes, err := w.WriteRune(r)
+		if err != nil {
+			t.Fatalf("WriteRune(0x%x) error: %s", r, err)
+		}
+		if nbytes != size {
+			t.Fatalf("WriteRune(0x%x) expected %d, got %d", r, size, nbytes)
+		}
+	}
+	w.Flush()
+
+	r := NewReader(byteBuf)
+	// Read them back with ReadRune
+	for r1 := rune(0); r1 < NRune; r1++ {
+		size := utf8.EncodeRune(buf, r1)
+		nr, nbytes, err := r.ReadRune()
+		if nr != r1 || nbytes != size || err != nil {
+			t.Fatalf("ReadRune(0x%x) got 0x%x,%d not 0x%x,%d (err=%s)", r1, nr, nbytes, r1, size, err)
+		}
+	}
+}
+
+func TestWriteInvalidRune(t *testing.T) {
+	// Invalid runes, including negative ones, should be written as the
+	// replacement character.
+	for _, r := range []rune{-1, utf8.MaxRune + 1} {
+		var buf strings.Builder
+		w := NewWriter(&buf)
+		w.WriteRune(r)
+		w.Flush()
+		if s := buf.String(); s != "\uFFFD" {
+			t.Errorf("WriteRune(%d) wrote %q, not replacement character", r, s)
+		}
+	}
+}
+
+func TestReadStringAllocs(t *testing.T) {
+	if asan.Enabled {
+		t.Skip("test allocates more with -asan; see #70079")
+	}
+	r := strings.NewReader("       foo       foo        42        42        42        42        42        42        42        42       4.2       4.2       4.2       4.2\n")
+	buf := NewReader(r)
+	allocs := testing.AllocsPerRun(100, func() {
+		r.Seek(0, io.SeekStart)
+		buf.Reset(r)
+
+		_, err := buf.ReadString('\n')
+		if err != nil {
+			t.Fatal(err)
+		}
+	})
+	if allocs != 1 {
+		t.Errorf("Unexpected number of allocations, got %f, want 1", allocs)
+	}
+}
+
+func TestWriter(t *testing.T) {
+	var data [8192]byte
+
+	for i := 0; i < len(data); i++ {
+		data[i] = byte(' ' + i%('~'-' '))
+	}
+	w := new(bytes.Buffer)
+	for i := 0; i < len(bufsizes); i++ {
+		for j := 0; j < len(bufsizes); j++ {
+			nwrite := bufsizes[i]
+			bs := bufsizes[j]
+
+			// Write nwrite bytes using buffer size bs.
+			// Check that the right amount makes it out
+			// and that the data is correct.
+
+			w.Reset()
+			buf := NewWriterSize(w, bs)
+			context := fmt.Sprintf("nwrite=%d bufsize=%d", nwrite, bs)
+			n, e1 := buf.Write(data[0:nwrite])
+			if e1 != nil || n != nwrite {
+				t.Errorf("%s: buf.Write %d = %d, %v", context, nwrite, n, e1)
+				continue
+			}
+			if e := buf.Flush(); e != nil {
+				t.Errorf("%s: buf.Flush = %v", context, e)
+			}
+
+			written := w.Bytes()
+			if len(written) != nwrite {
+				t.Errorf("%s: %d bytes written", context, len(written))
+			}
+			for l := 0; l < len(written); l++ {
+				if written[l] != data[l] {
+					t.Errorf("wrong bytes written")
+					t.Errorf("want=%q", data[:len(written)])
+					t.Errorf("have=%q", written)
+				}
+			}
+		}
+	}
+}
+
+func TestWriterAppend(t *testing.T) {
+	got := new(bytes.Buffer)
+	var want []byte
+	rn := rand.New(rand.NewSource(0))
+	w := NewWriterSize(got, 64)
+	for i := 0; i < 100; i++ {
+		// Obtain a buffer to append to.
+		b := w.AvailableBuffer()
+		if w.Available() != cap(b) {
+			t.Fatalf("Available() = %v, want %v", w.Available(), cap(b))
+		}
+
+		// While not recommended, it is valid to append to a shifted buffer.
+		// This forces Write to copy the input.
+		if rn.Intn(8) == 0 && cap(b) > 0 {
+			b = b[1:1:cap(b)]
+		}
+
+		// Append a random integer of varying width.
+		n := int64(rn.Intn(1 << rn.Intn(30)))
+		want = append(strconv.AppendInt(want, n, 10), ' ')
+		b = append(strconv.AppendInt(b, n, 10), ' ')
+		w.Write(b)
+	}
+	w.Flush()
+
+	if !bytes.Equal(got.Bytes(), want) {
+		t.Errorf("output mismatch:\ngot  %s\nwant %s", got.Bytes(), want)
+	}
+}
+
+// Check that write errors are returned properly.
+
+type errorWriterTest struct {
+	n, m   int
+	err    error
+	expect error
+}
+
+func (w errorWriterTest) Write(p []byte) (int, error) {
+	return len(p) * w.n / w.m, w.err
+}
+
+var errorWriterTests = []errorWriterTest{
+	{0, 1, nil, io.ErrShortWrite},
+	{1, 2, nil, io.ErrShortWrite},
+	{1, 1, nil, nil},
+	{0, 1, io.ErrClosedPipe, io.ErrClosedPipe},
+	{1, 2, io.ErrClosedPipe, io.ErrClosedPipe},
+	{1, 1, io.ErrClosedPipe, io.ErrClosedPipe},
+}
+
+func TestWriteErrors(t *testing.T) {
+	for _, w := range errorWriterTests {
+		buf := NewWriter(w)
+		_, e := buf.Write([]byte("hello world"))
+		if e != nil {
+			t.Errorf("Write hello to %v: %v", w, e)
+			continue
+		}
+		// Two flushes, to verify the error is sticky.
+		for i := 0; i < 2; i++ {
+			e = buf.Flush()
+			if e != w.expect {
+				t.Errorf("Flush %d/2 %v: got %v, wanted %v", i+1, w, e, w.expect)
+			}
+		}
+	}
+}
+
+func TestNewReaderSizeIdempotent(t *testing.T) {
+	const BufSize = 1000
+	b := NewReaderSize(strings.NewReader("hello world"), BufSize)
+	// Does it recognize itself?
+	b1 := NewReaderSize(b, BufSize)
+	if b1 != b {
+		t.Error("NewReaderSize did not detect underlying Reader")
+	}
+	// Does it wrap if existing buffer is too small?
+	b2 := NewReaderSize(b, 2*BufSize)
+	if b2 == b {
+		t.Error("NewReaderSize did not enlarge buffer")
+	}
+}
+
+func TestNewWriterSizeIdempotent(t *testing.T) {
+	const BufSize = 1000
+	b := NewWriterSize(new(bytes.Buffer), BufSize)
+	// Does it recognize itself?
+	b1 := NewWriterSize(b, BufSize)
+	if b1 != b {
+		t.Error("NewWriterSize did not detect underlying Writer")
+	}
+	// Does it wrap if existing buffer is too small?
+	b2 := NewWriterSize(b, 2*BufSize)
+	if b2 == b {
+		t.Error("NewWriterSize did not enlarge buffer")
+	}
+}
+
+func TestWriteString(t *testing.T) {
+	const BufSize = 8
+	buf := new(strings.Builder)
+	b := NewWriterSize(buf, BufSize)
+	b.WriteString("0")                         // easy
+	b.WriteString("123456")                    // still easy
+	b.WriteString("7890")                      // easy after flush
+	b.WriteString("abcdefghijklmnopqrstuvwxy") // hard
+	b.WriteString("z")
+	if err := b.Flush(); err != nil {
+		t.Error("WriteString", err)
+	}
+	s := "01234567890abcdefghijklmnopqrstuvwxyz"
+	if buf.String() != s {
+		t.Errorf("WriteString wants %q gets %q", s, buf.String())
+	}
+}
+
+func TestWriteStringStringWriter(t *testing.T) {
+	const BufSize = 8
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.WriteString("1234")
+		tw.check(t, "", "")
+		b.WriteString("56789012")   // longer than BufSize
+		tw.check(t, "12345678", "") // but not enough (after filling the partially-filled buffer)
+		b.Flush()
+		tw.check(t, "123456789012", "")
+	}
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.WriteString("123456789")   // long string, empty buffer:
+		tw.check(t, "", "123456789") // use WriteString
+	}
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.WriteString("abc")
+		tw.check(t, "", "")
+		b.WriteString("123456789012345")      // long string, non-empty buffer
+		tw.check(t, "abc12345", "6789012345") // use Write and then WriteString since the remaining part is still longer than BufSize
+	}
+	{
+		tw := &teststringwriter{}
+		b := NewWriterSize(tw, BufSize)
+		b.Write([]byte("abc")) // same as above, but use Write instead of WriteString
+		tw.check(t, "", "")
+		b.WriteString("123456789012345")
+		tw.check(t, "abc12345", "6789012345") // same as above
+	}
+}
+
+type teststringwriter struct {
+	write       string
+	writeString string
+}
+
+func (w *teststringwriter) Write(b []byte) (int, error) {
+	w.write += string(b)
+	return len(b), nil
+}
+
+func (w *teststringwriter) WriteString(s string) (int, error) {
+	w.writeString += s
+	return len(s), nil
+}
+
+func (w *teststringwriter) check(t *testing.T, write, writeString string) {
+	t.Helper()
+	if w.write != write {
+		t.Errorf("write: expected %q, got %q", write, w.write)
+	}
+	if w.writeString != writeString {
+		t.Errorf("writeString: expected %q, got %q", writeString, w.writeString)
+	}
+}
+
+func TestBufferFull(t *testing.T) {
+	const longString = "And now, hello, world! It is the time for all good men to come to the aid of their party"
+	buf := NewReaderSize(strings.NewReader(longString), minReadBufferSize)
+	line, err := buf.ReadSlice('!')
+	if string(line) != "And now, hello, " || err != ErrBufferFull {
+		t.Errorf("first ReadSlice(,) = %q, %v", line, err)
+	}
+	line, err = buf.ReadSlice('!')
+	if string(line) != "world!" || err != nil {
+		t.Errorf("second ReadSlice(,) = %q, %v", line, err)
+	}
+}
+
+func TestPeek(t *testing.T) {
+	p := make([]byte, 10)
+	// string is 16 (minReadBufferSize) long.
+	buf := NewReaderSize(strings.NewReader("abcdefghijklmnop"), minReadBufferSize)
+	if s, err := buf.Peek(1); string(s) != "a" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "a", string(s), err)
+	}
+	if s, err := buf.Peek(4); string(s) != "abcd" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "abcd", string(s), err)
+	}
+	if _, err := buf.Peek(-1); err != ErrNegativeCount {
+		t.Fatalf("want ErrNegativeCount got %v", err)
+	}
+	if s, err := buf.Peek(32); string(s) != "abcdefghijklmnop" || err != ErrBufferFull {
+		t.Fatalf("want %q, ErrBufFull got %q, err=%v", "abcdefghijklmnop", string(s), err)
+	}
+	if _, err := buf.Read(p[0:3]); string(p[0:3]) != "abc" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "abc", string(p[0:3]), err)
+	}
+	if s, err := buf.Peek(1); string(s) != "d" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "d", string(s), err)
+	}
+	if s, err := buf.Peek(2); string(s) != "de" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "de", string(s), err)
+	}
+	if _, err := buf.Read(p[0:3]); string(p[0:3]) != "def" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "def", string(p[0:3]), err)
+	}
+	if s, err := buf.Peek(4); string(s) != "ghij" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "ghij", string(s), err)
+	}
+	if _, err := buf.Read(p[0:]); string(p[0:]) != "ghijklmnop" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "ghijklmnop", string(p[0:minReadBufferSize]), err)
+	}
+	if s, err := buf.Peek(0); string(s) != "" || err != nil {
+		t.Fatalf("want %q got %q, err=%v", "", string(s), err)
+	}
+	if _, err := buf.Peek(1); err != io.EOF {
+		t.Fatalf("want EOF got %v", err)
+	}
+
+	// Test for issue 3022, not exposing a reader's error on a successful Peek.
+	buf = NewReaderSize(dataAndEOFReader("abcd"), 32)
+	if s, err := buf.Peek(2); string(s) != "ab" || err != nil {
+		t.Errorf(`Peek(2) on "abcd", EOF = %q, %v; want "ab", nil`, string(s), err)
+	}
+	if s, err := buf.Peek(4); string(s) != "abcd" || err != nil {
+		t.Errorf(`Peek(4) on "abcd", EOF = %q, %v; want "abcd", nil`, string(s), err)
+	}
+	if n, err := buf.Read(p[0:5]); string(p[0:n]) != "abcd" || err != nil {
+		t.Fatalf("Read after peek = %q, %v; want abcd, EOF", p[0:n], err)
+	}
+	if n, err := buf.Read(p[0:1]); string(p[0:n]) != "" || err != io.EOF {
+		t.Fatalf(`second Read after peek = %q, %v; want "", EOF`, p[0:n], err)
+	}
+}
+
+type dataAndEOFReader string
+
+func (r dataAndEOFReader) Read(p []byte) (int, error) {
+	return copy(p, r), io.EOF
+}
+
+func TestPeekThenUnreadRune(t *testing.T) {
+	// This sequence used to cause a crash.
+	r := NewReader(strings.NewReader("x"))
+	r.ReadRune()
+	r.Peek(1)
+	r.UnreadRune()
+	r.ReadRune() // Used to panic here
+}
+
+var testOutput = []byte("0123456789abcdefghijklmnopqrstuvwxy")
+var testInput = []byte("012\n345\n678\n9ab\ncde\nfgh\nijk\nlmn\nopq\nrst\nuvw\nxy")
+var testInputrn = []byte("012\r\n345\r\n678\r\n9ab\r\ncde\r\nfgh\r\nijk\r\nlmn\r\nopq\r\nrst\r\nuvw\r\nxy\r\n\n\r\n")
+
+// TestReader wraps a []byte and returns reads of a specific length.
+type testReader struct {
+	data   []byte
+	stride int
+}
+
+func (t *testReader) Read(buf []byte) (n int, err error) {
+	n = t.stride
+	if n > len(t.data) {
+		n = len(t.data)
+	}
+	if n > len(buf) {
+		n = len(buf)
+	}
+	copy(buf, t.data)
+	t.data = t.data[n:]
+	if len(t.data) == 0 {
+		err = io.EOF
+	}
+	return
+}
+
+func testReadLine(t *testing.T, input []byte) {
+	for stride := 1; stride < 2; stride++ {
+		done := 0
+		reader := testReader{input, stride}
+		l := NewReaderSize(&reader, len(input)+1)
+		for {
+			line, isPrefix, err := l.ReadLine()
+			if len(line) > 0 && err != nil {
+				t.Errorf("ReadLine returned both data and error: %s", err)
+			}
+			if isPrefix {
+				t.Errorf("ReadLine returned prefix")
+			}
+			if err != nil {
+				if err != io.EOF {
+					t.Fatalf("Got unknown error: %s", err)
+				}
+				break
+			}
+			if want := testOutput[done : done+len(line)]; !bytes.Equal(want, line) {
+				t.Errorf("Bad line at stride %d: want: %x got: %x", stride, want, line)
+			}
+			done += len(line)
+		}
+		if done != len(testOutput) {
+			t.Errorf("ReadLine didn't return everything: got: %d, want: %d (stride: %d)", done, len(testOutput), stride)
+		}
+	}
+}
+
+func TestReadLine(t *testing.T) {
+	testReadLine(t, testInput)
+	testReadLine(t, testInputrn)
+}
+
+func TestLineTooLong(t *testing.T) {
+	data := make([]byte, 0)
+	for i := 0; i < minReadBufferSize*5/2; i++ {
+		data = append(data, '0'+byte(i%10))
+	}
+	buf := bytes.NewReader(data)
+	l := NewReaderSize(buf, minReadBufferSize)
+	line, isPrefix, err := l.ReadLine()
+	if !isPrefix || !bytes.Equal(line, data[:minReadBufferSize]) || err != nil {
+		t.Errorf("bad result for first line: got %q want %q %v", line, data[:minReadBufferSize], err)
+	}
+	data = data[len(line):]
+	line, isPrefix, err = l.ReadLine()
+	if !isPrefix || !bytes.Equal(line, data[:minReadBufferSize]) || err != nil {
+		t.Errorf("bad result for second line: got %q want %q %v", line, data[:minReadBufferSize], err)
+	}
+	data = data[len(line):]
+	line, isPrefix, err = l.ReadLine()
+	if isPrefix || !bytes.Equal(line, data[:minReadBufferSize/2]) || err != nil {
+		t.Errorf("bad result for third line: got %q want %q %v", line, data[:minReadBufferSize/2], err)
+	}
+	line, isPrefix, err = l.ReadLine()
+	if isPrefix || err == nil {
+		t.Errorf("expected no more lines: %x %s", line, err)
+	}
+}
+
+func TestReadAfterLines(t *testing.T) {
+	line1 := "this is line1"
+	restData := "this is line2\nthis is line 3\n"
+	inbuf := bytes.NewReader([]byte(line1 + "\n" + restData))
+	outbuf := new(strings.Builder)
+	maxLineLength := len(line1) + len(restData)/2
+	l := NewReaderSize(inbuf, maxLineLength)
+	line, isPrefix, err := l.ReadLine()
+	if isPrefix || err != nil || string(line) != line1 {
+		t.Errorf("bad result for first line: isPrefix=%v err=%v line=%q", isPrefix, err, string(line))
+	}
+	n, err := io.Copy(outbuf, l)
+	if int(n) != len(restData) || err != nil {
+		t.Errorf("bad result for Read: n=%d err=%v", n, err)
+	}
+	if outbuf.String() != restData {
+		t.Errorf("bad result for Read: got %q; expected %q", outbuf.String(), restData)
+	}
+}
+
+func TestReadEmptyBuffer(t *testing.T) {
+	l := NewReaderSize(new(bytes.Buffer), minReadBufferSize)
+	line, isPrefix, err := l.ReadLine()
+	if err != io.EOF {
+		t.Errorf("expected EOF from ReadLine, got '%s' %t %s", line, isPrefix, err)
+	}
+}
+
+func TestLinesAfterRead(t *testing.T) {
+	l := NewReaderSize(bytes.NewReader([]byte("foo")), minReadBufferSize)
+	_, err := io.ReadAll(l)
+	if err != nil {
+		t.Error(err)
+		return
+	}
+
+	line, isPrefix, err := l.ReadLine()
+	if err != io.EOF {
+		t.Errorf("expected EOF from ReadLine, got '%s' %t %s", line, isPrefix, err)
+	}
+}
+
+func TestReadLineNonNilLineOrError(t *testing.T) {
+	r := NewReader(strings.NewReader("line 1\n"))
+	for i := 0; i < 2; i++ {
+		l, _, err := r.ReadLine()
+		if l != nil && err != nil {
+			t.Fatalf("on line %d/2; ReadLine=%#v, %v; want non-nil line or Error, but not both",
+				i+1, l, err)
+		}
+	}
+}
+
+type readLineResult struct {
+	line     []byte
+	isPrefix bool
+	err      error
+}
+
+var readLineNewlinesTests = []struct {
+	input  string
+	expect []readLineResult
+}{
+	{"012345678901234\r\n012345678901234\r\n", []readLineResult{
+		{[]byte("012345678901234"), true, nil},
+		{nil, false, nil},
+		{[]byte("012345678901234"), true, nil},
+		{nil, false, nil},
+		{nil, false, io.EOF},
+	}},
+	{"0123456789012345\r012345678901234\r", []readLineResult{
+		{[]byte("0123456789012345"), true, nil},
+		{[]byte("\r012345678901234"), true, nil},
+		{[]byte("\r"), false, nil},
+		{nil, false, io.EOF},
+	}},
+}
+
+func TestReadLineNewlines(t *testing.T) {
+	for _, e := range readLineNewlinesTests {
+		testReadLineNewlines(t, e.input, e.expect)
+	}
+}
+
+func testReadLineNewlines(t *testing.T, input string, expect []readLineResult) {
+	b := NewReaderSize(strings.NewReader(input), minReadBufferSize)
+	for i, e := range expect {
+		line, isPrefix, err := b.ReadLine()
+		if !bytes.Equal(line, e.line) {
+			t.Errorf("%q call %d, line == %q, want %q", input, i, line, e.line)
+			return
+		}
+		if isPrefix != e.isPrefix {
+			t.Errorf("%q call %d, isPrefix == %v, want %v", input, i, isPrefix, e.isPrefix)
+			return
+		}
+		if err != e.err {
+			t.Errorf("%q call %d, err == %v, want %v", input, i, err, e.err)
+			return
+		}
+	}
+}
+
+func createTestInput(n int) []byte {
+	input := make([]byte, n)
+	for i := range input {
+		// 101 and 251 are arbitrary prime numbers.
+		// The idea is to create an input sequence
+		// which doesn't repeat too frequently.
+		input[i] = byte(i % 251)
+		if i%101 == 0 {
+			input[i] ^= byte(i / 101)
+		}
+	}
+	return input
+}
+
+func TestReaderWriteTo(t *testing.T) {
+	input := createTestInput(8192)
+	r := NewReader(onlyReader{bytes.NewReader(input)})
+	w := new(bytes.Buffer)
+	if n, err := r.WriteTo(w); err != nil || n != int64(len(input)) {
+		t.Fatalf("r.WriteTo(w) = %d, %v, want %d, nil", n, err, len(input))
+	}
+
+	for i, val := range w.Bytes() {
+		if val != input[i] {
+			t.Errorf("after write: out[%d] = %#x, want %#x", i, val, input[i])
+		}
+	}
+}
+
+type errorWriterToTest struct {
+	rn, wn     int
+	rerr, werr error
+	expected   error
+}
+
+func (r errorWriterToTest) Read(p []byte) (int, error) {
+	return len(p) * r.rn, r.rerr
+}
+
+func (w errorWriterToTest) Write(p []byte) (int, error) {
+	return len(p) * w.wn, w.werr
+}
+
+var errorWriterToTests = []errorWriterToTest{
+	{1, 0, nil, io.ErrClosedPipe, io.ErrClosedPipe},
+	{0, 1, io.ErrClosedPipe, nil, io.ErrClosedPipe},
+	{0, 0, io.ErrUnexpectedEOF, io.ErrClosedPipe, io.ErrUnexpectedEOF},
+	{0, 1, io.EOF, nil, nil},
+}
+
+func TestReaderWriteToErrors(t *testing.T) {
+	for i, rw := range errorWriterToTests {
+		r := NewReader(rw)
+		if _, err := r.WriteTo(rw); err != rw.expected {
+			t.Errorf("r.WriteTo(errorWriterToTests[%d]) = _, %v, want _,%v", i, err, rw.expected)
+		}
+	}
+}
+
+func TestWriterReadFrom(t *testing.T) {
+	ws := []func(io.Writer) io.Writer{
+		func(w io.Writer) io.Writer { return onlyWriter{w} },
+		func(w io.Writer) io.Writer { return w },
+	}
+
+	rs := []func(io.Reader) io.Reader{
+		iotest.DataErrReader,
+		func(r io.Reader) io.Reader { return r },
+	}
+
+	for ri, rfunc := range rs {
+		for wi, wfunc := range ws {
+			input := createTestInput(8192)
+			b := new(strings.Builder)
+			w := NewWriter(wfunc(b))
+			r := rfunc(bytes.NewReader(input))
+			if n, err := w.ReadFrom(r); err != nil || n != int64(len(input)) {
+				t.Errorf("ws[%d],rs[%d]: w.ReadFrom(r) = %d, %v, want %d, nil", wi, ri, n, err, len(input))
+				continue
+			}
+			if err := w.Flush(); err != nil {
+				t.Errorf("Flush returned %v", err)
+				continue
+			}
+			if got, want := b.String(), string(input); got != want {
+				t.Errorf("ws[%d], rs[%d]:\ngot  %q\nwant %q\n", wi, ri, got, want)
+			}
+		}
+	}
+}
+
+type errorReaderFromTest struct {
+	rn, wn     int
+	rerr, werr error
+	expected   error
+}
+
+func (r errorReaderFromTest) Read(p []byte) (int, error) {
+	return len(p) * r.rn, r.rerr
+}
+
+func (w errorReaderFromTest) Write(p []byte) (int, error) {
+	return len(p) * w.wn, w.werr
+}
+
+var errorReaderFromTests = []errorReaderFromTest{
+	{0, 1, io.EOF, nil, nil},
+	{1, 1, io.EOF, nil, nil},
+	{0, 1, io.ErrClosedPipe, nil, io.ErrClosedPipe},
+	{0, 0, io.ErrClosedPipe, io.ErrShortWrite, io.ErrClosedPipe},
+	{1, 0, nil, io.ErrShortWrite, io.ErrShortWrite},
+}
+
+func TestWriterReadFromErrors(t *testing.T) {
+	for i, rw := range errorReaderFromTests {
+		w := NewWriter(rw)
+		if _, err := w.ReadFrom(rw); err != rw.expected {
+			t.Errorf("w.ReadFrom(errorReaderFromTests[%d]) = _, %v, want _,%v", i, err, rw.expected)
+		}
+	}
+}
+
+// TestWriterReadFromCounts tests that using io.Copy to copy into a
+// bufio.Writer does not prematurely flush the buffer. For example, when
+// buffering writes to a network socket, excessive network writes should be
+// avoided.
+func TestWriterReadFromCounts(t *testing.T) {
+	var w0 writeCountingDiscard
+	b0 := NewWriterSize(&w0, 1234)
+	b0.WriteString(strings.Repeat("x", 1000))
+	if w0 != 0 {
+		t.Fatalf("write 1000 'x's: got %d writes, want 0", w0)
+	}
+	b0.WriteString(strings.Repeat("x", 200))
+	if w0 != 0 {
+		t.Fatalf("write 1200 'x's: got %d writes, want 0", w0)
+	}
+	io.Copy(b0, onlyReader{strings.NewReader(strings.Repeat("x", 30))})
+	if w0 != 0 {
+		t.Fatalf("write 1230 'x's: got %d writes, want 0", w0)
+	}
+	io.Copy(b0, onlyReader{strings.NewReader(strings.Repeat("x", 9))})
+	if w0 != 1 {
+		t.Fatalf("write 1239 'x's: got %d writes, want 1", w0)
+	}
+
+	var w1 writeCountingDiscard
+	b1 := NewWriterSize(&w1, 1234)
+	b1.WriteString(strings.Repeat("x", 1200))
+	b1.Flush()
+	if w1 != 1 {
+		t.Fatalf("flush 1200 'x's: got %d writes, want 1", w1)
+	}
+	b1.WriteString(strings.Repeat("x", 89))
+	if w1 != 1 {
+		t.Fatalf("write 1200 + 89 'x's: got %d writes, want 1", w1)
+	}
+	io.Copy(b1, onlyReader{strings.NewReader(strings.Repeat("x", 700))})
+	if w1 != 1 {
+		t.Fatalf("write 1200 + 789 'x's: got %d writes, want 1", w1)
+	}
+	io.Copy(b1, onlyReader{strings.NewReader(strings.Repeat("x", 600))})
+	if w1 != 2 {
+		t.Fatalf("write 1200 + 1389 'x's: got %d writes, want 2", w1)
+	}
+	b1.Flush()
+	if w1 != 3 {
+		t.Fatalf("flush 1200 + 1389 'x's: got %d writes, want 3", w1)
+	}
+}
+
+// A writeCountingDiscard is like io.Discard and counts the number of times
+// Write is called on it.
+type writeCountingDiscard int
+
+func (w *writeCountingDiscard) Write(p []byte) (int, error) {
+	*w++
+	return len(p), nil
+}
+
+type negativeReader int
+
+func (r *negativeReader) Read([]byte) (int, error) { return -1, nil }
+
+func TestNegativeRead(t *testing.T) {
+	// should panic with a description pointing at the reader, not at itself.
+	// (should NOT panic with slice index error, for example.)
+	b := NewReader(new(negativeReader))
+	defer func() {
+		switch err := recover().(type) {
+		case nil:
+			t.Fatal("read did not panic")
+		case error:
+			if !strings.Contains(err.Error(), "reader returned negative count from Read") {
+				t.Fatalf("wrong panic: %v", err)
+			}
+		default:
+			t.Fatalf("unexpected panic value: %T(%v)", err, err)
+		}
+	}()
+	b.Read(make([]byte, 100))
+}
+
+var errFake = errors.New("fake error")
+
+type errorThenGoodReader struct {
+	didErr bool
+	nread  int
+}
+
+func (r *errorThenGoodReader) Read(p []byte) (int, error) {
+	r.nread++
+	if !r.didErr {
+		r.didErr = true
+		return 0, errFake
+	}
+	return len(p), nil
+}
+
+func TestReaderClearError(t *testing.T) {
+	r := &errorThenGoodReader{}
+	b := NewReader(r)
+	buf := make([]byte, 1)
+	if _, err := b.Read(nil); err != nil {
+		t.Fatalf("1st nil Read = %v; want nil", err)
+	}
+	if _, err := b.Read(buf); err != errFake {
+		t.Fatalf("1st Read = %v; want errFake", err)
+	}
+	if _, err := b.Read(nil); err != nil {
+		t.Fatalf("2nd nil Read = %v; want nil", err)
+	}
+	if _, err := b.Read(buf); err != nil {
+		t.Fatalf("3rd Read with buffer = %v; want nil", err)
+	}
+	if r.nread != 2 {
+		t.Errorf("num reads = %d; want 2", r.nread)
+	}
+}
+
+// Test for golang.org/issue/5947
+func TestWriterReadFromWhileFull(t *testing.T) {
+	buf := new(bytes.Buffer)
+	w := NewWriterSize(buf, 10)
+
+	// Fill buffer exactly.
+	n, err := w.Write([]byte("0123456789"))
+	if n != 10 || err != nil {
+		t.Fatalf("Write returned (%v, %v), want (10, nil)", n, err)
+	}
+
+	// Use ReadFrom to read in some data.
+	n2, err := w.ReadFrom(strings.NewReader("abcdef"))
+	if n2 != 6 || err != nil {
+		t.Fatalf("ReadFrom returned (%v, %v), want (6, nil)", n2, err)
+	}
+}
+
+type emptyThenNonEmptyReader struct {
+	r io.Reader
+	n int
+}
+
+func (r *emptyThenNonEmptyReader) Read(p []byte) (int, error) {
+	if r.n <= 0 {
+		return r.r.Read(p)
+	}
+	r.n--
+	return 0, nil
+}
+
+// Test for golang.org/issue/7611
+func TestWriterReadFromUntilEOF(t *testing.T) {
+	buf := new(bytes.Buffer)
+	w := NewWriterSize(buf, 5)
+
+	// Partially fill buffer
+	n, err := w.Write([]byte("0123"))
+	if n != 4 || err != nil {
+		t.Fatalf("Write returned (%v, %v), want (4, nil)", n, err)
+	}
+
+	// Use ReadFrom to read in some data.
+	r := &emptyThenNonEmptyReader{r: strings.NewReader("abcd"), n: 3}
+	n2, err := w.ReadFrom(r)
+	if n2 != 4 || err != nil {
+		t.Fatalf("ReadFrom returned (%v, %v), want (4, nil)", n2, err)
+	}
+	w.Flush()
+	if got, want := buf.String(), "0123abcd"; got != want {
+		t.Fatalf("buf.Bytes() returned %q, want %q", got, want)
+	}
+}
+
+func TestWriterReadFromErrNoProgress(t *testing.T) {
+	buf := new(bytes.Buffer)
+	w := NewWriterSize(buf, 5)
+
+	// Partially fill buffer
+	n, err := w.Write([]byte("0123"))
+	if n != 4 || err != nil {
+		t.Fatalf("Write returned (%v, %v), want (4, nil)", n, err)
+	}
+
+	// Use ReadFrom to read in some data.
+	r := &emptyThenNonEmptyReader{r: strings.NewReader("abcd"), n: 100}
+	n2, err := w.ReadFrom(r)
+	if n2 != 0 || err != io.ErrNoProgress {
+		t.Fatalf("buf.Bytes() returned (%v, %v), want (0, io.ErrNoProgress)", n2, err)
+	}
+}
+
+type readFromWriter struct {
+	buf           []byte
+	writeBytes    int
+	readFromBytes int
+}
+
+func (w *readFromWriter) Write(p []byte) (int, error) {
+	w.buf = append(w.buf, p...)
+	w.writeBytes += len(p)
+	return len(p), nil
+}
+
+func (w *readFromWriter) ReadFrom(r io.Reader) (int64, error) {
+	b, err := io.ReadAll(r)
+	w.buf = append(w.buf, b...)
+	w.readFromBytes += len(b)
+	return int64(len(b)), err
+}
+
+// Test that calling (*Writer).ReadFrom with a partially-filled buffer
+// fills the buffer before switching over to ReadFrom.
+func TestWriterReadFromWithBufferedData(t *testing.T) {
+	const bufsize = 16
+
+	input := createTestInput(64)
+	rfw := &readFromWriter{}
+	w := NewWriterSize(rfw, bufsize)
+
+	const writeSize = 8
+	if n, err := w.Write(input[:writeSize]); n != writeSize || err != nil {
+		t.Errorf("w.Write(%v bytes) = %v, %v; want %v, nil", writeSize, n, err, writeSize)
+	}
+	n, err := w.ReadFrom(bytes.NewReader(input[writeSize:]))
+	if wantn := len(input[writeSize:]); int(n) != wantn || err != nil {
+		t.Errorf("io.Copy(w, %v bytes) = %v, %v; want %v, nil", wantn, n, err, wantn)
+	}
+	if err := w.Flush(); err != nil {
+		t.Errorf("w.Flush() = %v, want nil", err)
+	}
+
+	if got, want := rfw.writeBytes, bufsize; got != want {
+		t.Errorf("wrote %v bytes with Write, want %v", got, want)
+	}
+	if got, want := rfw.readFromBytes, len(input)-bufsize; got != want {
+		t.Errorf("wrote %v bytes with ReadFrom, want %v", got, want)
+	}
+}
+
+func TestReadZero(t *testing.T) {
+	for _, size := range []int{100, 2} {
+		t.Run(fmt.Sprintf("bufsize=%d", size), func(t *testing.T) {
+			r := io.MultiReader(strings.NewReader("abc"), &emptyThenNonEmptyReader{r: strings.NewReader("def"), n: 1})
+			br := NewReaderSize(r, size)
+			want := func(s string, wantErr error) {
+				p := make([]byte, 50)
+				n, err := br.Read(p)
+				if err != wantErr || n != len(s) || string(p[:n]) != s {
+					t.Fatalf("read(%d) = %q, %v, want %q, %v", len(p), string(p[:n]), err, s, wantErr)
+				}
+				t.Logf("read(%d) = %q, %v", len(p), string(p[:n]), err)
+			}
+			want("abc", nil)
+			want("", nil)
+			want("def", nil)
+			want("", io.EOF)
+		})
+	}
+}
+
+func TestReaderReset(t *testing.T) {
+	checkAll := func(r *Reader, want string) {
+		t.Helper()
+		all, err := io.ReadAll(r)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if string(all) != want {
+			t.Errorf("ReadAll returned %q, want %q", all, want)
+		}
+	}
+
+	r := NewReader(strings.NewReader("foo foo"))
+	buf := make([]byte, 3)
+	r.Read(buf)
+	if string(buf) != "foo" {
+		t.Errorf("buf = %q; want foo", buf)
+	}
+
+	r.Reset(strings.NewReader("bar bar"))
+	checkAll(r, "bar bar")
+
+	*r = Reader{} // zero out the Reader
+	r.Reset(strings.NewReader("bar bar"))
+	checkAll(r, "bar bar")
+
+	// Wrap a reader and then Reset to that reader.
+	r.Reset(strings.NewReader("recur"))
+	r2 := NewReader(r)
+	checkAll(r2, "recur")
+	r.Reset(strings.NewReader("recur2"))
+	r2.Reset(r)
+	checkAll(r2, "recur2")
+}
+
+func TestWriterReset(t *testing.T) {
+	var buf1, buf2, buf3, buf4, buf5 strings.Builder
+	w := NewWriter(&buf1)
+	w.WriteString("foo")
+
+	w.Reset(&buf2) // and not flushed
+	w.WriteString("bar")
+	w.Flush()
+	if buf1.String() != "" {
+		t.Errorf("buf1 = %q; want empty", buf1.String())
+	}
+	if buf2.String() != "bar" {
+		t.Errorf("buf2 = %q; want bar", buf2.String())
+	}
+
+	*w = Writer{}  // zero out the Writer
+	w.Reset(&buf3) // and not flushed
+	w.WriteString("bar")
+	w.Flush()
+	if buf1.String() != "" {
+		t.Errorf("buf1 = %q; want empty", buf1.String())
+	}
+	if buf3.String() != "bar" {
+		t.Errorf("buf3 = %q; want bar", buf3.String())
+	}
+
+	// Wrap a writer and then Reset to that writer.
+	w.Reset(&buf4)
+	w2 := NewWriter(w)
+	w2.WriteString("recur")
+	w2.Flush()
+	if buf4.String() != "recur" {
+		t.Errorf("buf4 = %q, want %q", buf4.String(), "recur")
+	}
+	w.Reset(&buf5)
+	w2.Reset(w)
+	w2.WriteString("recur2")
+	w2.Flush()
+	if buf5.String() != "recur2" {
+		t.Errorf("buf5 = %q, want %q", buf5.String(), "recur2")
+	}
+}
+
+func TestReaderDiscard(t *testing.T) {
+	tests := []struct {
+		name     string
+		r        io.Reader
+		bufSize  int // 0 means 16
+		peekSize int
+
+		n int // input to Discard
+
+		want    int   // from Discard
+		wantErr error // from Discard
+
+		wantBuffered int
+	}{
+		{
+			name:         "normal case",
+			r:            strings.NewReader("abcdefghijklmnopqrstuvwxyz"),
+			peekSize:     16,
+			n:            6,
+			want:         6,
+			wantBuffered: 10,
+		},
+		{
+			name:         "discard causing read",
+			r:            strings.NewReader("abcdefghijklmnopqrstuvwxyz"),
+			n:            6,
+			want:         6,
+			wantBuffered: 10,
+		},
+		{
+			name:         "discard all without peek",
+			r:            strings.NewReader("abcdefghijklmnopqrstuvwxyz"),
+			n:            26,
+			want:         26,
+			wantBuffered: 0,
+		},
+		{
+			name:         "discard more than end",
+			r:            strings.NewReader("abcdefghijklmnopqrstuvwxyz"),
+			n:            27,
+			want:         26,
+			wantErr:      io.EOF,
+			wantBuffered: 0,
+		},
+		// Any error from filling shouldn't show up until we
+		// get past the valid bytes. Here we return 5 valid bytes at the same time
+		// as an error, but test that we don't see the error from Discard.
+		{
+			name: "fill error, discard less",
+			r: newScriptedReader(func(p []byte) (n int, err error) {
+				if len(p) < 5 {
+					panic("unexpected small read")
+				}
+				return 5, errors.New("5-then-error")
+			}),
+			n:            4,
+			want:         4,
+			wantErr:      nil,
+			wantBuffered: 1,
+		},
+		{
+			name: "fill error, discard equal",
+			r: newScriptedReader(func(p []byte) (n int, err error) {
+				if len(p) < 5 {
+					panic("unexpected small read")
+				}
+				return 5, errors.New("5-then-error")
+			}),
+			n:            5,
+			want:         5,
+			wantErr:      nil,
+			wantBuffered: 0,
+		},
+		{
+			name: "fill error, discard more",
+			r: newScriptedReader(func(p []byte) (n int, err error) {
+				if len(p) < 5 {
+					panic("unexpected small read")
+				}
+				return 5, errors.New("5-then-error")
+			}),
+			n:            6,
+			want:         5,
+			wantErr:      errors.New("5-then-error"),
+			wantBuffered: 0,
+		},
+		// Discard of 0 shouldn't cause a read:
+		{
+			name:         "discard zero",
+			r:            newScriptedReader(), // will panic on Read
+			n:            0,
+			want:         0,
+			wantErr:      nil,
+			wantBuffered: 0,
+		},
+		{
+			name:         "discard negative",
+			r:            newScriptedReader(), // will panic on Read
+			n:            -1,
+			want:         0,
+			wantErr:      ErrNegativeCount,
+			wantBuffered: 0,
+		},
+	}
+	for _, tt := range tests {
+		br := NewReaderSize(tt.r, tt.bufSize)
+		if tt.peekSize > 0 {
+			peekBuf, err := br.Peek(tt.peekSize)
+			if err != nil {
+				t.Errorf("%s: Peek(%d): %v", tt.name, tt.peekSize, err)
+				continue
+			}
+			if len(peekBuf) != tt.peekSize {
+				t.Errorf("%s: len(Peek(%d)) = %v; want %v", tt.name, tt.peekSize, len(peekBuf), tt.peekSize)
+				continue
+			}
+		}
+		discarded, err := br.Discard(tt.n)
+		if ge, we := fmt.Sprint(err), fmt.Sprint(tt.wantErr); discarded != tt.want || ge != we {
+			t.Errorf("%s: Discard(%d) = (%v, %v); want (%v, %v)", tt.name, tt.n, discarded, ge, tt.want, we)
+			continue
+		}
+		if bn := br.Buffered(); bn != tt.wantBuffered {
+			t.Errorf("%s: after Discard, Buffered = %d; want %d", tt.name, bn, tt.wantBuffered)
+		}
+	}
+
+}
+
+func TestReaderSize(t *testing.T) {
+	if got, want := NewReader(nil).Size(), DefaultBufSize; got != want {
+		t.Errorf("NewReader's Reader.Size = %d; want %d", got, want)
+	}
+	if got, want := NewReaderSize(nil, 1234).Size(), 1234; got != want {
+		t.Errorf("NewReaderSize's Reader.Size = %d; want %d", got, want)
+	}
+}
+
+func TestWriterSize(t *testing.T) {
+	if got, want := NewWriter(nil).Size(), DefaultBufSize; got != want {
+		t.Errorf("NewWriter's Writer.Size = %d; want %d", got, want)
+	}
+	if got, want := NewWriterSize(nil, 1234).Size(), 1234; got != want {
+		t.Errorf("NewWriterSize's Writer.Size = %d; want %d", got, want)
+	}
+}
+
+// An onlyReader only implements io.Reader, no matter what other methods the underlying implementation may have.
+type onlyReader struct {
+	io.Reader
+}
+
+// An onlyWriter only implements io.Writer, no matter what other methods the underlying implementation may have.
+type onlyWriter struct {
+	io.Writer
+}
+
+// A scriptedReader is an io.Reader that executes its steps sequentially.
+type scriptedReader []func(p []byte) (n int, err error)
+
+func (sr *scriptedReader) Read(p []byte) (n int, err error) {
+	if len(*sr) == 0 {
+		panic("too many Read calls on scripted Reader. No steps remain.")
+	}
+	step := (*sr)[0]
+	*sr = (*sr)[1:]
+	return step(p)
+}
+
+func newScriptedReader(steps ...func(p []byte) (n int, err error)) io.Reader {
+	sr := scriptedReader(steps)
+	return &sr
+}
+
+// eofReader returns the number of bytes read and io.EOF for the read that consumes the last of the content.
+type eofReader struct {
+	buf []byte
+}
+
+func (r *eofReader) Read(p []byte) (int, error) {
+	read := copy(p, r.buf)
+	r.buf = r.buf[read:]
+
+	switch read {
+	case 0, len(r.buf):
+		// As allowed in the documentation, this will return io.EOF
+		// in the same call that consumes the last of the data.
+		// https://godoc.org/io#Reader
+		return read, io.EOF
+	}
+
+	return read, nil
+}
+
+func TestPartialReadEOF(t *testing.T) {
+	src := make([]byte, 10)
+	eofR := &eofReader{buf: src}
+	r := NewReader(eofR)
+
+	// Start by reading 5 of the 10 available bytes.
+	dest := make([]byte, 5)
+	read, err := r.Read(dest)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if n := len(dest); read != n {
+		t.Fatalf("read %d bytes; wanted %d bytes", read, n)
+	}
+
+	// The Reader should have buffered all the content from the io.Reader.
+	if n := len(eofR.buf); n != 0 {
+		t.Fatalf("got %d bytes left in bufio.Reader source; want 0 bytes", n)
+	}
+	// To prove the point, check that there are still 5 bytes available to read.
+	if n := r.Buffered(); n != 5 {
+		t.Fatalf("got %d bytes buffered in bufio.Reader; want 5 bytes", n)
+	}
+
+	// This is the second read of 0 bytes.
+	read, err = r.Read([]byte{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if read != 0 {
+		t.Fatalf("read %d bytes; want 0 bytes", read)
+	}
+}
+
+type writerWithReadFromError struct{}
+
+func (w writerWithReadFromError) ReadFrom(r io.Reader) (int64, error) {
+	return 0, errors.New("writerWithReadFromError error")
+}
+
+func (w writerWithReadFromError) Write(b []byte) (n int, err error) {
+	return 10, nil
+}
+
+func TestWriterReadFromMustSetUnderlyingError(t *testing.T) {
+	var wr = NewWriter(writerWithReadFromError{})
+	if _, err := wr.ReadFrom(strings.NewReader("test2")); err == nil {
+		t.Fatal("expected ReadFrom returns error, got nil")
+	}
+	if _, err := wr.Write([]byte("123")); err == nil {
+		t.Fatal("expected Write returns error, got nil")
+	}
+}
+
+type writeErrorOnlyWriter struct{}
+
+func (w writeErrorOnlyWriter) Write(p []byte) (n int, err error) {
+	return 0, errors.New("writeErrorOnlyWriter error")
+}
+
+// Ensure that previous Write errors are immediately returned
+// on any ReadFrom. See golang.org/issue/35194.
+func TestWriterReadFromMustReturnUnderlyingError(t *testing.T) {
+	var wr = NewWriter(writeErrorOnlyWriter{})
+	s := "test1"
+	wantBuffered := len(s)
+	if _, err := wr.WriteString(s); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if err := wr.Flush(); err == nil {
+		t.Error("expected flush error, got nil")
+	}
+	if _, err := wr.ReadFrom(strings.NewReader("test2")); err == nil {
+		t.Fatal("expected error, got nil")
+	}
+	if buffered := wr.Buffered(); buffered != wantBuffered {
+		t.Fatalf("Buffered = %v; want %v", buffered, wantBuffered)
+	}
+}
+
+func BenchmarkReaderCopyOptimal(b *testing.B) {
+	// Optimal case is where the underlying reader implements io.WriterTo
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := NewReader(srcBuf)
+	dstBuf := new(bytes.Buffer)
+	dst := onlyWriter{dstBuf}
+	for i := 0; i < b.N; i++ {
+		srcBuf.Reset()
+		src.Reset(srcBuf)
+		dstBuf.Reset()
+		io.Copy(dst, src)
+	}
+}
+
+func BenchmarkReaderCopyUnoptimal(b *testing.B) {
+	// Unoptimal case is where the underlying reader doesn't implement io.WriterTo
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := NewReader(onlyReader{srcBuf})
+	dstBuf := new(bytes.Buffer)
+	dst := onlyWriter{dstBuf}
+	for i := 0; i < b.N; i++ {
+		srcBuf.Reset()
+		src.Reset(onlyReader{srcBuf})
+		dstBuf.Reset()
+		io.Copy(dst, src)
+	}
+}
+
+func BenchmarkReaderCopyNoWriteTo(b *testing.B) {
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	srcReader := NewReader(srcBuf)
+	src := onlyReader{srcReader}
+	dstBuf := new(bytes.Buffer)
+	dst := onlyWriter{dstBuf}
+	for i := 0; i < b.N; i++ {
+		srcBuf.Reset()
+		srcReader.Reset(srcBuf)
+		dstBuf.Reset()
+		io.Copy(dst, src)
+	}
+}
+
+func BenchmarkReaderWriteToOptimal(b *testing.B) {
+	const bufSize = 16 << 10
+	buf := make([]byte, bufSize)
+	r := bytes.NewReader(buf)
+	srcReader := NewReaderSize(onlyReader{r}, 1<<10)
+	if _, ok := io.Discard.(io.ReaderFrom); !ok {
+		b.Fatal("io.Discard doesn't support ReaderFrom")
+	}
+	for i := 0; i < b.N; i++ {
+		r.Seek(0, io.SeekStart)
+		srcReader.Reset(onlyReader{r})
+		n, err := srcReader.WriteTo(io.Discard)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if n != bufSize {
+			b.Fatalf("n = %d; want %d", n, bufSize)
+		}
+	}
+}
+
+func BenchmarkReaderReadString(b *testing.B) {
+	r := strings.NewReader("       foo       foo        42        42        42        42        42        42        42        42       4.2       4.2       4.2       4.2\n")
+	buf := NewReader(r)
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		r.Seek(0, io.SeekStart)
+		buf.Reset(r)
+
+		_, err := buf.ReadString('\n')
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func BenchmarkWriterCopyOptimal(b *testing.B) {
+	// Optimal case is where the underlying writer implements io.ReaderFrom
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := onlyReader{srcBuf}
+	dstBuf := new(bytes.Buffer)
+	dst := NewWriter(dstBuf)
+	for i := 0; i < b.N; i++ {
+		srcBuf.Reset()
+		dstBuf.Reset()
+		dst.Reset(dstBuf)
+		io.Copy(dst, src)
+	}
+}
+
+func BenchmarkWriterCopyUnoptimal(b *testing.B) {
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := onlyReader{srcBuf}
+	dstBuf := new(bytes.Buffer)
+	dst := NewWriter(onlyWriter{dstBuf})
+	for i := 0; i < b.N; i++ {
+		srcBuf.Reset()
+		dstBuf.Reset()
+		dst.Reset(onlyWriter{dstBuf})
+		io.Copy(dst, src)
+	}
+}
+
+func BenchmarkWriterCopyNoReadFrom(b *testing.B) {
+	srcBuf := bytes.NewBuffer(make([]byte, 8192))
+	src := onlyReader{srcBuf}
+	dstBuf := new(bytes.Buffer)
+	dstWriter := NewWriter(dstBuf)
+	dst := onlyWriter{dstWriter}
+	for i := 0; i < b.N; i++ {
+		srcBuf.Reset()
+		dstBuf.Reset()
+		dstWriter.Reset(dstBuf)
+		io.Copy(dst, src)
+	}
+}
+
+func BenchmarkReaderEmpty(b *testing.B) {
+	b.ReportAllocs()
+	str := strings.Repeat("x", 16<<10)
+	for i := 0; i < b.N; i++ {
+		br := NewReader(strings.NewReader(str))
+		n, err := io.Copy(io.Discard, br)
+		if err != nil {
+			b.Fatal(err)
+		}
+		if n != int64(len(str)) {
+			b.Fatal("wrong length")
+		}
+	}
+}
+
+func BenchmarkWriterEmpty(b *testing.B) {
+	b.ReportAllocs()
+	str := strings.Repeat("x", 1<<10)
+	bs := []byte(str)
+	for i := 0; i < b.N; i++ {
+		bw := NewWriter(io.Discard)
+		bw.Flush()
+		bw.WriteByte('a')
+		bw.Flush()
+		bw.WriteRune('B')
+		bw.Flush()
+		bw.Write(bs)
+		bw.Flush()
+		bw.WriteString(str)
+		bw.Flush()
+	}
+}
+
+func BenchmarkWriterFlush(b *testing.B) {
+	b.ReportAllocs()
+	bw := NewWriter(io.Discard)
+	str := strings.Repeat("x", 50)
+	for i := 0; i < b.N; i++ {
+		bw.WriteString(str)
+		bw.Flush()
+	}
+}

go/src/bufio/example_test.go

@@ -0,0 +1,200 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bufio_test
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+func ExampleWriter() {
+	w := bufio.NewWriter(os.Stdout)
+	fmt.Fprint(w, "Hello, ")
+	fmt.Fprint(w, "world!")
+	w.Flush() // Don't forget to flush!
+	// Output: Hello, world!
+}
+
+func ExampleWriter_AvailableBuffer() {
+	w := bufio.NewWriter(os.Stdout)
+	for _, i := range []int64{1, 2, 3, 4} {
+		b := w.AvailableBuffer()
+		b = strconv.AppendInt(b, i, 10)
+		b = append(b, ' ')
+		w.Write(b)
+	}
+	w.Flush()
+	// Output: 1 2 3 4
+}
+
+// ExampleWriter_ReadFrom demonstrates how to use the ReadFrom method of Writer.
+func ExampleWriter_ReadFrom() {
+	var buf bytes.Buffer
+	writer := bufio.NewWriter(&buf)
+
+	data := "Hello, world!\nThis is a ReadFrom example."
+	reader := strings.NewReader(data)
+
+	n, err := writer.ReadFrom(reader)
+	if err != nil {
+		fmt.Println("ReadFrom Error:", err)
+		return
+	}
+
+	if err = writer.Flush(); err != nil {
+		fmt.Println("Flush Error:", err)
+		return
+	}
+
+	fmt.Println("Bytes written:", n)
+	fmt.Println("Buffer contents:", buf.String())
+	// Output:
+	// Bytes written: 41
+	// Buffer contents: Hello, world!
+	// This is a ReadFrom example.
+}
+
+// The simplest use of a Scanner, to read standard input as a set of lines.
+func ExampleScanner_lines() {
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		fmt.Println(scanner.Text()) // Println will add back the final '\n'
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stderr, "reading standard input:", err)
+	}
+}
+
+// Return the most recent call to Scan as a []byte.
+func ExampleScanner_Bytes() {
+	scanner := bufio.NewScanner(strings.NewReader("gopher"))
+	for scanner.Scan() {
+		fmt.Println(len(scanner.Bytes()) == 6)
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stderr, "shouldn't see an error scanning a string")
+	}
+	// Output:
+	// true
+}
+
+// Use a Scanner to implement a simple word-count utility by scanning the
+// input as a sequence of space-delimited tokens.
+func ExampleScanner_words() {
+	// An artificial input source.
+	const input = "Now is the winter of our discontent,\nMade glorious summer by this sun of York.\n"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+	// Set the split function for the scanning operation.
+	scanner.Split(bufio.ScanWords)
+	// Count the words.
+	count := 0
+	for scanner.Scan() {
+		count++
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stderr, "reading input:", err)
+	}
+	fmt.Printf("%d\n", count)
+	// Output: 15
+}
+
+// Use a Scanner with a custom split function (built by wrapping ScanWords) to validate
+// 32-bit decimal input.
+func ExampleScanner_custom() {
+	// An artificial input source.
+	const input = "1234 5678 1234567901234567890"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+	// Create a custom split function by wrapping the existing ScanWords function.
+	split := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+		advance, token, err = bufio.ScanWords(data, atEOF)
+		if err == nil && token != nil {
+			_, err = strconv.ParseInt(string(token), 10, 32)
+		}
+		return
+	}
+	// Set the split function for the scanning operation.
+	scanner.Split(split)
+	// Validate the input
+	for scanner.Scan() {
+		fmt.Printf("%s\n", scanner.Text())
+	}
+
+	if err := scanner.Err(); err != nil {
+		fmt.Printf("Invalid input: %s", err)
+	}
+	// Output:
+	// 1234
+	// 5678
+	// Invalid input: strconv.ParseInt: parsing "1234567901234567890": value out of range
+}
+
+// Use a Scanner with a custom split function to parse a comma-separated
+// list with an empty final value.
+func ExampleScanner_emptyFinalToken() {
+	// Comma-separated list; last entry is empty.
+	const input = "1,2,3,4,"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+	// Define a split function that separates on commas.
+	onComma := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+		for i := 0; i < len(data); i++ {
+			if data[i] == ',' {
+				return i + 1, data[:i], nil
+			}
+		}
+		if !atEOF {
+			return 0, nil, nil
+		}
+		// There is one final token to be delivered, which may be the empty string.
+		// Returning bufio.ErrFinalToken here tells Scan there are no more tokens after this
+		// but does not trigger an error to be returned from Scan itself.
+		return 0, data, bufio.ErrFinalToken
+	}
+	scanner.Split(onComma)
+	// Scan.
+	for scanner.Scan() {
+		fmt.Printf("%q ", scanner.Text())
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stderr, "reading input:", err)
+	}
+	// Output: "1" "2" "3" "4" ""
+}
+
+// Use a Scanner with a custom split function to parse a comma-separated
+// list with an empty final value but stops at the token "STOP".
+func ExampleScanner_earlyStop() {
+	onComma := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+		i := bytes.IndexByte(data, ',')
+		if i == -1 {
+			if !atEOF {
+				return 0, nil, nil
+			}
+			// If we have reached the end, return the last token.
+			return 0, data, bufio.ErrFinalToken
+		}
+		// If the token is "STOP", stop the scanning and ignore the rest.
+		if string(data[:i]) == "STOP" {
+			return i + 1, nil, bufio.ErrFinalToken
+		}
+		// Otherwise, return the token before the comma.
+		return i + 1, data[:i], nil
+	}
+	const input = "1,2,STOP,4,"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+	scanner.Split(onComma)
+	for scanner.Scan() {
+		fmt.Printf("Got a token %q\n", scanner.Text())
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stderr, "reading input:", err)
+	}
+	// Output:
+	// Got a token "1"
+	// Got a token "2"
+}

go/src/bufio/export_test.go

@@ -0,0 +1,29 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bufio
+
+// Exported for testing only.
+import (
+	"unicode/utf8"
+)
+
+var IsSpace = isSpace
+
+const DefaultBufSize = defaultBufSize
+
+func (s *Scanner) MaxTokenSize(n int) {
+	if n < utf8.UTFMax || n > 1e9 {
+		panic("bad max token size")
+	}
+	if n < len(s.buf) {
+		s.buf = make([]byte, n)
+	}
+	s.maxTokenSize = n
+}
+
+// ErrOrEOF is like Err, but returns EOF. Used to test a corner case.
+func (s *Scanner) ErrOrEOF() error {
+	return s.err
+}

go/src/bufio/net_test.go

@@ -0,0 +1,96 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build unix
+
+package bufio_test
+
+import (
+	"bufio"
+	"io"
+	"net"
+	"path/filepath"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// TestCopyUnixpacket tests that we can use bufio when copying
+// across a unixpacket socket. This used to fail due to an unnecessary
+// empty Write call that was interpreted as an EOF.
+func TestCopyUnixpacket(t *testing.T) {
+	tmpDir := t.TempDir()
+	socket := filepath.Join(tmpDir, "unixsock")
+
+	// Start a unixpacket server.
+	addr := &net.UnixAddr{
+		Name: socket,
+		Net:  "unixpacket",
+	}
+	server, err := net.ListenUnix("unixpacket", addr)
+	if err != nil {
+		t.Skipf("skipping test because opening a unixpacket socket failed: %v", err)
+	}
+
+	// Start a goroutine for the server to accept one connection
+	// and read all the data sent on the connection,
+	// reporting the number of bytes read on ch.
+	ch := make(chan int, 1)
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+
+		tot := 0
+		defer func() {
+			ch <- tot
+		}()
+
+		serverConn, err := server.Accept()
+		if err != nil {
+			t.Error(err)
+			return
+		}
+
+		buf := make([]byte, 1024)
+		for {
+			n, err := serverConn.Read(buf)
+			tot += n
+			if err == io.EOF {
+				return
+			}
+			if err != nil {
+				t.Error(err)
+				return
+			}
+		}
+	}()
+
+	clientConn, err := net.DialUnix("unixpacket", nil, addr)
+	if err != nil {
+		// Leaves the server goroutine hanging. Oh well.
+		t.Fatal(err)
+	}
+
+	defer wg.Wait()
+	defer clientConn.Close()
+
+	const data = "data"
+	r := bufio.NewReader(strings.NewReader(data))
+	n, err := io.Copy(clientConn, r)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if n != int64(len(data)) {
+		t.Errorf("io.Copy returned %d, want %d", n, len(data))
+	}
+
+	clientConn.Close()
+	tot := <-ch
+
+	if tot != len(data) {
+		t.Errorf("server read %d, want %d", tot, len(data))
+	}
+}

go/src/bufio/scan.go

@@ -0,0 +1,427 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bufio
+
+import (
+	"bytes"
+	"errors"
+	"io"
+	"unicode/utf8"
+)
+
+// Scanner provides a convenient interface for reading data such as
+// a file of newline-delimited lines of text. Successive calls to
+// the [Scanner.Scan] method will step through the 'tokens' of a file, skipping
+// the bytes between the tokens. The specification of a token is
+// defined by a split function of type [SplitFunc]; the default split
+// function breaks the input into lines with line termination stripped. [Scanner.Split]
+// functions are defined in this package for scanning a file into
+// lines, bytes, UTF-8-encoded runes, and space-delimited words. The
+// client may instead provide a custom split function.
+//
+// Scanning stops unrecoverably at EOF, the first I/O error, or a token too
+// large to fit in the [Scanner.Buffer]. When a scan stops, the reader may have
+// advanced arbitrarily far past the last token. Programs that need more
+// control over error handling or large tokens, or must run sequential scans
+// on a reader, should use [bufio.Reader] instead.
+type Scanner struct {
+	r            io.Reader // The reader provided by the client.
+	split        SplitFunc // The function to split the tokens.
+	maxTokenSize int       // Maximum size of a token; modified by tests.
+	token        []byte    // Last token returned by split.
+	buf          []byte    // Buffer used as argument to split.
+	start        int       // First non-processed byte in buf.
+	end          int       // End of data in buf.
+	err          error     // Sticky error.
+	empties      int       // Count of successive empty tokens.
+	scanCalled   bool      // Scan has been called; buffer is in use.
+	done         bool      // Scan has finished.
+}
+
+// SplitFunc is the signature of the split function used to tokenize the
+// input. The arguments are an initial substring of the remaining unprocessed
+// data and a flag, atEOF, that reports whether the [Reader] has no more data
+// to give. The return values are the number of bytes to advance the input
+// and the next token to return to the user, if any, plus an error, if any.
+//
+// Scanning stops if the function returns an error, in which case some of
+// the input may be discarded. If that error is [ErrFinalToken], scanning
+// stops with no error. A non-nil token delivered with [ErrFinalToken]
+// will be the last token, and a nil token with [ErrFinalToken]
+// immediately stops the scanning.
+//
+// Otherwise, the [Scanner] advances the input. If the token is not nil,
+// the [Scanner] returns it to the user. If the token is nil, the
+// Scanner reads more data and continues scanning; if there is no more
+// data--if atEOF was true--the [Scanner] returns. If the data does not
+// yet hold a complete token, for instance if it has no newline while
+// scanning lines, a [SplitFunc] can return (0, nil, nil) to signal the
+// [Scanner] to read more data into the slice and try again with a
+// longer slice starting at the same point in the input.
+//
+// The function is never called with an empty data slice unless atEOF
+// is true. If atEOF is true, however, data may be non-empty and,
+// as always, holds unprocessed text.
+type SplitFunc func(data []byte, atEOF bool) (advance int, token []byte, err error)
+
+// Errors returned by Scanner.
+var (
+	ErrTooLong         = errors.New("bufio.Scanner: token too long")
+	ErrNegativeAdvance = errors.New("bufio.Scanner: SplitFunc returns negative advance count")
+	ErrAdvanceTooFar   = errors.New("bufio.Scanner: SplitFunc returns advance count beyond input")
+	ErrBadReadCount    = errors.New("bufio.Scanner: Read returned impossible count")
+)
+
+const (
+	// MaxScanTokenSize is the maximum size used to buffer a token
+	// unless the user provides an explicit buffer with [Scanner.Buffer].
+	// The actual maximum token size may be smaller as the buffer
+	// may need to include, for instance, a newline.
+	MaxScanTokenSize = 64 * 1024
+
+	startBufSize = 4096 // Size of initial allocation for buffer.
+)
+
+// NewScanner returns a new [Scanner] to read from r.
+// The split function defaults to [ScanLines].
+func NewScanner(r io.Reader) *Scanner {
+	return &Scanner{
+		r:            r,
+		split:        ScanLines,
+		maxTokenSize: MaxScanTokenSize,
+	}
+}
+
+// Err returns the first non-EOF error that was encountered by the [Scanner].
+func (s *Scanner) Err() error {
+	if s.err == io.EOF {
+		return nil
+	}
+	return s.err
+}
+
+// Bytes returns the most recent token generated by a call to [Scanner.Scan].
+// The underlying array may point to data that will be overwritten
+// by a subsequent call to Scan. It does no allocation.
+func (s *Scanner) Bytes() []byte {
+	return s.token
+}
+
+// Text returns the most recent token generated by a call to [Scanner.Scan]
+// as a newly allocated string holding its bytes.
+func (s *Scanner) Text() string {
+	return string(s.token)
+}
+
+// ErrFinalToken is a special sentinel error value. It is intended to be
+// returned by a Split function to indicate that the scanning should stop
+// with no error. If the token being delivered with this error is not nil,
+// the token is the last token.
+//
+// The value is useful to stop processing early or when it is necessary to
+// deliver a final empty token (which is different from a nil token).
+// One could achieve the same behavior with a custom error value but
+// providing one here is tidier.
+// See the emptyFinalToken example for a use of this value.
+var ErrFinalToken = errors.New("final token")
+
+// Scan advances the [Scanner] to the next token, which will then be
+// available through the [Scanner.Bytes] or [Scanner.Text] method. It returns false when
+// there are no more tokens, either by reaching the end of the input or an error.
+// After Scan returns false, the [Scanner.Err] method will return any error that
+// occurred during scanning, except that if it was [io.EOF], [Scanner.Err]
+// will return nil.
+// Scan panics if the split function returns too many empty
+// tokens without advancing the input. This is a common error mode for
+// scanners.
+func (s *Scanner) Scan() bool {
+	if s.done {
+		return false
+	}
+	s.scanCalled = true
+	// Loop until we have a token.
+	for {
+		// See if we can get a token with what we already have.
+		// If we've run out of data but have an error, give the split function
+		// a chance to recover any remaining, possibly empty token.
+		if s.end > s.start || s.err != nil {
+			advance, token, err := s.split(s.buf[s.start:s.end], s.err != nil)
+			if err != nil {
+				if err == ErrFinalToken {
+					s.token = token
+					s.done = true
+					// When token is not nil, it means the scanning stops
+					// with a trailing token, and thus the return value
+					// should be true to indicate the existence of the token.
+					return token != nil
+				}
+				s.setErr(err)
+				return false
+			}
+			if !s.advance(advance) {
+				return false
+			}
+			s.token = token
+			if token != nil {
+				if s.err == nil || advance > 0 {
+					s.empties = 0
+				} else {
+					// Returning tokens not advancing input at EOF.
+					s.empties++
+					if s.empties > maxConsecutiveEmptyReads {
+						panic("bufio.Scan: too many empty tokens without progressing")
+					}
+				}
+				return true
+			}
+		}
+		// We cannot generate a token with what we are holding.
+		// If we've already hit EOF or an I/O error, we are done.
+		if s.err != nil {
+			// Shut it down.
+			s.start = 0
+			s.end = 0
+			return false
+		}
+		// Must read more data.
+		// First, shift data to beginning of buffer if there's lots of empty space
+		// or space is needed.
+		if s.start > 0 && (s.end == len(s.buf) || s.start > len(s.buf)/2) {
+			copy(s.buf, s.buf[s.start:s.end])
+			s.end -= s.start
+			s.start = 0
+		}
+		// Is the buffer full? If so, resize.
+		if s.end == len(s.buf) {
+			// Guarantee no overflow in the multiplication below.
+			const maxInt = int(^uint(0) >> 1)
+			if len(s.buf) >= s.maxTokenSize || len(s.buf) > maxInt/2 {
+				s.setErr(ErrTooLong)
+				return false
+			}
+			newSize := len(s.buf) * 2
+			if newSize == 0 {
+				newSize = startBufSize
+			}
+			newSize = min(newSize, s.maxTokenSize)
+			newBuf := make([]byte, newSize)
+			copy(newBuf, s.buf[s.start:s.end])
+			s.buf = newBuf
+			s.end -= s.start
+			s.start = 0
+		}
+		// Finally we can read some input. Make sure we don't get stuck with
+		// a misbehaving Reader. Officially we don't need to do this, but let's
+		// be extra careful: Scanner is for safe, simple jobs.
+		for loop := 0; ; {
+			n, err := s.r.Read(s.buf[s.end:len(s.buf)])
+			if n < 0 || len(s.buf)-s.end < n {
+				s.setErr(ErrBadReadCount)
+				break
+			}
+			s.end += n
+			if err != nil {
+				s.setErr(err)
+				break
+			}
+			if n > 0 {
+				s.empties = 0
+				break
+			}
+			loop++
+			if loop > maxConsecutiveEmptyReads {
+				s.setErr(io.ErrNoProgress)
+				break
+			}
+		}
+	}
+}
+
+// advance consumes n bytes of the buffer. It reports whether the advance was legal.
+func (s *Scanner) advance(n int) bool {
+	if n < 0 {
+		s.setErr(ErrNegativeAdvance)
+		return false
+	}
+	if n > s.end-s.start {
+		s.setErr(ErrAdvanceTooFar)
+		return false
+	}
+	s.start += n
+	return true
+}
+
+// setErr records the first error encountered.
+func (s *Scanner) setErr(err error) {
+	if s.err == nil || s.err == io.EOF {
+		s.err = err
+	}
+}
+
+// Buffer controls memory allocation by the Scanner.
+// It sets the initial buffer to use when scanning
+// and the maximum size of buffer that may be allocated during scanning.
+// The contents of the buffer are ignored.
+//
+// The maximum token size must be less than the larger of max and cap(buf).
+// If max <= cap(buf), [Scanner.Scan] will use this buffer only and do no allocation.
+//
+// By default, [Scanner.Scan] uses an internal buffer and sets the
+// maximum token size to [MaxScanTokenSize].
+//
+// Buffer panics if it is called after scanning has started.
+func (s *Scanner) Buffer(buf []byte, max int) {
+	if s.scanCalled {
+		panic("Buffer called after Scan")
+	}
+	s.buf = buf[0:cap(buf)]
+	s.maxTokenSize = max
+}
+
+// Split sets the split function for the [Scanner].
+// The default split function is [ScanLines].
+//
+// Split panics if it is called after scanning has started.
+func (s *Scanner) Split(split SplitFunc) {
+	if s.scanCalled {
+		panic("Split called after Scan")
+	}
+	s.split = split
+}
+
+// Split functions
+
+// ScanBytes is a split function for a [Scanner] that returns each byte as a token.
+func ScanBytes(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+	return 1, data[0:1], nil
+}
+
+var errorRune = []byte(string(utf8.RuneError))
+
+// ScanRunes is a split function for a [Scanner] that returns each
+// UTF-8-encoded rune as a token. The sequence of runes returned is
+// equivalent to that from a range loop over the input as a string, which
+// means that erroneous UTF-8 encodings translate to U+FFFD = "\xef\xbf\xbd".
+// Because of the Scan interface, this makes it impossible for the client to
+// distinguish correctly encoded replacement runes from encoding errors.
+func ScanRunes(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+
+	// Fast path 1: ASCII.
+	if data[0] < utf8.RuneSelf {
+		return 1, data[0:1], nil
+	}
+
+	// Fast path 2: Correct UTF-8 decode without error.
+	_, width := utf8.DecodeRune(data)
+	if width > 1 {
+		// It's a valid encoding. Width cannot be one for a correctly encoded
+		// non-ASCII rune.
+		return width, data[0:width], nil
+	}
+
+	// We know it's an error: we have width==1 and implicitly r==utf8.RuneError.
+	// Is the error because there wasn't a full rune to be decoded?
+	// FullRune distinguishes correctly between erroneous and incomplete encodings.
+	if !atEOF && !utf8.FullRune(data) {
+		// Incomplete; get more bytes.
+		return 0, nil, nil
+	}
+
+	// We have a real UTF-8 encoding error. Return a properly encoded error rune
+	// but advance only one byte. This matches the behavior of a range loop over
+	// an incorrectly encoded string.
+	return 1, errorRune, nil
+}
+
+// dropCR drops a terminal \r from the data.
+func dropCR(data []byte) []byte {
+	if len(data) > 0 && data[len(data)-1] == '\r' {
+		return data[0 : len(data)-1]
+	}
+	return data
+}
+
+// ScanLines is a split function for a [Scanner] that returns each line of
+// text, stripped of any trailing end-of-line marker. The returned line may
+// be empty. The end-of-line marker is one optional carriage return followed
+// by one mandatory newline. In regular expression notation, it is `\r?\n`.
+// The last non-empty line of input will be returned even if it has no
+// newline.
+func ScanLines(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if atEOF && len(data) == 0 {
+		return 0, nil, nil
+	}
+	if i := bytes.IndexByte(data, '\n'); i >= 0 {
+		// We have a full newline-terminated line.
+		return i + 1, dropCR(data[0:i]), nil
+	}
+	// If we're at EOF, we have a final, non-terminated line. Return it.
+	if atEOF {
+		return len(data), dropCR(data), nil
+	}
+	// Request more data.
+	return 0, nil, nil
+}
+
+// isSpace reports whether the character is a Unicode white space character.
+// We avoid dependency on the unicode package, but check validity of the implementation
+// in the tests.
+func isSpace(r rune) bool {
+	if r <= '\u00FF' {
+		// Obvious ASCII ones: \t through \r plus space. Plus two Latin-1 oddballs.
+		switch r {
+		case ' ', '\t', '\n', '\v', '\f', '\r':
+			return true
+		case '\u0085', '\u00A0':
+			return true
+		}
+		return false
+	}
+	// High-valued ones.
+	if '\u2000' <= r && r <= '\u200a' {
+		return true
+	}
+	switch r {
+	case '\u1680', '\u2028', '\u2029', '\u202f', '\u205f', '\u3000':
+		return true
+	}
+	return false
+}
+
+// ScanWords is a split function for a [Scanner] that returns each
+// space-separated word of text, with surrounding spaces deleted. It will
+// never return an empty string. The definition of space is set by
+// unicode.IsSpace.
+func ScanWords(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	// Skip leading spaces.
+	start := 0
+	for width := 0; start < len(data); start += width {
+		var r rune
+		r, width = utf8.DecodeRune(data[start:])
+		if !isSpace(r) {
+			break
+		}
+	}
+	// Scan until space, marking end of word.
+	for width, i := 0, start; i < len(data); i += width {
+		var r rune
+		r, width = utf8.DecodeRune(data[i:])
+		if isSpace(r) {
+			return i + width, data[start:i], nil
+		}
+	}
+	// If we're at EOF, we have a final, non-empty, non-terminated word. Return it.
+	if atEOF && len(data) > start {
+		return len(data), data[start:], nil
+	}
+	// Request more data.
+	return start, nil, nil
+}

go/src/bufio/scan_test.go

@@ -0,0 +1,596 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bufio_test
+
+import (
+	. "bufio"
+	"bytes"
+	"errors"
+	"io"
+	"strings"
+	"testing"
+	"unicode"
+	"unicode/utf8"
+)
+
+const smallMaxTokenSize = 256 // Much smaller for more efficient testing.
+
+// Test white space table matches the Unicode definition.
+func TestSpace(t *testing.T) {
+	for r := rune(0); r <= utf8.MaxRune; r++ {
+		if IsSpace(r) != unicode.IsSpace(r) {
+			t.Fatalf("white space property disagrees: %#U should be %t", r, unicode.IsSpace(r))
+		}
+	}
+}
+
+var scanTests = []string{
+	"",
+	"a",
+	"¼",
+	"☹",
+	"\x81",   // UTF-8 error
+	"\uFFFD", // correctly encoded RuneError
+	"abcdefgh",
+	"abc def\n\t\tgh    ",
+	"abc¼☹\x81\uFFFD日本語\x82abc",
+}
+
+func TestScanByte(t *testing.T) {
+	for n, test := range scanTests {
+		buf := strings.NewReader(test)
+		s := NewScanner(buf)
+		s.Split(ScanBytes)
+		var i int
+		for i = 0; s.Scan(); i++ {
+			if b := s.Bytes(); len(b) != 1 || b[0] != test[i] {
+				t.Errorf("#%d: %d: expected %q got %q", n, i, test, b)
+			}
+		}
+		if i != len(test) {
+			t.Errorf("#%d: termination expected at %d; got %d", n, len(test), i)
+		}
+		err := s.Err()
+		if err != nil {
+			t.Errorf("#%d: %v", n, err)
+		}
+	}
+}
+
+// Test that the rune splitter returns same sequence of runes (not bytes) as for range string.
+func TestScanRune(t *testing.T) {
+	for n, test := range scanTests {
+		buf := strings.NewReader(test)
+		s := NewScanner(buf)
+		s.Split(ScanRunes)
+		var i, runeCount int
+		var expect rune
+		// Use a string range loop to validate the sequence of runes.
+		for i, expect = range test {
+			if !s.Scan() {
+				break
+			}
+			runeCount++
+			got, _ := utf8.DecodeRune(s.Bytes())
+			if got != expect {
+				t.Errorf("#%d: %d: expected %q got %q", n, i, expect, got)
+			}
+		}
+		if s.Scan() {
+			t.Errorf("#%d: scan ran too long, got %q", n, s.Text())
+		}
+		testRuneCount := utf8.RuneCountInString(test)
+		if runeCount != testRuneCount {
+			t.Errorf("#%d: termination expected at %d; got %d", n, testRuneCount, runeCount)
+		}
+		err := s.Err()
+		if err != nil {
+			t.Errorf("#%d: %v", n, err)
+		}
+	}
+}
+
+var wordScanTests = []string{
+	"",
+	" ",
+	"\n",
+	"a",
+	" a ",
+	"abc def",
+	" abc def ",
+	" abc\tdef\nghi\rjkl\fmno\vpqr\u0085stu\u00a0\n",
+}
+
+// Test that the word splitter returns the same data as strings.Fields.
+func TestScanWords(t *testing.T) {
+	for n, test := range wordScanTests {
+		buf := strings.NewReader(test)
+		s := NewScanner(buf)
+		s.Split(ScanWords)
+		words := strings.Fields(test)
+		var wordCount int
+		for wordCount = 0; wordCount < len(words); wordCount++ {
+			if !s.Scan() {
+				break
+			}
+			got := s.Text()
+			if got != words[wordCount] {
+				t.Errorf("#%d: %d: expected %q got %q", n, wordCount, words[wordCount], got)
+			}
+		}
+		if s.Scan() {
+			t.Errorf("#%d: scan ran too long, got %q", n, s.Text())
+		}
+		if wordCount != len(words) {
+			t.Errorf("#%d: termination expected at %d; got %d", n, len(words), wordCount)
+		}
+		err := s.Err()
+		if err != nil {
+			t.Errorf("#%d: %v", n, err)
+		}
+	}
+}
+
+// slowReader is a reader that returns only a few bytes at a time, to test the incremental
+// reads in Scanner.Scan.
+type slowReader struct {
+	max int
+	buf io.Reader
+}
+
+func (sr *slowReader) Read(p []byte) (n int, err error) {
+	if len(p) > sr.max {
+		p = p[0:sr.max]
+	}
+	return sr.buf.Read(p)
+}
+
+// genLine writes to buf a predictable but non-trivial line of text of length
+// n, including the terminal newline and an occasional carriage return.
+// If addNewline is false, the \r and \n are not emitted.
+func genLine(buf *bytes.Buffer, lineNum, n int, addNewline bool) {
+	buf.Reset()
+	doCR := lineNum%5 == 0
+	if doCR {
+		n--
+	}
+	for i := 0; i < n-1; i++ { // Stop early for \n.
+		c := 'a' + byte(lineNum+i)
+		if c == '\n' || c == '\r' { // Don't confuse us.
+			c = 'N'
+		}
+		buf.WriteByte(c)
+	}
+	if addNewline {
+		if doCR {
+			buf.WriteByte('\r')
+		}
+		buf.WriteByte('\n')
+	}
+}
+
+// Test the line splitter, including some carriage returns but no long lines.
+func TestScanLongLines(t *testing.T) {
+	// Build a buffer of lots of line lengths up to but not exceeding smallMaxTokenSize.
+	tmp := new(bytes.Buffer)
+	buf := new(bytes.Buffer)
+	lineNum := 0
+	j := 0
+	for i := 0; i < 2*smallMaxTokenSize; i++ {
+		genLine(tmp, lineNum, j, true)
+		if j < smallMaxTokenSize {
+			j++
+		} else {
+			j--
+		}
+		buf.Write(tmp.Bytes())
+		lineNum++
+	}
+	s := NewScanner(&slowReader{1, buf})
+	s.Split(ScanLines)
+	s.MaxTokenSize(smallMaxTokenSize)
+	j = 0
+	for lineNum := 0; s.Scan(); lineNum++ {
+		genLine(tmp, lineNum, j, false)
+		if j < smallMaxTokenSize {
+			j++
+		} else {
+			j--
+		}
+		line := tmp.String() // We use the string-valued token here, for variety.
+		if s.Text() != line {
+			t.Errorf("%d: bad line: %d %d\n%.100q\n%.100q\n", lineNum, len(s.Bytes()), len(line), s.Text(), line)
+		}
+	}
+	err := s.Err()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+// Test that the line splitter errors out on a long line.
+func TestScanLineTooLong(t *testing.T) {
+	const smallMaxTokenSize = 256 // Much smaller for more efficient testing.
+	// Build a buffer of lots of line lengths up to but not exceeding smallMaxTokenSize.
+	tmp := new(bytes.Buffer)
+	buf := new(bytes.Buffer)
+	lineNum := 0
+	j := 0
+	for i := 0; i < 2*smallMaxTokenSize; i++ {
+		genLine(tmp, lineNum, j, true)
+		j++
+		buf.Write(tmp.Bytes())
+		lineNum++
+	}
+	s := NewScanner(&slowReader{3, buf})
+	s.Split(ScanLines)
+	s.MaxTokenSize(smallMaxTokenSize)
+	j = 0
+	for lineNum := 0; s.Scan(); lineNum++ {
+		genLine(tmp, lineNum, j, false)
+		if j < smallMaxTokenSize {
+			j++
+		} else {
+			j--
+		}
+		line := tmp.Bytes()
+		if !bytes.Equal(s.Bytes(), line) {
+			t.Errorf("%d: bad line: %d %d\n%.100q\n%.100q\n", lineNum, len(s.Bytes()), len(line), s.Bytes(), line)
+		}
+	}
+	err := s.Err()
+	if err != ErrTooLong {
+		t.Fatalf("expected ErrTooLong; got %s", err)
+	}
+}
+
+// Test that the line splitter handles a final line without a newline.
+func testNoNewline(text string, lines []string, t *testing.T) {
+	buf := strings.NewReader(text)
+	s := NewScanner(&slowReader{7, buf})
+	s.Split(ScanLines)
+	for lineNum := 0; s.Scan(); lineNum++ {
+		line := lines[lineNum]
+		if s.Text() != line {
+			t.Errorf("%d: bad line: %d %d\n%.100q\n%.100q\n", lineNum, len(s.Bytes()), len(line), s.Bytes(), line)
+		}
+	}
+	err := s.Err()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+// Test that the line splitter handles a final line without a newline.
+func TestScanLineNoNewline(t *testing.T) {
+	const text = "abcdefghijklmn\nopqrstuvwxyz"
+	lines := []string{
+		"abcdefghijklmn",
+		"opqrstuvwxyz",
+	}
+	testNoNewline(text, lines, t)
+}
+
+// Test that the line splitter handles a final line with a carriage return but no newline.
+func TestScanLineReturnButNoNewline(t *testing.T) {
+	const text = "abcdefghijklmn\nopqrstuvwxyz\r"
+	lines := []string{
+		"abcdefghijklmn",
+		"opqrstuvwxyz",
+	}
+	testNoNewline(text, lines, t)
+}
+
+// Test that the line splitter handles a final empty line.
+func TestScanLineEmptyFinalLine(t *testing.T) {
+	const text = "abcdefghijklmn\nopqrstuvwxyz\n\n"
+	lines := []string{
+		"abcdefghijklmn",
+		"opqrstuvwxyz",
+		"",
+	}
+	testNoNewline(text, lines, t)
+}
+
+// Test that the line splitter handles a final empty line with a carriage return but no newline.
+func TestScanLineEmptyFinalLineWithCR(t *testing.T) {
+	const text = "abcdefghijklmn\nopqrstuvwxyz\n\r"
+	lines := []string{
+		"abcdefghijklmn",
+		"opqrstuvwxyz",
+		"",
+	}
+	testNoNewline(text, lines, t)
+}
+
+var testError = errors.New("testError")
+
+// Test the correct error is returned when the split function errors out.
+func TestSplitError(t *testing.T) {
+	// Create a split function that delivers a little data, then a predictable error.
+	numSplits := 0
+	const okCount = 7
+	errorSplit := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+		if atEOF {
+			panic("didn't get enough data")
+		}
+		if numSplits >= okCount {
+			return 0, nil, testError
+		}
+		numSplits++
+		return 1, data[0:1], nil
+	}
+	// Read the data.
+	const text = "abcdefghijklmnopqrstuvwxyz"
+	buf := strings.NewReader(text)
+	s := NewScanner(&slowReader{1, buf})
+	s.Split(errorSplit)
+	var i int
+	for i = 0; s.Scan(); i++ {
+		if len(s.Bytes()) != 1 || text[i] != s.Bytes()[0] {
+			t.Errorf("#%d: expected %q got %q", i, text[i], s.Bytes()[0])
+		}
+	}
+	// Check correct termination location and error.
+	if i != okCount {
+		t.Errorf("unexpected termination; expected %d tokens got %d", okCount, i)
+	}
+	err := s.Err()
+	if err != testError {
+		t.Fatalf("expected %q got %v", testError, err)
+	}
+}
+
+// Test that an EOF is overridden by a user-generated scan error.
+func TestErrAtEOF(t *testing.T) {
+	s := NewScanner(strings.NewReader("1 2 33"))
+	// This splitter will fail on last entry, after s.err==EOF.
+	split := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+		advance, token, err = ScanWords(data, atEOF)
+		if len(token) > 1 {
+			if s.ErrOrEOF() != io.EOF {
+				t.Fatal("not testing EOF")
+			}
+			err = testError
+		}
+		return
+	}
+	s.Split(split)
+	for s.Scan() {
+	}
+	if s.Err() != testError {
+		t.Fatal("wrong error:", s.Err())
+	}
+}
+
+// Test for issue 5268.
+type alwaysError struct{}
+
+func (alwaysError) Read(p []byte) (int, error) {
+	return 0, io.ErrUnexpectedEOF
+}
+
+func TestNonEOFWithEmptyRead(t *testing.T) {
+	scanner := NewScanner(alwaysError{})
+	for scanner.Scan() {
+		t.Fatal("read should fail")
+	}
+	err := scanner.Err()
+	if err != io.ErrUnexpectedEOF {
+		t.Errorf("unexpected error: %v", err)
+	}
+}
+
+// Test that Scan finishes if we have endless empty reads.
+type endlessZeros struct{}
+
+func (endlessZeros) Read(p []byte) (int, error) {
+	return 0, nil
+}
+
+func TestBadReader(t *testing.T) {
+	scanner := NewScanner(endlessZeros{})
+	for scanner.Scan() {
+		t.Fatal("read should fail")
+	}
+	err := scanner.Err()
+	if err != io.ErrNoProgress {
+		t.Errorf("unexpected error: %v", err)
+	}
+}
+
+func TestScanWordsExcessiveWhiteSpace(t *testing.T) {
+	const word = "ipsum"
+	s := strings.Repeat(" ", 4*smallMaxTokenSize) + word
+	scanner := NewScanner(strings.NewReader(s))
+	scanner.MaxTokenSize(smallMaxTokenSize)
+	scanner.Split(ScanWords)
+	if !scanner.Scan() {
+		t.Fatalf("scan failed: %v", scanner.Err())
+	}
+	if token := scanner.Text(); token != word {
+		t.Fatalf("unexpected token: %v", token)
+	}
+}
+
+// Test that empty tokens, including at end of line or end of file, are found by the scanner.
+// Issue 8672: Could miss final empty token.
+
+func commaSplit(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	for i := 0; i < len(data); i++ {
+		if data[i] == ',' {
+			return i + 1, data[:i], nil
+		}
+	}
+	return 0, data, ErrFinalToken
+}
+
+func testEmptyTokens(t *testing.T, text string, values []string) {
+	s := NewScanner(strings.NewReader(text))
+	s.Split(commaSplit)
+	var i int
+	for i = 0; s.Scan(); i++ {
+		if i >= len(values) {
+			t.Fatalf("got %d fields, expected %d", i+1, len(values))
+		}
+		if s.Text() != values[i] {
+			t.Errorf("%d: expected %q got %q", i, values[i], s.Text())
+		}
+	}
+	if i != len(values) {
+		t.Fatalf("got %d fields, expected %d", i, len(values))
+	}
+	if err := s.Err(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestEmptyTokens(t *testing.T) {
+	testEmptyTokens(t, "1,2,3,", []string{"1", "2", "3", ""})
+}
+
+func TestWithNoEmptyTokens(t *testing.T) {
+	testEmptyTokens(t, "1,2,3", []string{"1", "2", "3"})
+}
+
+func loopAtEOFSplit(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if len(data) > 0 {
+		return 1, data[:1], nil
+	}
+	return 0, data, nil
+}
+
+func TestDontLoopForever(t *testing.T) {
+	s := NewScanner(strings.NewReader("abc"))
+	s.Split(loopAtEOFSplit)
+	// Expect a panic
+	defer func() {
+		err := recover()
+		if err == nil {
+			t.Fatal("should have panicked")
+		}
+		if msg, ok := err.(string); !ok || !strings.Contains(msg, "empty tokens") {
+			panic(err)
+		}
+	}()
+	for count := 0; s.Scan(); count++ {
+		if count > 1000 {
+			t.Fatal("looping")
+		}
+	}
+	if s.Err() != nil {
+		t.Fatal("after scan:", s.Err())
+	}
+}
+
+func TestBlankLines(t *testing.T) {
+	s := NewScanner(strings.NewReader(strings.Repeat("\n", 1000)))
+	for count := 0; s.Scan(); count++ {
+		if count > 2000 {
+			t.Fatal("looping")
+		}
+	}
+	if s.Err() != nil {
+		t.Fatal("after scan:", s.Err())
+	}
+}
+
+type countdown int
+
+func (c *countdown) split(data []byte, atEOF bool) (advance int, token []byte, err error) {
+	if *c > 0 {
+		*c--
+		return 1, data[:1], nil
+	}
+	return 0, nil, nil
+}
+
+// Check that the looping-at-EOF check doesn't trigger for merely empty tokens.
+func TestEmptyLinesOK(t *testing.T) {
+	c := countdown(10000)
+	s := NewScanner(strings.NewReader(strings.Repeat("\n", 10000)))
+	s.Split(c.split)
+	for s.Scan() {
+	}
+	if s.Err() != nil {
+		t.Fatal("after scan:", s.Err())
+	}
+	if c != 0 {
+		t.Fatalf("stopped with %d left to process", c)
+	}
+}
+
+// Make sure we can read a huge token if a big enough buffer is provided.
+func TestHugeBuffer(t *testing.T) {
+	text := strings.Repeat("x", 2*MaxScanTokenSize)
+	s := NewScanner(strings.NewReader(text + "\n"))
+	s.Buffer(make([]byte, 100), 3*MaxScanTokenSize)
+	for s.Scan() {
+		token := s.Text()
+		if token != text {
+			t.Errorf("scan got incorrect token of length %d", len(token))
+		}
+	}
+	if s.Err() != nil {
+		t.Fatal("after scan:", s.Err())
+	}
+}
+
+// negativeEOFReader returns an invalid -1 at the end, as though it
+// were wrapping the read system call.
+type negativeEOFReader int
+
+func (r *negativeEOFReader) Read(p []byte) (int, error) {
+	if *r > 0 {
+		c := int(*r)
+		if c > len(p) {
+			c = len(p)
+		}
+		for i := 0; i < c; i++ {
+			p[i] = 'a'
+		}
+		p[c-1] = '\n'
+		*r -= negativeEOFReader(c)
+		return c, nil
+	}
+	return -1, io.EOF
+}
+
+// Test that the scanner doesn't panic and returns ErrBadReadCount
+// on a reader that returns a negative count of bytes read (issue 38053).
+func TestNegativeEOFReader(t *testing.T) {
+	r := negativeEOFReader(10)
+	scanner := NewScanner(&r)
+	c := 0
+	for scanner.Scan() {
+		c++
+		if c > 1 {
+			t.Error("read too many lines")
+			break
+		}
+	}
+	if got, want := scanner.Err(), ErrBadReadCount; got != want {
+		t.Errorf("scanner.Err: got %v, want %v", got, want)
+	}
+}
+
+// largeReader returns an invalid count that is larger than the number
+// of bytes requested.
+type largeReader struct{}
+
+func (largeReader) Read(p []byte) (int, error) {
+	return len(p) + 1, nil
+}
+
+// Test that the scanner doesn't panic and returns ErrBadReadCount
+// on a reader that returns an impossibly large count of bytes read (issue 38053).
+func TestLargeReader(t *testing.T) {
+	scanner := NewScanner(largeReader{})
+	for scanner.Scan() {
+	}
+	if got, want := scanner.Err(), ErrBadReadCount; got != want {
+		t.Errorf("scanner.Err: got %v, want %v", got, want)
+	}
+}

go/src/builtin/builtin.go

@@ -0,0 +1,319 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package builtin provides documentation for Go's predeclared identifiers.
+The items documented here are not actually in package builtin
+but their descriptions here allow godoc to present documentation
+for the language's special identifiers.
+*/
+package builtin
+
+import "cmp"
+
+// bool is the set of boolean values, true and false.
+type bool bool
+
+// true and false are the two untyped boolean values.
+const (
+	true  = 0 == 0 // Untyped bool.
+	false = 0 != 0 // Untyped bool.
+)
+
+// uint8 is the set of all unsigned 8-bit integers.
+// Range: 0 through 255.
+type uint8 uint8
+
+// uint16 is the set of all unsigned 16-bit integers.
+// Range: 0 through 65535.
+type uint16 uint16
+
+// uint32 is the set of all unsigned 32-bit integers.
+// Range: 0 through 4294967295.
+type uint32 uint32
+
+// uint64 is the set of all unsigned 64-bit integers.
+// Range: 0 through 18446744073709551615.
+type uint64 uint64
+
+// int8 is the set of all signed 8-bit integers.
+// Range: -128 through 127.
+type int8 int8
+
+// int16 is the set of all signed 16-bit integers.
+// Range: -32768 through 32767.
+type int16 int16
+
+// int32 is the set of all signed 32-bit integers.
+// Range: -2147483648 through 2147483647.
+type int32 int32
+
+// int64 is the set of all signed 64-bit integers.
+// Range: -9223372036854775808 through 9223372036854775807.
+type int64 int64
+
+// float32 is the set of all IEEE 754 32-bit floating-point numbers.
+type float32 float32
+
+// float64 is the set of all IEEE 754 64-bit floating-point numbers.
+type float64 float64
+
+// complex64 is the set of all complex numbers with float32 real and
+// imaginary parts.
+type complex64 complex64
+
+// complex128 is the set of all complex numbers with float64 real and
+// imaginary parts.
+type complex128 complex128
+
+// string is the set of all strings of 8-bit bytes, conventionally but not
+// necessarily representing UTF-8-encoded text. A string may be empty, but
+// not nil. Values of string type are immutable.
+type string string
+
+// int is a signed integer type that is at least 32 bits in size. It is a
+// distinct type, however, and not an alias for, say, int32.
+type int int
+
+// uint is an unsigned integer type that is at least 32 bits in size. It is a
+// distinct type, however, and not an alias for, say, uint32.
+type uint uint
+
+// uintptr is an integer type that is large enough to hold the bit pattern of
+// any pointer.
+type uintptr uintptr
+
+// byte is an alias for uint8 and is equivalent to uint8 in all ways. It is
+// used, by convention, to distinguish byte values from 8-bit unsigned
+// integer values.
+type byte = uint8
+
+// rune is an alias for int32 and is equivalent to int32 in all ways. It is
+// used, by convention, to distinguish character values from integer values.
+type rune = int32
+
+// any is an alias for interface{} and is equivalent to interface{} in all ways.
+type any = interface{}
+
+// comparable is an interface that is implemented by all comparable types
+// (booleans, numbers, strings, pointers, channels, arrays of comparable types,
+// structs whose fields are all comparable types).
+// The comparable interface may only be used as a type parameter constraint,
+// not as the type of a variable.
+type comparable interface{ comparable }
+
+// iota is a predeclared identifier representing the untyped integer ordinal
+// number of the current const specification in a (usually parenthesized)
+// const declaration. It is zero-indexed.
+const iota = 0 // Untyped int.
+
+// nil is a predeclared identifier representing the zero value for a
+// pointer, channel, func, interface, map, or slice type.
+var nil Type // Type must be a pointer, channel, func, interface, map, or slice type
+
+// Type is here for the purposes of documentation only. It is a stand-in
+// for any Go type, but represents the same type for any given function
+// invocation.
+type Type int
+
+// Type1 is here for the purposes of documentation only. It is a stand-in
+// for any Go type, but represents the same type for any given function
+// invocation.
+type Type1 int
+
+// TypeOrExpr is here for the purposes of documentation only. It is a stand-in
+// for either a Go type or an expression.
+type TypeOrExpr int
+
+// IntegerType is here for the purposes of documentation only. It is a stand-in
+// for any integer type: int, uint, int8 etc.
+type IntegerType int
+
+// FloatType is here for the purposes of documentation only. It is a stand-in
+// for either float type: float32 or float64.
+type FloatType float32
+
+// ComplexType is here for the purposes of documentation only. It is a
+// stand-in for either complex type: complex64 or complex128.
+type ComplexType complex64
+
+// The append built-in function appends elements to the end of a slice. If
+// it has sufficient capacity, the destination is resliced to accommodate the
+// new elements. If it does not, a new underlying array will be allocated.
+// Append returns the updated slice. It is therefore necessary to store the
+// result of append, often in the variable holding the slice itself:
+//
+//	slice = append(slice, elem1, elem2)
+//	slice = append(slice, anotherSlice...)
+//
+// As a special case, it is legal to append a string to a byte slice, like this:
+//
+//	slice = append([]byte("hello "), "world"...)
+func append(slice []Type, elems ...Type) []Type
+
+// The copy built-in function copies elements from a source slice into a
+// destination slice. (As a special case, it also will copy bytes from a
+// string to a slice of bytes.) The source and destination may overlap. Copy
+// returns the number of elements copied, which will be the minimum of
+// len(src) and len(dst).
+func copy(dst, src []Type) int
+
+// The delete built-in function deletes the element with the specified key
+// (m[key]) from the map. If m is nil or there is no such element, delete
+// is a no-op.
+func delete(m map[Type]Type1, key Type)
+
+// The len built-in function returns the length of v, according to its type:
+//
+//   - Array: the number of elements in v.
+//   - Pointer to array: the number of elements in *v (even if v is nil).
+//   - Slice, or map: the number of elements in v; if v is nil, len(v) is zero.
+//   - String: the number of bytes in v.
+//   - Channel: the number of elements queued (unread) in the channel buffer;
+//     if v is nil, len(v) is zero.
+//
+// For some arguments, such as a string literal or a simple array expression, the
+// result can be a constant. See the Go language specification's "Length and
+// capacity" section for details.
+func len(v Type) int
+
+// The cap built-in function returns the capacity of v, according to its type:
+//
+//   - Array: the number of elements in v (same as len(v)).
+//   - Pointer to array: the number of elements in *v (same as len(v)).
+//   - Slice: the maximum length the slice can reach when resliced;
+//     if v is nil, cap(v) is zero.
+//   - Channel: the channel buffer capacity, in units of elements;
+//     if v is nil, cap(v) is zero.
+//
+// For some arguments, such as a simple array expression, the result can be a
+// constant. See the Go language specification's "Length and capacity" section for
+// details.
+func cap(v Type) int
+
+// The make built-in function allocates and initializes an object of type
+// slice, map, or chan (only). Like new, the first argument is a type, not a
+// value. Unlike new, make's return type is the same as the type of its
+// argument, not a pointer to it. The specification of the result depends on
+// the type:
+//
+//   - Slice: The size specifies the length. The capacity of the slice is
+//     equal to its length. A second integer argument may be provided to
+//     specify a different capacity; it must be no smaller than the
+//     length. For example, make([]int, 0, 10) allocates an underlying array
+//     of size 10 and returns a slice of length 0 and capacity 10 that is
+//     backed by this underlying array.
+//   - Map: An empty map is allocated with enough space to hold the
+//     specified number of elements. The size may be omitted, in which case
+//     a small starting size is allocated.
+//   - Channel: The channel's buffer is initialized with the specified
+//     buffer capacity. If zero, or the size is omitted, the channel is
+//     unbuffered.
+func make(t Type, size ...IntegerType) Type
+
+// The max built-in function returns the largest value of a fixed number of
+// arguments of [cmp.Ordered] types. There must be at least one argument.
+// If T is a floating-point type and any of the arguments are NaNs,
+// max will return NaN.
+func max[T cmp.Ordered](x T, y ...T) T
+
+// The min built-in function returns the smallest value of a fixed number of
+// arguments of [cmp.Ordered] types. There must be at least one argument.
+// If T is a floating-point type and any of the arguments are NaNs,
+// min will return NaN.
+func min[T cmp.Ordered](x T, y ...T) T
+
+// The built-in function new allocates a new, initialized variable and returns
+// a pointer to it. It accepts a single argument, which may be either a type
+// or an expression.
+// If the argument is a type T, then new(T) allocates a variable of type T
+// initialized to its zero value.
+// Otherwise, the argument is an expression x and new(x) allocates a variable
+// of the type of x initialized to the value of x. If that value is an untyped
+// constant, it is first implicitly converted to its default type.
+func new(TypeOrExpr) *Type
+
+// The complex built-in function constructs a complex value from two
+// floating-point values. The real and imaginary parts must be of the same
+// size, either float32 or float64 (or assignable to them), and the return
+// value will be the corresponding complex type (complex64 for float32,
+// complex128 for float64).
+func complex(r, i FloatType) ComplexType
+
+// The real built-in function returns the real part of the complex number c.
+// The return value will be floating point type corresponding to the type of c.
+func real(c ComplexType) FloatType
+
+// The imag built-in function returns the imaginary part of the complex
+// number c. The return value will be floating point type corresponding to
+// the type of c.
+func imag(c ComplexType) FloatType
+
+// The clear built-in function clears maps and slices.
+// For maps, clear deletes all entries, resulting in an empty map.
+// For slices, clear sets all elements up to the length of the slice
+// to the zero value of the respective element type. If the argument
+// type is a type parameter, the type parameter's type set must
+// contain only map or slice types, and clear performs the operation
+// implied by the type argument. If t is nil, clear is a no-op.
+func clear[T ~[]Type | ~map[Type]Type1](t T)
+
+// The close built-in function closes a channel, which must be either
+// bidirectional or send-only. It should be executed only by the sender,
+// never the receiver, and has the effect of shutting down the channel after
+// the last sent value is received. After the last value has been received
+// from a closed channel c, any receive from c will succeed without
+// blocking, returning the zero value for the channel element. The form
+//
+//	x, ok := <-c
+//
+// will also set ok to false for a closed and empty channel.
+func close(c chan<- Type)
+
+// The panic built-in function stops normal execution of the current
+// goroutine. When a function F calls panic, normal execution of F stops
+// immediately. Any functions whose execution was deferred by F are run in
+// the usual way, and then F returns to its caller. To the caller G, the
+// invocation of F then behaves like a call to panic, terminating G's
+// execution and running any deferred functions. This continues until all
+// functions in the executing goroutine have stopped, in reverse order. At
+// that point, the program is terminated with a non-zero exit code. This
+// termination sequence is called panicking and can be controlled by the
+// built-in function recover.
+//
+// Starting in Go 1.21, calling panic with a nil interface value or an
+// untyped nil causes a run-time error (a different panic).
+// The GODEBUG setting panicnil=1 disables the run-time error.
+func panic(v any)
+
+// The recover built-in function allows a program to manage behavior of a
+// panicking goroutine. Executing a call to recover inside a deferred
+// function (but not any function called by it) stops the panicking sequence
+// by restoring normal execution and retrieves the error value passed to the
+// call of panic. If recover is called outside the deferred function it will
+// not stop a panicking sequence. In this case, or when the goroutine is not
+// panicking, recover returns nil.
+//
+// Prior to Go 1.21, recover would also return nil if panic is called with
+// a nil argument. See [panic] for details.
+func recover() any
+
+// The print built-in function formats its arguments in an
+// implementation-specific way and writes the result to standard error.
+// Print is useful for bootstrapping and debugging; it is not guaranteed
+// to stay in the language.
+func print(args ...Type)
+
+// The println built-in function formats its arguments in an
+// implementation-specific way and writes the result to standard error.
+// Spaces are always added between arguments and a newline is appended.
+// Println is useful for bootstrapping and debugging; it is not guaranteed
+// to stay in the language.
+func println(args ...Type)
+
+// The error built-in interface type is the conventional interface for
+// representing an error condition, with the nil value representing no error.
+type error interface {
+	Error() string
+}

go/src/bytes/boundary_test.go

@@ -0,0 +1,115 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+//
+//go:build linux
+
+package bytes_test
+
+import (
+	. "bytes"
+	"syscall"
+	"testing"
+)
+
+// This file tests the situation where byte operations are checking
+// data very near to a page boundary. We want to make sure those
+// operations do not read across the boundary and cause a page
+// fault where they shouldn't.
+
+// These tests run only on linux. The code being tested is
+// not OS-specific, so it does not need to be tested on all
+// operating systems.
+
+// dangerousSlice returns a slice which is immediately
+// preceded and followed by a faulting page.
+func dangerousSlice(t *testing.T) []byte {
+	pagesize := syscall.Getpagesize()
+	b, err := syscall.Mmap(0, 0, 3*pagesize, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANONYMOUS|syscall.MAP_PRIVATE)
+	if err != nil {
+		t.Fatalf("mmap failed %s", err)
+	}
+	err = syscall.Mprotect(b[:pagesize], syscall.PROT_NONE)
+	if err != nil {
+		t.Fatalf("mprotect low failed %s\n", err)
+	}
+	err = syscall.Mprotect(b[2*pagesize:], syscall.PROT_NONE)
+	if err != nil {
+		t.Fatalf("mprotect high failed %s\n", err)
+	}
+	return b[pagesize : 2*pagesize]
+}
+
+func TestEqualNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	b := dangerousSlice(t)
+	for i := range b {
+		b[i] = 'A'
+	}
+	for i := 0; i <= len(b); i++ {
+		Equal(b[:i], b[len(b)-i:])
+		Equal(b[len(b)-i:], b[:i])
+	}
+}
+
+func TestIndexByteNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	b := dangerousSlice(t)
+	for i := range b {
+		idx := IndexByte(b[i:], 1)
+		if idx != -1 {
+			t.Fatalf("IndexByte(b[%d:])=%d, want -1\n", i, idx)
+		}
+	}
+}
+
+func TestIndexNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	q := dangerousSlice(t)
+	if len(q) > 64 {
+		// Only worry about when we're near the end of a page.
+		q = q[len(q)-64:]
+	}
+	b := dangerousSlice(t)
+	if len(b) > 256 {
+		// Only worry about when we're near the end of a page.
+		b = b[len(b)-256:]
+	}
+	for j := 1; j < len(q); j++ {
+		q[j-1] = 1 // difference is only found on the last byte
+		for i := range b {
+			idx := Index(b[i:], q[:j])
+			if idx != -1 {
+				t.Fatalf("Index(b[%d:], q[:%d])=%d, want -1\n", i, j, idx)
+			}
+		}
+		q[j-1] = 0
+	}
+
+	// Test differing alignments and sizes of q which always end on a page boundary.
+	q[len(q)-1] = 1 // difference is only found on the last byte
+	for j := 0; j < len(q); j++ {
+		for i := range b {
+			idx := Index(b[i:], q[j:])
+			if idx != -1 {
+				t.Fatalf("Index(b[%d:], q[%d:])=%d, want -1\n", i, j, idx)
+			}
+		}
+	}
+	q[len(q)-1] = 0
+}
+
+func TestCountNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	b := dangerousSlice(t)
+	for i := range b {
+		c := Count(b[i:], []byte{1})
+		if c != 0 {
+			t.Fatalf("Count(b[%d:], {1})=%d, want 0\n", i, c)
+		}
+		c = Count(b[:i], []byte{0})
+		if c != i {
+			t.Fatalf("Count(b[:%d], {0})=%d, want %d\n", i, c, i)
+		}
+	}
+}

go/src/bytes/buffer.go

@@ -0,0 +1,500 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes
+
+// Simple byte buffer for marshaling data.
+
+import (
+	"errors"
+	"io"
+	"unicode/utf8"
+)
+
+// smallBufferSize is an initial allocation minimal capacity.
+const smallBufferSize = 64
+
+// A Buffer is a variable-sized buffer of bytes with [Buffer.Read] and [Buffer.Write] methods.
+// The zero value for Buffer is an empty buffer ready to use.
+type Buffer struct {
+	buf      []byte // contents are the bytes buf[off : len(buf)]
+	off      int    // read at &buf[off], write at &buf[len(buf)]
+	lastRead readOp // last read operation, so that Unread* can work correctly.
+
+	// Copying and modifying a non-zero Buffer is prone to error,
+	// but we cannot employ the noCopy trick used by WaitGroup and Mutex,
+	// which causes vet's copylocks checker to report misuse, as vet
+	// cannot reliably distinguish the zero and non-zero cases.
+	// See #26462, #25907, #47276, #48398 for history.
+}
+
+// The readOp constants describe the last action performed on
+// the buffer, so that UnreadRune and UnreadByte can check for
+// invalid usage. opReadRuneX constants are chosen such that
+// converted to int they correspond to the rune size that was read.
+type readOp int8
+
+// Don't use iota for these, as the values need to correspond with the
+// names and comments, which is easier to see when being explicit.
+const (
+	opRead      readOp = -1 // Any other read operation.
+	opInvalid   readOp = 0  // Non-read operation.
+	opReadRune1 readOp = 1  // Read rune of size 1.
+	opReadRune2 readOp = 2  // Read rune of size 2.
+	opReadRune3 readOp = 3  // Read rune of size 3.
+	opReadRune4 readOp = 4  // Read rune of size 4.
+)
+
+// ErrTooLarge is passed to panic if memory cannot be allocated to store data in a buffer.
+var ErrTooLarge = errors.New("bytes.Buffer: too large")
+var errNegativeRead = errors.New("bytes.Buffer: reader returned negative count from Read")
+
+const maxInt = int(^uint(0) >> 1)
+
+// Bytes returns a slice of length b.Len() holding the unread portion of the buffer.
+// The slice is valid for use only until the next buffer modification (that is,
+// only until the next call to a method like [Buffer.Read], [Buffer.Write], [Buffer.Reset], or [Buffer.Truncate]).
+// The slice aliases the buffer content at least until the next buffer modification,
+// so immediate changes to the slice will affect the result of future reads.
+func (b *Buffer) Bytes() []byte { return b.buf[b.off:] }
+
+// AvailableBuffer returns an empty buffer with b.Available() capacity.
+// This buffer is intended to be appended to and
+// passed to an immediately succeeding [Buffer.Write] call.
+// The buffer is only valid until the next write operation on b.
+func (b *Buffer) AvailableBuffer() []byte { return b.buf[len(b.buf):] }
+
+// String returns the contents of the unread portion of the buffer
+// as a string. If the [Buffer] is a nil pointer, it returns "<nil>".
+//
+// To build strings more efficiently, see the [strings.Builder] type.
+func (b *Buffer) String() string {
+	if b == nil {
+		// Special case, useful in debugging.
+		return "<nil>"
+	}
+	return string(b.buf[b.off:])
+}
+
+// Peek returns the next n bytes without advancing the buffer.
+// If Peek returns fewer than n bytes, it also returns [io.EOF].
+// The slice is only valid until the next call to a read or write method.
+// The slice aliases the buffer content at least until the next buffer modification,
+// so immediate changes to the slice will affect the result of future reads.
+func (b *Buffer) Peek(n int) ([]byte, error) {
+	if b.Len() < n {
+		return b.buf[b.off:], io.EOF
+	}
+	return b.buf[b.off : b.off+n], nil
+}
+
+// empty reports whether the unread portion of the buffer is empty.
+func (b *Buffer) empty() bool { return len(b.buf) <= b.off }
+
+// Len returns the number of bytes of the unread portion of the buffer;
+// b.Len() == len(b.Bytes()).
+func (b *Buffer) Len() int { return len(b.buf) - b.off }
+
+// Cap returns the capacity of the buffer's underlying byte slice, that is, the
+// total space allocated for the buffer's data.
+func (b *Buffer) Cap() int { return cap(b.buf) }
+
+// Available returns how many bytes are unused in the buffer.
+func (b *Buffer) Available() int { return cap(b.buf) - len(b.buf) }
+
+// Truncate discards all but the first n unread bytes from the buffer
+// but continues to use the same allocated storage.
+// It panics if n is negative or greater than the length of the buffer.
+func (b *Buffer) Truncate(n int) {
+	if n == 0 {
+		b.Reset()
+		return
+	}
+	b.lastRead = opInvalid
+	if n < 0 || n > b.Len() {
+		panic("bytes.Buffer: truncation out of range")
+	}
+	b.buf = b.buf[:b.off+n]
+}
+
+// Reset resets the buffer to be empty,
+// but it retains the underlying storage for use by future writes.
+// Reset is the same as [Buffer.Truncate](0).
+func (b *Buffer) Reset() {
+	b.buf = b.buf[:0]
+	b.off = 0
+	b.lastRead = opInvalid
+}
+
+// tryGrowByReslice is an inlineable version of grow for the fast-case where the
+// internal buffer only needs to be resliced.
+// It returns the index where bytes should be written and whether it succeeded.
+func (b *Buffer) tryGrowByReslice(n int) (int, bool) {
+	if l := len(b.buf); n <= cap(b.buf)-l {
+		b.buf = b.buf[:l+n]
+		return l, true
+	}
+	return 0, false
+}
+
+// grow grows the buffer to guarantee space for n more bytes.
+// It returns the index where bytes should be written.
+// If the buffer can't grow it will panic with ErrTooLarge.
+func (b *Buffer) grow(n int) int {
+	m := b.Len()
+	// If buffer is empty, reset to recover space.
+	if m == 0 && b.off != 0 {
+		b.Reset()
+	}
+	// Try to grow by means of a reslice.
+	if i, ok := b.tryGrowByReslice(n); ok {
+		return i
+	}
+	if b.buf == nil && n <= smallBufferSize {
+		b.buf = make([]byte, n, smallBufferSize)
+		return 0
+	}
+	c := cap(b.buf)
+	if n <= c/2-m {
+		// We can slide things down instead of allocating a new
+		// slice. We only need m+n <= c to slide, but
+		// we instead let capacity get twice as large so we
+		// don't spend all our time copying.
+		copy(b.buf, b.buf[b.off:])
+	} else if c > maxInt-c-n {
+		panic(ErrTooLarge)
+	} else {
+		// Add b.off to account for b.buf[:b.off] being sliced off the front.
+		b.buf = growSlice(b.buf[b.off:], b.off+n)
+	}
+	// Restore b.off and len(b.buf).
+	b.off = 0
+	b.buf = b.buf[:m+n]
+	return m
+}
+
+// Grow grows the buffer's capacity, if necessary, to guarantee space for
+// another n bytes. After Grow(n), at least n bytes can be written to the
+// buffer without another allocation.
+// If n is negative, Grow will panic.
+// If the buffer can't grow it will panic with [ErrTooLarge].
+func (b *Buffer) Grow(n int) {
+	if n < 0 {
+		panic("bytes.Buffer.Grow: negative count")
+	}
+	m := b.grow(n)
+	b.buf = b.buf[:m]
+}
+
+// Write appends the contents of p to the buffer, growing the buffer as
+// needed. The return value n is the length of p; err is always nil. If the
+// buffer becomes too large, Write will panic with [ErrTooLarge].
+func (b *Buffer) Write(p []byte) (n int, err error) {
+	b.lastRead = opInvalid
+	m, ok := b.tryGrowByReslice(len(p))
+	if !ok {
+		m = b.grow(len(p))
+	}
+	return copy(b.buf[m:], p), nil
+}
+
+// WriteString appends the contents of s to the buffer, growing the buffer as
+// needed. The return value n is the length of s; err is always nil. If the
+// buffer becomes too large, WriteString will panic with [ErrTooLarge].
+func (b *Buffer) WriteString(s string) (n int, err error) {
+	b.lastRead = opInvalid
+	m, ok := b.tryGrowByReslice(len(s))
+	if !ok {
+		m = b.grow(len(s))
+	}
+	return copy(b.buf[m:], s), nil
+}
+
+// MinRead is the minimum slice size passed to a [Buffer.Read] call by
+// [Buffer.ReadFrom]. As long as the [Buffer] has at least MinRead bytes beyond
+// what is required to hold the contents of r, [Buffer.ReadFrom] will not grow the
+// underlying buffer.
+const MinRead = 512
+
+// ReadFrom reads data from r until EOF and appends it to the buffer, growing
+// the buffer as needed. The return value n is the number of bytes read. Any
+// error except io.EOF encountered during the read is also returned. If the
+// buffer becomes too large, ReadFrom will panic with [ErrTooLarge].
+func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
+	b.lastRead = opInvalid
+	for {
+		i := b.grow(MinRead)
+		b.buf = b.buf[:i]
+		m, e := r.Read(b.buf[i:cap(b.buf)])
+		if m < 0 {
+			panic(errNegativeRead)
+		}
+
+		b.buf = b.buf[:i+m]
+		n += int64(m)
+		if e == io.EOF {
+			return n, nil // e is EOF, so return nil explicitly
+		}
+		if e != nil {
+			return n, e
+		}
+	}
+}
+
+// growSlice grows b by n, preserving the original content of b.
+// If the allocation fails, it panics with ErrTooLarge.
+func growSlice(b []byte, n int) []byte {
+	defer func() {
+		if recover() != nil {
+			panic(ErrTooLarge)
+		}
+	}()
+	// TODO(http://golang.org/issue/51462): We should rely on the append-make
+	// pattern so that the compiler can call runtime.growslice. For example:
+	//	return append(b, make([]byte, n)...)
+	// This avoids unnecessary zero-ing of the first len(b) bytes of the
+	// allocated slice, but this pattern causes b to escape onto the heap.
+	//
+	// Instead use the append-make pattern with a nil slice to ensure that
+	// we allocate buffers rounded up to the closest size class.
+	c := len(b) + n // ensure enough space for n elements
+	if c < 2*cap(b) {
+		// The growth rate has historically always been 2x. In the future,
+		// we could rely purely on append to determine the growth rate.
+		c = 2 * cap(b)
+	}
+	b2 := append([]byte(nil), make([]byte, c)...)
+	i := copy(b2, b)
+	return b2[:i]
+}
+
+// WriteTo writes data to w until the buffer is drained or an error occurs.
+// The return value n is the number of bytes written; it always fits into an
+// int, but it is int64 to match the [io.WriterTo] interface. Any error
+// encountered during the write is also returned.
+func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
+	b.lastRead = opInvalid
+	if nBytes := b.Len(); nBytes > 0 {
+		m, e := w.Write(b.buf[b.off:])
+		if m > nBytes {
+			panic("bytes.Buffer.WriteTo: invalid Write count")
+		}
+		b.off += m
+		n = int64(m)
+		if e != nil {
+			return n, e
+		}
+		// all bytes should have been written, by definition of
+		// Write method in io.Writer
+		if m != nBytes {
+			return n, io.ErrShortWrite
+		}
+	}
+	// Buffer is now empty; reset.
+	b.Reset()
+	return n, nil
+}
+
+// WriteByte appends the byte c to the buffer, growing the buffer as needed.
+// The returned error is always nil, but is included to match [bufio.Writer]'s
+// WriteByte. If the buffer becomes too large, WriteByte will panic with
+// [ErrTooLarge].
+func (b *Buffer) WriteByte(c byte) error {
+	b.lastRead = opInvalid
+	m, ok := b.tryGrowByReslice(1)
+	if !ok {
+		m = b.grow(1)
+	}
+	b.buf[m] = c
+	return nil
+}
+
+// WriteRune appends the UTF-8 encoding of Unicode code point r to the
+// buffer, returning its length and an error, which is always nil but is
+// included to match [bufio.Writer]'s WriteRune. The buffer is grown as needed;
+// if it becomes too large, WriteRune will panic with [ErrTooLarge].
+func (b *Buffer) WriteRune(r rune) (n int, err error) {
+	// Compare as uint32 to correctly handle negative runes.
+	if uint32(r) < utf8.RuneSelf {
+		b.WriteByte(byte(r))
+		return 1, nil
+	}
+	b.lastRead = opInvalid
+	m, ok := b.tryGrowByReslice(utf8.UTFMax)
+	if !ok {
+		m = b.grow(utf8.UTFMax)
+	}
+	b.buf = utf8.AppendRune(b.buf[:m], r)
+	return len(b.buf) - m, nil
+}
+
+// Read reads the next len(p) bytes from the buffer or until the buffer
+// is drained. The return value n is the number of bytes read. If the
+// buffer has no data to return, err is [io.EOF] (unless len(p) is zero);
+// otherwise it is nil.
+func (b *Buffer) Read(p []byte) (n int, err error) {
+	b.lastRead = opInvalid
+	if b.empty() {
+		// Buffer is empty, reset to recover space.
+		b.Reset()
+		if len(p) == 0 {
+			return 0, nil
+		}
+		return 0, io.EOF
+	}
+	n = copy(p, b.buf[b.off:])
+	b.off += n
+	if n > 0 {
+		b.lastRead = opRead
+	}
+	return n, nil
+}
+
+// Next returns a slice containing the next n bytes from the buffer,
+// advancing the buffer as if the bytes had been returned by [Buffer.Read].
+// If there are fewer than n bytes in the buffer, Next returns the entire buffer.
+// The slice is only valid until the next call to a read or write method.
+func (b *Buffer) Next(n int) []byte {
+	b.lastRead = opInvalid
+	m := b.Len()
+	if n > m {
+		n = m
+	}
+	data := b.buf[b.off : b.off+n]
+	b.off += n
+	if n > 0 {
+		b.lastRead = opRead
+	}
+	return data
+}
+
+// ReadByte reads and returns the next byte from the buffer.
+// If no byte is available, it returns error [io.EOF].
+func (b *Buffer) ReadByte() (byte, error) {
+	if b.empty() {
+		// Buffer is empty, reset to recover space.
+		b.Reset()
+		return 0, io.EOF
+	}
+	c := b.buf[b.off]
+	b.off++
+	b.lastRead = opRead
+	return c, nil
+}
+
+// ReadRune reads and returns the next UTF-8-encoded
+// Unicode code point from the buffer.
+// If no bytes are available, the error returned is io.EOF.
+// If the bytes are an erroneous UTF-8 encoding, it
+// consumes one byte and returns U+FFFD, 1.
+func (b *Buffer) ReadRune() (r rune, size int, err error) {
+	if b.empty() {
+		// Buffer is empty, reset to recover space.
+		b.Reset()
+		return 0, 0, io.EOF
+	}
+	c := b.buf[b.off]
+	if c < utf8.RuneSelf {
+		b.off++
+		b.lastRead = opReadRune1
+		return rune(c), 1, nil
+	}
+	r, n := utf8.DecodeRune(b.buf[b.off:])
+	b.off += n
+	b.lastRead = readOp(n)
+	return r, n, nil
+}
+
+// UnreadRune unreads the last rune returned by [Buffer.ReadRune].
+// If the most recent read or write operation on the buffer was
+// not a successful [Buffer.ReadRune], UnreadRune returns an error.  (In this regard
+// it is stricter than [Buffer.UnreadByte], which will unread the last byte
+// from any read operation.)
+func (b *Buffer) UnreadRune() error {
+	if b.lastRead <= opInvalid {
+		return errors.New("bytes.Buffer: UnreadRune: previous operation was not a successful ReadRune")
+	}
+	if b.off >= int(b.lastRead) {
+		b.off -= int(b.lastRead)
+	}
+	b.lastRead = opInvalid
+	return nil
+}
+
+var errUnreadByte = errors.New("bytes.Buffer: UnreadByte: previous operation was not a successful read")
+
+// UnreadByte unreads the last byte returned by the most recent successful
+// read operation that read at least one byte. If a write has happened since
+// the last read, if the last read returned an error, or if the read read zero
+// bytes, UnreadByte returns an error.
+func (b *Buffer) UnreadByte() error {
+	if b.lastRead == opInvalid {
+		return errUnreadByte
+	}
+	b.lastRead = opInvalid
+	if b.off > 0 {
+		b.off--
+	}
+	return nil
+}
+
+// ReadBytes reads until the first occurrence of delim in the input,
+// returning a slice containing the data up to and including the delimiter.
+// If ReadBytes encounters an error before finding a delimiter,
+// it returns the data read before the error and the error itself (often [io.EOF]).
+// ReadBytes returns err != nil if and only if the returned data does not end in
+// delim.
+func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
+	slice, err := b.readSlice(delim)
+	// return a copy of slice. The buffer's backing array may
+	// be overwritten by later calls.
+	line = append(line, slice...)
+	return line, err
+}
+
+// readSlice is like ReadBytes but returns a reference to internal buffer data.
+func (b *Buffer) readSlice(delim byte) (line []byte, err error) {
+	i := IndexByte(b.buf[b.off:], delim)
+	end := b.off + i + 1
+	if i < 0 {
+		end = len(b.buf)
+		err = io.EOF
+	}
+	line = b.buf[b.off:end]
+	b.off = end
+	b.lastRead = opRead
+	return line, err
+}
+
+// ReadString reads until the first occurrence of delim in the input,
+// returning a string containing the data up to and including the delimiter.
+// If ReadString encounters an error before finding a delimiter,
+// it returns the data read before the error and the error itself (often [io.EOF]).
+// ReadString returns err != nil if and only if the returned data does not end
+// in delim.
+func (b *Buffer) ReadString(delim byte) (line string, err error) {
+	slice, err := b.readSlice(delim)
+	return string(slice), err
+}
+
+// NewBuffer creates and initializes a new [Buffer] using buf as its
+// initial contents. The new [Buffer] takes ownership of buf, and the
+// caller should not use buf after this call. NewBuffer is intended to
+// prepare a [Buffer] to read existing data. It can also be used to set
+// the initial size of the internal buffer for writing. To do that,
+// buf should have the desired capacity but a length of zero.
+//
+// In most cases, new([Buffer]) (or just declaring a [Buffer] variable) is
+// sufficient to initialize a [Buffer].
+func NewBuffer(buf []byte) *Buffer { return &Buffer{buf: buf} }
+
+// NewBufferString creates and initializes a new [Buffer] using string s as its
+// initial contents. It is intended to prepare a buffer to read an existing
+// string.
+//
+// In most cases, new([Buffer]) (or just declaring a [Buffer] variable) is
+// sufficient to initialize a [Buffer].
+func NewBufferString(s string) *Buffer {
+	return &Buffer{buf: []byte(s)}
+}

go/src/bytes/buffer_test.go

@@ -0,0 +1,781 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes_test
+
+import (
+	. "bytes"
+	"fmt"
+	"internal/testenv"
+	"io"
+	"math/rand"
+	"strconv"
+	"testing"
+	"unicode/utf8"
+)
+
+const N = 10000       // make this bigger for a larger (and slower) test
+var testString string // test data for write tests
+var testBytes []byte  // test data; same as testString but as a slice.
+
+type negativeReader struct{}
+
+func (r *negativeReader) Read([]byte) (int, error) { return -1, nil }
+
+func init() {
+	testBytes = make([]byte, N)
+	for i := 0; i < N; i++ {
+		testBytes[i] = 'a' + byte(i%26)
+	}
+	testString = string(testBytes)
+}
+
+// Verify that contents of buf match the string s.
+func check(t *testing.T, testname string, buf *Buffer, s string) {
+	bytes := buf.Bytes()
+	str := buf.String()
+	if buf.Len() != len(bytes) {
+		t.Errorf("%s: buf.Len() == %d, len(buf.Bytes()) == %d", testname, buf.Len(), len(bytes))
+	}
+
+	if buf.Len() != len(str) {
+		t.Errorf("%s: buf.Len() == %d, len(buf.String()) == %d", testname, buf.Len(), len(str))
+	}
+
+	if buf.Len() != len(s) {
+		t.Errorf("%s: buf.Len() == %d, len(s) == %d", testname, buf.Len(), len(s))
+	}
+
+	if string(bytes) != s {
+		t.Errorf("%s: string(buf.Bytes()) == %q, s == %q", testname, string(bytes), s)
+	}
+}
+
+// Fill buf through n writes of string fus.
+// The initial contents of buf corresponds to the string s;
+// the result is the final contents of buf returned as a string.
+func fillString(t *testing.T, testname string, buf *Buffer, s string, n int, fus string) string {
+	check(t, testname+" (fill 1)", buf, s)
+	for ; n > 0; n-- {
+		m, err := buf.WriteString(fus)
+		if m != len(fus) {
+			t.Errorf(testname+" (fill 2): m == %d, expected %d", m, len(fus))
+		}
+		if err != nil {
+			t.Errorf(testname+" (fill 3): err should always be nil, found err == %s", err)
+		}
+		s += fus
+		check(t, testname+" (fill 4)", buf, s)
+	}
+	return s
+}
+
+// Fill buf through n writes of byte slice fub.
+// The initial contents of buf corresponds to the string s;
+// the result is the final contents of buf returned as a string.
+func fillBytes(t *testing.T, testname string, buf *Buffer, s string, n int, fub []byte) string {
+	check(t, testname+" (fill 1)", buf, s)
+	for ; n > 0; n-- {
+		m, err := buf.Write(fub)
+		if m != len(fub) {
+			t.Errorf(testname+" (fill 2): m == %d, expected %d", m, len(fub))
+		}
+		if err != nil {
+			t.Errorf(testname+" (fill 3): err should always be nil, found err == %s", err)
+		}
+		s += string(fub)
+		check(t, testname+" (fill 4)", buf, s)
+	}
+	return s
+}
+
+func TestNewBuffer(t *testing.T) {
+	buf := NewBuffer(testBytes)
+	check(t, "NewBuffer", buf, testString)
+}
+
+var buf Buffer
+
+// Calling NewBuffer and immediately shallow copying the Buffer struct
+// should not result in any allocations.
+// This can be used to reset the underlying []byte of an existing Buffer.
+func TestNewBufferShallow(t *testing.T) {
+	testenv.SkipIfOptimizationOff(t)
+	n := testing.AllocsPerRun(1000, func() {
+		buf = *NewBuffer(testBytes)
+	})
+	if n > 0 {
+		t.Errorf("allocations occurred while shallow copying")
+	}
+	check(t, "NewBuffer", &buf, testString)
+}
+
+func TestNewBufferString(t *testing.T) {
+	buf := NewBufferString(testString)
+	check(t, "NewBufferString", buf, testString)
+}
+
+// Empty buf through repeated reads into fub.
+// The initial contents of buf corresponds to the string s.
+func empty(t *testing.T, testname string, buf *Buffer, s string, fub []byte) {
+	check(t, testname+" (empty 1)", buf, s)
+
+	for {
+		n, err := buf.Read(fub)
+		if n == 0 {
+			break
+		}
+		if err != nil {
+			t.Errorf(testname+" (empty 2): err should always be nil, found err == %s", err)
+		}
+		s = s[n:]
+		check(t, testname+" (empty 3)", buf, s)
+	}
+
+	check(t, testname+" (empty 4)", buf, "")
+}
+
+func TestBasicOperations(t *testing.T) {
+	var buf Buffer
+
+	for i := 0; i < 5; i++ {
+		check(t, "TestBasicOperations (1)", &buf, "")
+
+		buf.Reset()
+		check(t, "TestBasicOperations (2)", &buf, "")
+
+		buf.Truncate(0)
+		check(t, "TestBasicOperations (3)", &buf, "")
+
+		n, err := buf.Write(testBytes[0:1])
+		if want := 1; err != nil || n != want {
+			t.Errorf("Write: got (%d, %v), want (%d, %v)", n, err, want, nil)
+		}
+		check(t, "TestBasicOperations (4)", &buf, "a")
+
+		buf.WriteByte(testString[1])
+		check(t, "TestBasicOperations (5)", &buf, "ab")
+
+		n, err = buf.Write(testBytes[2:26])
+		if want := 24; err != nil || n != want {
+			t.Errorf("Write: got (%d, %v), want (%d, %v)", n, err, want, nil)
+		}
+		check(t, "TestBasicOperations (6)", &buf, testString[0:26])
+
+		buf.Truncate(26)
+		check(t, "TestBasicOperations (7)", &buf, testString[0:26])
+
+		buf.Truncate(20)
+		check(t, "TestBasicOperations (8)", &buf, testString[0:20])
+
+		empty(t, "TestBasicOperations (9)", &buf, testString[0:20], make([]byte, 5))
+		empty(t, "TestBasicOperations (10)", &buf, "", make([]byte, 100))
+
+		buf.WriteByte(testString[1])
+		c, err := buf.ReadByte()
+		if want := testString[1]; err != nil || c != want {
+			t.Errorf("ReadByte: got (%q, %v), want (%q, %v)", c, err, want, nil)
+		}
+		c, err = buf.ReadByte()
+		if err != io.EOF {
+			t.Errorf("ReadByte: got (%q, %v), want (%q, %v)", c, err, byte(0), io.EOF)
+		}
+	}
+}
+
+func TestLargeStringWrites(t *testing.T) {
+	var buf Buffer
+	limit := 30
+	if testing.Short() {
+		limit = 9
+	}
+	for i := 3; i < limit; i += 3 {
+		s := fillString(t, "TestLargeWrites (1)", &buf, "", 5, testString)
+		empty(t, "TestLargeStringWrites (2)", &buf, s, make([]byte, len(testString)/i))
+	}
+	check(t, "TestLargeStringWrites (3)", &buf, "")
+}
+
+func TestLargeByteWrites(t *testing.T) {
+	var buf Buffer
+	limit := 30
+	if testing.Short() {
+		limit = 9
+	}
+	for i := 3; i < limit; i += 3 {
+		s := fillBytes(t, "TestLargeWrites (1)", &buf, "", 5, testBytes)
+		empty(t, "TestLargeByteWrites (2)", &buf, s, make([]byte, len(testString)/i))
+	}
+	check(t, "TestLargeByteWrites (3)", &buf, "")
+}
+
+func TestLargeStringReads(t *testing.T) {
+	var buf Buffer
+	for i := 3; i < 30; i += 3 {
+		s := fillString(t, "TestLargeReads (1)", &buf, "", 5, testString[:len(testString)/i])
+		empty(t, "TestLargeReads (2)", &buf, s, make([]byte, len(testString)))
+	}
+	check(t, "TestLargeStringReads (3)", &buf, "")
+}
+
+func TestLargeByteReads(t *testing.T) {
+	var buf Buffer
+	for i := 3; i < 30; i += 3 {
+		s := fillBytes(t, "TestLargeReads (1)", &buf, "", 5, testBytes[:len(testBytes)/i])
+		empty(t, "TestLargeReads (2)", &buf, s, make([]byte, len(testString)))
+	}
+	check(t, "TestLargeByteReads (3)", &buf, "")
+}
+
+func TestMixedReadsAndWrites(t *testing.T) {
+	var buf Buffer
+	s := ""
+	for i := 0; i < 50; i++ {
+		wlen := rand.Intn(len(testString))
+		if i%2 == 0 {
+			s = fillString(t, "TestMixedReadsAndWrites (1)", &buf, s, 1, testString[0:wlen])
+		} else {
+			s = fillBytes(t, "TestMixedReadsAndWrites (1)", &buf, s, 1, testBytes[0:wlen])
+		}
+
+		rlen := rand.Intn(len(testString))
+		fub := make([]byte, rlen)
+		n, _ := buf.Read(fub)
+		s = s[n:]
+	}
+	empty(t, "TestMixedReadsAndWrites (2)", &buf, s, make([]byte, buf.Len()))
+}
+
+func TestCapWithPreallocatedSlice(t *testing.T) {
+	buf := NewBuffer(make([]byte, 10))
+	n := buf.Cap()
+	if n != 10 {
+		t.Errorf("expected 10, got %d", n)
+	}
+}
+
+func TestCapWithSliceAndWrittenData(t *testing.T) {
+	buf := NewBuffer(make([]byte, 0, 10))
+	buf.Write([]byte("test"))
+	n := buf.Cap()
+	if n != 10 {
+		t.Errorf("expected 10, got %d", n)
+	}
+}
+
+func TestNil(t *testing.T) {
+	var b *Buffer
+	if b.String() != "<nil>" {
+		t.Errorf("expected <nil>; got %q", b.String())
+	}
+}
+
+func TestReadFrom(t *testing.T) {
+	var buf Buffer
+	for i := 3; i < 30; i += 3 {
+		s := fillBytes(t, "TestReadFrom (1)", &buf, "", 5, testBytes[:len(testBytes)/i])
+		var b Buffer
+		b.ReadFrom(&buf)
+		empty(t, "TestReadFrom (2)", &b, s, make([]byte, len(testString)))
+	}
+}
+
+type panicReader struct{ panic bool }
+
+func (r panicReader) Read(p []byte) (int, error) {
+	if r.panic {
+		panic("oops")
+	}
+	return 0, io.EOF
+}
+
+// Make sure that an empty Buffer remains empty when
+// it is "grown" before a Read that panics
+func TestReadFromPanicReader(t *testing.T) {
+
+	// First verify non-panic behaviour
+	var buf Buffer
+	i, err := buf.ReadFrom(panicReader{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if i != 0 {
+		t.Fatalf("unexpected return from bytes.ReadFrom (1): got: %d, want %d", i, 0)
+	}
+	check(t, "TestReadFromPanicReader (1)", &buf, "")
+
+	// Confirm that when Reader panics, the empty buffer remains empty
+	var buf2 Buffer
+	defer func() {
+		recover()
+		check(t, "TestReadFromPanicReader (2)", &buf2, "")
+	}()
+	buf2.ReadFrom(panicReader{panic: true})
+}
+
+func TestReadFromNegativeReader(t *testing.T) {
+	var b Buffer
+	defer func() {
+		switch err := recover().(type) {
+		case nil:
+			t.Fatal("bytes.Buffer.ReadFrom didn't panic")
+		case error:
+			// this is the error string of errNegativeRead
+			wantError := "bytes.Buffer: reader returned negative count from Read"
+			if err.Error() != wantError {
+				t.Fatalf("recovered panic: got %v, want %v", err.Error(), wantError)
+			}
+		default:
+			t.Fatalf("unexpected panic value: %#v", err)
+		}
+	}()
+
+	b.ReadFrom(new(negativeReader))
+}
+
+func TestWriteTo(t *testing.T) {
+	var buf Buffer
+	for i := 3; i < 30; i += 3 {
+		s := fillBytes(t, "TestWriteTo (1)", &buf, "", 5, testBytes[:len(testBytes)/i])
+		var b Buffer
+		buf.WriteTo(&b)
+		empty(t, "TestWriteTo (2)", &b, s, make([]byte, len(testString)))
+	}
+}
+
+func TestWriteAppend(t *testing.T) {
+	var got Buffer
+	var want []byte
+	for i := 0; i < 1000; i++ {
+		b := got.AvailableBuffer()
+		b = strconv.AppendInt(b, int64(i), 10)
+		want = strconv.AppendInt(want, int64(i), 10)
+		got.Write(b)
+	}
+	if !Equal(got.Bytes(), want) {
+		t.Fatalf("Bytes() = %q, want %q", &got, want)
+	}
+
+	// With a sufficiently sized buffer, there should be no allocations.
+	n := testing.AllocsPerRun(100, func() {
+		got.Reset()
+		for i := 0; i < 1000; i++ {
+			b := got.AvailableBuffer()
+			b = strconv.AppendInt(b, int64(i), 10)
+			got.Write(b)
+		}
+	})
+	if n > 0 {
+		t.Errorf("allocations occurred while appending")
+	}
+}
+
+func TestRuneIO(t *testing.T) {
+	const NRune = 1000
+	// Built a test slice while we write the data
+	b := make([]byte, utf8.UTFMax*NRune)
+	var buf Buffer
+	n := 0
+	for r := rune(0); r < NRune; r++ {
+		size := utf8.EncodeRune(b[n:], r)
+		nbytes, err := buf.WriteRune(r)
+		if err != nil {
+			t.Fatalf("WriteRune(%U) error: %s", r, err)
+		}
+		if nbytes != size {
+			t.Fatalf("WriteRune(%U) expected %d, got %d", r, size, nbytes)
+		}
+		n += size
+	}
+	b = b[0:n]
+
+	// Check the resulting bytes
+	if !Equal(buf.Bytes(), b) {
+		t.Fatalf("incorrect result from WriteRune: %q not %q", buf.Bytes(), b)
+	}
+
+	p := make([]byte, utf8.UTFMax)
+	// Read it back with ReadRune
+	for r := rune(0); r < NRune; r++ {
+		size := utf8.EncodeRune(p, r)
+		nr, nbytes, err := buf.ReadRune()
+		if nr != r || nbytes != size || err != nil {
+			t.Fatalf("ReadRune(%U) got %U,%d not %U,%d (err=%s)", r, nr, nbytes, r, size, err)
+		}
+	}
+
+	// Check that UnreadRune works
+	buf.Reset()
+
+	// check at EOF
+	if err := buf.UnreadRune(); err == nil {
+		t.Fatal("UnreadRune at EOF: got no error")
+	}
+	if _, _, err := buf.ReadRune(); err == nil {
+		t.Fatal("ReadRune at EOF: got no error")
+	}
+	if err := buf.UnreadRune(); err == nil {
+		t.Fatal("UnreadRune after ReadRune at EOF: got no error")
+	}
+
+	// check not at EOF
+	buf.Write(b)
+	for r := rune(0); r < NRune; r++ {
+		r1, size, _ := buf.ReadRune()
+		if err := buf.UnreadRune(); err != nil {
+			t.Fatalf("UnreadRune(%U) got error %q", r, err)
+		}
+		r2, nbytes, err := buf.ReadRune()
+		if r1 != r2 || r1 != r || nbytes != size || err != nil {
+			t.Fatalf("ReadRune(%U) after UnreadRune got %U,%d not %U,%d (err=%s)", r, r2, nbytes, r, size, err)
+		}
+	}
+}
+
+func TestWriteInvalidRune(t *testing.T) {
+	// Invalid runes, including negative ones, should be written as
+	// utf8.RuneError.
+	for _, r := range []rune{-1, utf8.MaxRune + 1} {
+		var buf Buffer
+		buf.WriteRune(r)
+		check(t, fmt.Sprintf("TestWriteInvalidRune (%d)", r), &buf, "\uFFFD")
+	}
+}
+
+func TestNext(t *testing.T) {
+	b := []byte{0, 1, 2, 3, 4}
+	tmp := make([]byte, 5)
+	for i := 0; i <= 5; i++ {
+		for j := i; j <= 5; j++ {
+			for k := 0; k <= 6; k++ {
+				// 0 <= i <= j <= 5; 0 <= k <= 6
+				// Check that if we start with a buffer
+				// of length j at offset i and ask for
+				// Next(k), we get the right bytes.
+				buf := NewBuffer(b[0:j])
+				n, _ := buf.Read(tmp[0:i])
+				if n != i {
+					t.Fatalf("Read %d returned %d", i, n)
+				}
+				bb := buf.Next(k)
+				want := k
+				if want > j-i {
+					want = j - i
+				}
+				if len(bb) != want {
+					t.Fatalf("in %d,%d: len(Next(%d)) == %d", i, j, k, len(bb))
+				}
+				for l, v := range bb {
+					if v != byte(l+i) {
+						t.Fatalf("in %d,%d: Next(%d)[%d] = %d, want %d", i, j, k, l, v, l+i)
+					}
+				}
+			}
+		}
+	}
+}
+
+var readBytesTests = []struct {
+	buffer   string
+	delim    byte
+	expected []string
+	err      error
+}{
+	{"", 0, []string{""}, io.EOF},
+	{"a\x00", 0, []string{"a\x00"}, nil},
+	{"abbbaaaba", 'b', []string{"ab", "b", "b", "aaab"}, nil},
+	{"hello\x01world", 1, []string{"hello\x01"}, nil},
+	{"foo\nbar", 0, []string{"foo\nbar"}, io.EOF},
+	{"alpha\nbeta\ngamma\n", '\n', []string{"alpha\n", "beta\n", "gamma\n"}, nil},
+	{"alpha\nbeta\ngamma", '\n', []string{"alpha\n", "beta\n", "gamma"}, io.EOF},
+}
+
+func TestReadBytes(t *testing.T) {
+	for _, test := range readBytesTests {
+		buf := NewBufferString(test.buffer)
+		var err error
+		for _, expected := range test.expected {
+			var bytes []byte
+			bytes, err = buf.ReadBytes(test.delim)
+			if string(bytes) != expected {
+				t.Errorf("expected %q, got %q", expected, bytes)
+			}
+			if err != nil {
+				break
+			}
+		}
+		if err != test.err {
+			t.Errorf("expected error %v, got %v", test.err, err)
+		}
+	}
+}
+
+func TestReadString(t *testing.T) {
+	for _, test := range readBytesTests {
+		buf := NewBufferString(test.buffer)
+		var err error
+		for _, expected := range test.expected {
+			var s string
+			s, err = buf.ReadString(test.delim)
+			if s != expected {
+				t.Errorf("expected %q, got %q", expected, s)
+			}
+			if err != nil {
+				break
+			}
+		}
+		if err != test.err {
+			t.Errorf("expected error %v, got %v", test.err, err)
+		}
+	}
+}
+
+var peekTests = []struct {
+	buffer   string
+	skip     int
+	n        int
+	expected string
+	err      error
+}{
+	{"", 0, 0, "", nil},
+	{"aaa", 0, 3, "aaa", nil},
+	{"foobar", 0, 2, "fo", nil},
+	{"a", 0, 2, "a", io.EOF},
+	{"helloworld", 4, 3, "owo", nil},
+	{"helloworld", 5, 5, "world", nil},
+	{"helloworld", 5, 6, "world", io.EOF},
+	{"helloworld", 10, 1, "", io.EOF},
+}
+
+func TestPeek(t *testing.T) {
+	for _, test := range peekTests {
+		buf := NewBufferString(test.buffer)
+		buf.Next(test.skip)
+		bytes, err := buf.Peek(test.n)
+		if string(bytes) != test.expected {
+			t.Errorf("expected %q, got %q", test.expected, bytes)
+		}
+		if err != test.err {
+			t.Errorf("expected error %v, got %v", test.err, err)
+		}
+		if buf.Len() != len(test.buffer)-test.skip {
+			t.Errorf("bad length after peek: %d, want %d", buf.Len(), len(test.buffer)-test.skip)
+		}
+	}
+}
+
+func BenchmarkReadString(b *testing.B) {
+	const n = 32 << 10
+
+	data := make([]byte, n)
+	data[n-1] = 'x'
+	b.SetBytes(int64(n))
+	for i := 0; i < b.N; i++ {
+		buf := NewBuffer(data)
+		_, err := buf.ReadString('x')
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
+func TestGrow(t *testing.T) {
+	x := []byte{'x'}
+	y := []byte{'y'}
+	tmp := make([]byte, 72)
+	for _, growLen := range []int{0, 100, 1000, 10000, 100000} {
+		for _, startLen := range []int{0, 100, 1000, 10000, 100000} {
+			xBytes := Repeat(x, startLen)
+
+			buf := NewBuffer(xBytes)
+			// If we read, this affects buf.off, which is good to test.
+			readBytes, _ := buf.Read(tmp)
+			yBytes := Repeat(y, growLen)
+			allocs := testing.AllocsPerRun(100, func() {
+				buf.Grow(growLen)
+				buf.Write(yBytes)
+			})
+			// Check no allocation occurs in write, as long as we're single-threaded.
+			if allocs != 0 {
+				t.Errorf("allocation occurred during write")
+			}
+			// Check that buffer has correct data.
+			if !Equal(buf.Bytes()[0:startLen-readBytes], xBytes[readBytes:]) {
+				t.Errorf("bad initial data at %d %d", startLen, growLen)
+			}
+			if !Equal(buf.Bytes()[startLen-readBytes:startLen-readBytes+growLen], yBytes) {
+				t.Errorf("bad written data at %d %d", startLen, growLen)
+			}
+		}
+	}
+}
+
+func TestGrowOverflow(t *testing.T) {
+	defer func() {
+		if err := recover(); err != ErrTooLarge {
+			t.Errorf("after too-large Grow, recover() = %v; want %v", err, ErrTooLarge)
+		}
+	}()
+
+	buf := NewBuffer(make([]byte, 1))
+	const maxInt = int(^uint(0) >> 1)
+	buf.Grow(maxInt)
+}
+
+// Was a bug: used to give EOF reading empty slice at EOF.
+func TestReadEmptyAtEOF(t *testing.T) {
+	b := new(Buffer)
+	slice := make([]byte, 0)
+	n, err := b.Read(slice)
+	if err != nil {
+		t.Errorf("read error: %v", err)
+	}
+	if n != 0 {
+		t.Errorf("wrong count; got %d want 0", n)
+	}
+}
+
+func TestUnreadByte(t *testing.T) {
+	b := new(Buffer)
+
+	// check at EOF
+	if err := b.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte at EOF: got no error")
+	}
+	if _, err := b.ReadByte(); err == nil {
+		t.Fatal("ReadByte at EOF: got no error")
+	}
+	if err := b.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte after ReadByte at EOF: got no error")
+	}
+
+	// check not at EOF
+	b.WriteString("abcdefghijklmnopqrstuvwxyz")
+
+	// after unsuccessful read
+	if n, err := b.Read(nil); n != 0 || err != nil {
+		t.Fatalf("Read(nil) = %d,%v; want 0,nil", n, err)
+	}
+	if err := b.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte after Read(nil): got no error")
+	}
+
+	// after successful read
+	if _, err := b.ReadBytes('m'); err != nil {
+		t.Fatalf("ReadBytes: %v", err)
+	}
+	if err := b.UnreadByte(); err != nil {
+		t.Fatalf("UnreadByte: %v", err)
+	}
+	c, err := b.ReadByte()
+	if err != nil {
+		t.Fatalf("ReadByte: %v", err)
+	}
+	if c != 'm' {
+		t.Errorf("ReadByte = %q; want %q", c, 'm')
+	}
+}
+
+// Tests that we occasionally compact. Issue 5154.
+func TestBufferGrowth(t *testing.T) {
+	var b Buffer
+	buf := make([]byte, 1024)
+	b.Write(buf[0:1])
+	var cap0 int
+	for i := 0; i < 5<<10; i++ {
+		b.Write(buf)
+		b.Read(buf)
+		if i == 0 {
+			cap0 = b.Cap()
+		}
+	}
+	cap1 := b.Cap()
+	// (*Buffer).grow allows for 2x capacity slop before sliding,
+	// so set our error threshold at 3x.
+	if cap1 > cap0*3 {
+		t.Errorf("buffer cap = %d; too big (grew from %d)", cap1, cap0)
+	}
+}
+
+func BenchmarkWriteByte(b *testing.B) {
+	const n = 4 << 10
+	b.SetBytes(n)
+	buf := NewBuffer(make([]byte, n))
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		for i := 0; i < n; i++ {
+			buf.WriteByte('x')
+		}
+	}
+}
+
+func BenchmarkWriteRune(b *testing.B) {
+	const n = 4 << 10
+	const r = '☺'
+	b.SetBytes(int64(n * utf8.RuneLen(r)))
+	buf := NewBuffer(make([]byte, n*utf8.UTFMax))
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		for i := 0; i < n; i++ {
+			buf.WriteRune(r)
+		}
+	}
+}
+
+// From Issue 5154.
+func BenchmarkBufferNotEmptyWriteRead(b *testing.B) {
+	buf := make([]byte, 1024)
+	for i := 0; i < b.N; i++ {
+		var b Buffer
+		b.Write(buf[0:1])
+		for i := 0; i < 5<<10; i++ {
+			b.Write(buf)
+			b.Read(buf)
+		}
+	}
+}
+
+// Check that we don't compact too often. From Issue 5154.
+func BenchmarkBufferFullSmallReads(b *testing.B) {
+	buf := make([]byte, 1024)
+	for i := 0; i < b.N; i++ {
+		var b Buffer
+		b.Write(buf)
+		for b.Len()+20 < b.Cap() {
+			b.Write(buf[:10])
+		}
+		for i := 0; i < 5<<10; i++ {
+			b.Read(buf[:1])
+			b.Write(buf[:1])
+		}
+	}
+}
+
+func BenchmarkBufferWriteBlock(b *testing.B) {
+	block := make([]byte, 1024)
+	for _, n := range []int{1 << 12, 1 << 16, 1 << 20} {
+		b.Run(fmt.Sprintf("N%d", n), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				var bb Buffer
+				for bb.Len() < n {
+					bb.Write(block)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkBufferAppendNoCopy(b *testing.B) {
+	var bb Buffer
+	bb.Grow(16 << 20)
+	b.SetBytes(int64(bb.Available()))
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		bb.Reset()
+		b := bb.AvailableBuffer()
+		b = b[:cap(b)] // use max capacity to simulate a large append operation
+		bb.Write(b)    // should be nearly infinitely fast
+	}
+}

go/src/bytes/bytes.go

@@ -0,0 +1,1415 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package bytes implements functions for the manipulation of byte slices.
+// It is analogous to the facilities of the [strings] package.
+package bytes
+
+import (
+	"internal/bytealg"
+	"math/bits"
+	"unicode"
+	"unicode/utf8"
+	_ "unsafe" // for linkname
+)
+
+// Equal reports whether a and b
+// are the same length and contain the same bytes.
+// A nil argument is equivalent to an empty slice.
+func Equal(a, b []byte) bool {
+	// Neither cmd/compile nor gccgo allocates for these string conversions.
+	return string(a) == string(b)
+}
+
+// Compare returns an integer comparing two byte slices lexicographically.
+// The result will be 0 if a == b, -1 if a < b, and +1 if a > b.
+// A nil argument is equivalent to an empty slice.
+func Compare(a, b []byte) int {
+	return bytealg.Compare(a, b)
+}
+
+// explode splits s into a slice of UTF-8 sequences, one per Unicode code point (still slices of bytes),
+// up to a maximum of n byte slices. Invalid UTF-8 sequences are chopped into individual bytes.
+func explode(s []byte, n int) [][]byte {
+	if n <= 0 || n > len(s) {
+		n = len(s)
+	}
+	a := make([][]byte, n)
+	var size int
+	na := 0
+	for len(s) > 0 {
+		if na+1 >= n {
+			a[na] = s
+			na++
+			break
+		}
+		_, size = utf8.DecodeRune(s)
+		a[na] = s[0:size:size]
+		s = s[size:]
+		na++
+	}
+	return a[0:na]
+}
+
+// Count counts the number of non-overlapping instances of sep in s.
+// If sep is an empty slice, Count returns 1 + the number of UTF-8-encoded code points in s.
+func Count(s, sep []byte) int {
+	// special case
+	if len(sep) == 0 {
+		return utf8.RuneCount(s) + 1
+	}
+	if len(sep) == 1 {
+		return bytealg.Count(s, sep[0])
+	}
+	n := 0
+	for {
+		i := Index(s, sep)
+		if i == -1 {
+			return n
+		}
+		n++
+		s = s[i+len(sep):]
+	}
+}
+
+// Contains reports whether subslice is within b.
+func Contains(b, subslice []byte) bool {
+	return Index(b, subslice) != -1
+}
+
+// ContainsAny reports whether any of the UTF-8-encoded code points in chars are within b.
+func ContainsAny(b []byte, chars string) bool {
+	return IndexAny(b, chars) >= 0
+}
+
+// ContainsRune reports whether the rune is contained in the UTF-8-encoded byte slice b.
+func ContainsRune(b []byte, r rune) bool {
+	return IndexRune(b, r) >= 0
+}
+
+// ContainsFunc reports whether any of the UTF-8-encoded code points r within b satisfy f(r).
+func ContainsFunc(b []byte, f func(rune) bool) bool {
+	return IndexFunc(b, f) >= 0
+}
+
+// IndexByte returns the index of the first instance of c in b, or -1 if c is not present in b.
+func IndexByte(b []byte, c byte) int {
+	return bytealg.IndexByte(b, c)
+}
+
+func indexBytePortable(s []byte, c byte) int {
+	for i, b := range s {
+		if b == c {
+			return i
+		}
+	}
+	return -1
+}
+
+// LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
+func LastIndex(s, sep []byte) int {
+	n := len(sep)
+	switch {
+	case n == 0:
+		return len(s)
+	case n == 1:
+		return bytealg.LastIndexByte(s, sep[0])
+	case n == len(s):
+		if Equal(s, sep) {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
+	}
+	return bytealg.LastIndexRabinKarp(s, sep)
+}
+
+// LastIndexByte returns the index of the last instance of c in s, or -1 if c is not present in s.
+func LastIndexByte(s []byte, c byte) int {
+	return bytealg.LastIndexByte(s, c)
+}
+
+// IndexRune interprets s as a sequence of UTF-8-encoded code points.
+// It returns the byte index of the first occurrence in s of the given rune.
+// It returns -1 if rune is not present in s.
+// If r is [utf8.RuneError], it returns the first instance of any
+// invalid UTF-8 byte sequence.
+func IndexRune(s []byte, r rune) int {
+	const haveFastIndex = bytealg.MaxBruteForce > 0
+	switch {
+	case 0 <= r && r < utf8.RuneSelf:
+		return IndexByte(s, byte(r))
+	case r == utf8.RuneError:
+		for i := 0; i < len(s); {
+			r1, n := utf8.DecodeRune(s[i:])
+			if r1 == utf8.RuneError {
+				return i
+			}
+			i += n
+		}
+		return -1
+	case !utf8.ValidRune(r):
+		return -1
+	default:
+		// Search for rune r using the last byte of its UTF-8 encoded form.
+		// The distribution of the last byte is more uniform compared to the
+		// first byte which has a 78% chance of being [240, 243, 244].
+		var b [utf8.UTFMax]byte
+		n := utf8.EncodeRune(b[:], r)
+		last := n - 1
+		i := last
+		fails := 0
+		for i < len(s) {
+			if s[i] != b[last] {
+				o := IndexByte(s[i+1:], b[last])
+				if o < 0 {
+					return -1
+				}
+				i += o + 1
+			}
+			// Step backwards comparing bytes.
+			for j := 1; j < n; j++ {
+				if s[i-j] != b[last-j] {
+					goto next
+				}
+			}
+			return i - last
+		next:
+			fails++
+			i++
+			if (haveFastIndex && fails > bytealg.Cutover(i)) && i < len(s) ||
+				(!haveFastIndex && fails >= 4+i>>4 && i < len(s)) {
+				goto fallback
+			}
+		}
+		return -1
+
+	fallback:
+		// Switch to bytealg.Index, if available, or a brute force search when
+		// IndexByte returns too many false positives.
+		if haveFastIndex {
+			if j := bytealg.Index(s[i-last:], b[:n]); j >= 0 {
+				return i + j - last
+			}
+		} else {
+			// If bytealg.Index is not available a brute force search is
+			// ~1.5-3x faster than Rabin-Karp since n is small.
+			c0 := b[last]
+			c1 := b[last-1] // There are at least 2 chars to match
+		loop:
+			for ; i < len(s); i++ {
+				if s[i] == c0 && s[i-1] == c1 {
+					for k := 2; k < n; k++ {
+						if s[i-k] != b[last-k] {
+							continue loop
+						}
+					}
+					return i - last
+				}
+			}
+		}
+		return -1
+	}
+}
+
+// IndexAny interprets s as a sequence of UTF-8-encoded Unicode code points.
+// It returns the byte index of the first occurrence in s of any of the Unicode
+// code points in chars. It returns -1 if chars is empty or if there is no code
+// point in common.
+func IndexAny(s []byte, chars string) int {
+	if chars == "" {
+		// Avoid scanning all of s.
+		return -1
+	}
+	if len(s) == 1 {
+		r := rune(s[0])
+		if r >= utf8.RuneSelf {
+			// search utf8.RuneError.
+			for _, r = range chars {
+				if r == utf8.RuneError {
+					return 0
+				}
+			}
+			return -1
+		}
+		if bytealg.IndexByteString(chars, s[0]) >= 0 {
+			return 0
+		}
+		return -1
+	}
+	if len(chars) == 1 {
+		r := rune(chars[0])
+		if r >= utf8.RuneSelf {
+			r = utf8.RuneError
+		}
+		return IndexRune(s, r)
+	}
+	if len(s) > 8 {
+		if as, isASCII := makeASCIISet(chars); isASCII {
+			for i, c := range s {
+				if as.contains(c) {
+					return i
+				}
+			}
+			return -1
+		}
+	}
+	var width int
+	for i := 0; i < len(s); i += width {
+		r := rune(s[i])
+		if r < utf8.RuneSelf {
+			if bytealg.IndexByteString(chars, s[i]) >= 0 {
+				return i
+			}
+			width = 1
+			continue
+		}
+		r, width = utf8.DecodeRune(s[i:])
+		if r != utf8.RuneError {
+			// r is 2 to 4 bytes
+			if len(chars) == width {
+				if chars == string(r) {
+					return i
+				}
+				continue
+			}
+			// Use bytealg.IndexString for performance if available.
+			if bytealg.MaxLen >= width {
+				if bytealg.IndexString(chars, string(r)) >= 0 {
+					return i
+				}
+				continue
+			}
+		}
+		for _, ch := range chars {
+			if r == ch {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
+// LastIndexAny interprets s as a sequence of UTF-8-encoded Unicode code
+// points. It returns the byte index of the last occurrence in s of any of
+// the Unicode code points in chars. It returns -1 if chars is empty or if
+// there is no code point in common.
+func LastIndexAny(s []byte, chars string) int {
+	if chars == "" {
+		// Avoid scanning all of s.
+		return -1
+	}
+	if len(s) > 8 {
+		if as, isASCII := makeASCIISet(chars); isASCII {
+			for i := len(s) - 1; i >= 0; i-- {
+				if as.contains(s[i]) {
+					return i
+				}
+			}
+			return -1
+		}
+	}
+	if len(s) == 1 {
+		r := rune(s[0])
+		if r >= utf8.RuneSelf {
+			for _, r = range chars {
+				if r == utf8.RuneError {
+					return 0
+				}
+			}
+			return -1
+		}
+		if bytealg.IndexByteString(chars, s[0]) >= 0 {
+			return 0
+		}
+		return -1
+	}
+	if len(chars) == 1 {
+		cr := rune(chars[0])
+		if cr >= utf8.RuneSelf {
+			cr = utf8.RuneError
+		}
+		for i := len(s); i > 0; {
+			r, size := utf8.DecodeLastRune(s[:i])
+			i -= size
+			if r == cr {
+				return i
+			}
+		}
+		return -1
+	}
+	for i := len(s); i > 0; {
+		r := rune(s[i-1])
+		if r < utf8.RuneSelf {
+			if bytealg.IndexByteString(chars, s[i-1]) >= 0 {
+				return i - 1
+			}
+			i--
+			continue
+		}
+		r, size := utf8.DecodeLastRune(s[:i])
+		i -= size
+		if r != utf8.RuneError {
+			// r is 2 to 4 bytes
+			if len(chars) == size {
+				if chars == string(r) {
+					return i
+				}
+				continue
+			}
+			// Use bytealg.IndexString for performance if available.
+			if bytealg.MaxLen >= size {
+				if bytealg.IndexString(chars, string(r)) >= 0 {
+					return i
+				}
+				continue
+			}
+		}
+		for _, ch := range chars {
+			if r == ch {
+				return i
+			}
+		}
+	}
+	return -1
+}
+
+// Generic split: splits after each instance of sep,
+// including sepSave bytes of sep in the subslices.
+func genSplit(s, sep []byte, sepSave, n int) [][]byte {
+	if n == 0 {
+		return nil
+	}
+	if len(sep) == 0 {
+		return explode(s, n)
+	}
+	if n < 0 {
+		n = Count(s, sep) + 1
+	}
+	if n > len(s)+1 {
+		n = len(s) + 1
+	}
+
+	a := make([][]byte, n)
+	n--
+	i := 0
+	for i < n {
+		m := Index(s, sep)
+		if m < 0 {
+			break
+		}
+		a[i] = s[: m+sepSave : m+sepSave]
+		s = s[m+len(sep):]
+		i++
+	}
+	a[i] = s
+	return a[:i+1]
+}
+
+// SplitN slices s into subslices separated by sep and returns a slice of
+// the subslices between those separators.
+// If sep is empty, SplitN splits after each UTF-8 sequence.
+// The count determines the number of subslices to return:
+//   - n > 0: at most n subslices; the last subslice will be the unsplit remainder;
+//   - n == 0: the result is nil (zero subslices);
+//   - n < 0: all subslices.
+//
+// To split around the first instance of a separator, see [Cut].
+func SplitN(s, sep []byte, n int) [][]byte { return genSplit(s, sep, 0, n) }
+
+// SplitAfterN slices s into subslices after each instance of sep and
+// returns a slice of those subslices.
+// If sep is empty, SplitAfterN splits after each UTF-8 sequence.
+// The count determines the number of subslices to return:
+//   - n > 0: at most n subslices; the last subslice will be the unsplit remainder;
+//   - n == 0: the result is nil (zero subslices);
+//   - n < 0: all subslices.
+func SplitAfterN(s, sep []byte, n int) [][]byte {
+	return genSplit(s, sep, len(sep), n)
+}
+
+// Split slices s into all subslices separated by sep and returns a slice of
+// the subslices between those separators.
+// If sep is empty, Split splits after each UTF-8 sequence.
+// It is equivalent to SplitN with a count of -1.
+//
+// To split around the first instance of a separator, see [Cut].
+func Split(s, sep []byte) [][]byte { return genSplit(s, sep, 0, -1) }
+
+// SplitAfter slices s into all subslices after each instance of sep and
+// returns a slice of those subslices.
+// If sep is empty, SplitAfter splits after each UTF-8 sequence.
+// It is equivalent to SplitAfterN with a count of -1.
+func SplitAfter(s, sep []byte) [][]byte {
+	return genSplit(s, sep, len(sep), -1)
+}
+
+var asciiSpace = [256]uint8{'\t': 1, '\n': 1, '\v': 1, '\f': 1, '\r': 1, ' ': 1}
+
+// Fields interprets s as a sequence of UTF-8-encoded code points.
+// It splits the slice s around each instance of one or more consecutive white space
+// characters, as defined by [unicode.IsSpace], returning a slice of subslices of s or an
+// empty slice if s contains only white space. Every element of the returned slice is
+// non-empty. Unlike [Split], leading and trailing runs of white space characters
+// are discarded.
+func Fields(s []byte) [][]byte {
+	// First count the fields.
+	// This is an exact count if s is ASCII, otherwise it is an approximation.
+	n := 0
+	wasSpace := 1
+	// setBits is used to track which bits are set in the bytes of s.
+	setBits := uint8(0)
+	for i := 0; i < len(s); i++ {
+		r := s[i]
+		setBits |= r
+		isSpace := int(asciiSpace[r])
+		n += wasSpace & ^isSpace
+		wasSpace = isSpace
+	}
+
+	if setBits >= utf8.RuneSelf {
+		// Some runes in the input slice are not ASCII.
+		return FieldsFunc(s, unicode.IsSpace)
+	}
+
+	// ASCII fast path
+	a := make([][]byte, n)
+	na := 0
+	fieldStart := 0
+	i := 0
+	// Skip spaces in the front of the input.
+	for i < len(s) && asciiSpace[s[i]] != 0 {
+		i++
+	}
+	fieldStart = i
+	for i < len(s) {
+		if asciiSpace[s[i]] == 0 {
+			i++
+			continue
+		}
+		a[na] = s[fieldStart:i:i]
+		na++
+		i++
+		// Skip spaces in between fields.
+		for i < len(s) && asciiSpace[s[i]] != 0 {
+			i++
+		}
+		fieldStart = i
+	}
+	if fieldStart < len(s) { // Last field might end at EOF.
+		a[na] = s[fieldStart:len(s):len(s)]
+	}
+	return a
+}
+
+// FieldsFunc interprets s as a sequence of UTF-8-encoded code points.
+// It splits the slice s at each run of code points c satisfying f(c) and
+// returns a slice of subslices of s. If all code points in s satisfy f(c), or
+// len(s) == 0, an empty slice is returned. Every element of the returned slice is
+// non-empty. Unlike [Split], leading and trailing runs of code points
+// satisfying f(c) are discarded.
+//
+// FieldsFunc makes no guarantees about the order in which it calls f(c)
+// and assumes that f always returns the same value for a given c.
+func FieldsFunc(s []byte, f func(rune) bool) [][]byte {
+	// A span is used to record a slice of s of the form s[start:end].
+	// The start index is inclusive and the end index is exclusive.
+	type span struct {
+		start int
+		end   int
+	}
+	spans := make([]span, 0, 32)
+
+	// Find the field start and end indices.
+	// Doing this in a separate pass (rather than slicing the string s
+	// and collecting the result substrings right away) is significantly
+	// more efficient, possibly due to cache effects.
+	start := -1 // valid span start if >= 0
+	for i := 0; i < len(s); {
+		r, size := utf8.DecodeRune(s[i:])
+		if f(r) {
+			if start >= 0 {
+				spans = append(spans, span{start, i})
+				start = -1
+			}
+		} else {
+			if start < 0 {
+				start = i
+			}
+		}
+		i += size
+	}
+
+	// Last field might end at EOF.
+	if start >= 0 {
+		spans = append(spans, span{start, len(s)})
+	}
+
+	// Create subslices from recorded field indices.
+	a := make([][]byte, len(spans))
+	for i, span := range spans {
+		a[i] = s[span.start:span.end:span.end]
+	}
+
+	return a
+}
+
+// Join concatenates the elements of s to create a new byte slice. The separator
+// sep is placed between elements in the resulting slice.
+func Join(s [][]byte, sep []byte) []byte {
+	if len(s) == 0 {
+		return []byte{}
+	}
+	if len(s) == 1 {
+		// Just return a copy.
+		return append([]byte(nil), s[0]...)
+	}
+
+	var n int
+	if len(sep) > 0 {
+		if len(sep) >= maxInt/(len(s)-1) {
+			panic("bytes: Join output length overflow")
+		}
+		n += len(sep) * (len(s) - 1)
+	}
+	for _, v := range s {
+		if len(v) > maxInt-n {
+			panic("bytes: Join output length overflow")
+		}
+		n += len(v)
+	}
+
+	b := bytealg.MakeNoZero(n)[:n:n]
+	bp := copy(b, s[0])
+	for _, v := range s[1:] {
+		bp += copy(b[bp:], sep)
+		bp += copy(b[bp:], v)
+	}
+	return b
+}
+
+// HasPrefix reports whether the byte slice s begins with prefix.
+func HasPrefix(s, prefix []byte) bool {
+	return len(s) >= len(prefix) && Equal(s[:len(prefix)], prefix)
+}
+
+// HasSuffix reports whether the byte slice s ends with suffix.
+func HasSuffix(s, suffix []byte) bool {
+	return len(s) >= len(suffix) && Equal(s[len(s)-len(suffix):], suffix)
+}
+
+// Map returns a copy of the byte slice s with all its characters modified
+// according to the mapping function. If mapping returns a negative value, the character is
+// dropped from the byte slice with no replacement. The characters in s and the
+// output are interpreted as UTF-8-encoded code points.
+func Map(mapping func(r rune) rune, s []byte) []byte {
+	// In the worst case, the slice can grow when mapped, making
+	// things unpleasant. But it's so rare we barge in assuming it's
+	// fine. It could also shrink but that falls out naturally.
+	b := make([]byte, 0, len(s))
+	for i := 0; i < len(s); {
+		r, wid := utf8.DecodeRune(s[i:])
+		r = mapping(r)
+		if r >= 0 {
+			b = utf8.AppendRune(b, r)
+		}
+		i += wid
+	}
+	return b
+}
+
+// Despite being an exported symbol,
+// Repeat is linknamed by widely used packages.
+// Notable members of the hall of shame include:
+//   - gitee.com/quant1x/num
+//
+// Do not remove or change the type signature.
+// See go.dev/issue/67401.
+//
+// Note that this comment is not part of the doc comment.
+//
+//go:linkname Repeat
+
+// Repeat returns a new byte slice consisting of count copies of b.
+//
+// It panics if count is negative or if the result of (len(b) * count)
+// overflows.
+func Repeat(b []byte, count int) []byte {
+	if count == 0 {
+		return []byte{}
+	}
+
+	// Since we cannot return an error on overflow,
+	// we should panic if the repeat will generate an overflow.
+	// See golang.org/issue/16237.
+	if count < 0 {
+		panic("bytes: negative Repeat count")
+	}
+	hi, lo := bits.Mul(uint(len(b)), uint(count))
+	if hi > 0 || lo > uint(maxInt) {
+		panic("bytes: Repeat output length overflow")
+	}
+	n := int(lo) // lo = len(b) * count
+
+	if len(b) == 0 {
+		return []byte{}
+	}
+
+	// Past a certain chunk size it is counterproductive to use
+	// larger chunks as the source of the write, as when the source
+	// is too large we are basically just thrashing the CPU D-cache.
+	// So if the result length is larger than an empirically-found
+	// limit (8KB), we stop growing the source string once the limit
+	// is reached and keep reusing the same source string - that
+	// should therefore be always resident in the L1 cache - until we
+	// have completed the construction of the result.
+	// This yields significant speedups (up to +100%) in cases where
+	// the result length is large (roughly, over L2 cache size).
+	const chunkLimit = 8 * 1024
+	chunkMax := n
+	if chunkMax > chunkLimit {
+		chunkMax = chunkLimit / len(b) * len(b)
+		if chunkMax == 0 {
+			chunkMax = len(b)
+		}
+	}
+	nb := bytealg.MakeNoZero(n)[:n:n]
+	bp := copy(nb, b)
+	for bp < n {
+		chunk := min(bp, chunkMax)
+		bp += copy(nb[bp:], nb[:chunk])
+	}
+	return nb
+}
+
+// ToUpper returns a copy of the byte slice s with all Unicode letters mapped to
+// their upper case.
+func ToUpper(s []byte) []byte {
+	isASCII, hasLower := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasLower = hasLower || ('a' <= c && c <= 'z')
+	}
+
+	if isASCII { // optimize for ASCII-only byte slices.
+		if !hasLower {
+			// Just return a copy.
+			return append([]byte(""), s...)
+		}
+		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'a' <= c && c <= 'z' {
+				c -= 'a' - 'A'
+			}
+			b[i] = c
+		}
+		return b
+	}
+	return Map(unicode.ToUpper, s)
+}
+
+// ToLower returns a copy of the byte slice s with all Unicode letters mapped to
+// their lower case.
+func ToLower(s []byte) []byte {
+	isASCII, hasUpper := true, false
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			isASCII = false
+			break
+		}
+		hasUpper = hasUpper || ('A' <= c && c <= 'Z')
+	}
+
+	if isASCII { // optimize for ASCII-only byte slices.
+		if !hasUpper {
+			return append([]byte(""), s...)
+		}
+		b := bytealg.MakeNoZero(len(s))[:len(s):len(s)]
+		for i := 0; i < len(s); i++ {
+			c := s[i]
+			if 'A' <= c && c <= 'Z' {
+				c += 'a' - 'A'
+			}
+			b[i] = c
+		}
+		return b
+	}
+	return Map(unicode.ToLower, s)
+}
+
+// ToTitle treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their title case.
+func ToTitle(s []byte) []byte { return Map(unicode.ToTitle, s) }
+
+// ToUpperSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
+// upper case, giving priority to the special casing rules.
+func ToUpperSpecial(c unicode.SpecialCase, s []byte) []byte {
+	return Map(c.ToUpper, s)
+}
+
+// ToLowerSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
+// lower case, giving priority to the special casing rules.
+func ToLowerSpecial(c unicode.SpecialCase, s []byte) []byte {
+	return Map(c.ToLower, s)
+}
+
+// ToTitleSpecial treats s as UTF-8-encoded bytes and returns a copy with all the Unicode letters mapped to their
+// title case, giving priority to the special casing rules.
+func ToTitleSpecial(c unicode.SpecialCase, s []byte) []byte {
+	return Map(c.ToTitle, s)
+}
+
+// ToValidUTF8 treats s as UTF-8-encoded bytes and returns a copy with each run of bytes
+// representing invalid UTF-8 replaced with the bytes in replacement, which may be empty.
+func ToValidUTF8(s, replacement []byte) []byte {
+	b := make([]byte, 0, len(s)+len(replacement))
+	invalid := false // previous byte was from an invalid UTF-8 sequence
+	for i := 0; i < len(s); {
+		c := s[i]
+		if c < utf8.RuneSelf {
+			i++
+			invalid = false
+			b = append(b, c)
+			continue
+		}
+		_, wid := utf8.DecodeRune(s[i:])
+		if wid == 1 {
+			i++
+			if !invalid {
+				invalid = true
+				b = append(b, replacement...)
+			}
+			continue
+		}
+		invalid = false
+		b = append(b, s[i:i+wid]...)
+		i += wid
+	}
+	return b
+}
+
+// isSeparator reports whether the rune could mark a word boundary.
+// TODO: update when package unicode captures more of the properties.
+func isSeparator(r rune) bool {
+	// ASCII alphanumerics and underscore are not separators
+	if r <= 0x7F {
+		switch {
+		case '0' <= r && r <= '9':
+			return false
+		case 'a' <= r && r <= 'z':
+			return false
+		case 'A' <= r && r <= 'Z':
+			return false
+		case r == '_':
+			return false
+		}
+		return true
+	}
+	// Letters and digits are not separators
+	if unicode.IsLetter(r) || unicode.IsDigit(r) {
+		return false
+	}
+	// Otherwise, all we can do for now is treat spaces as separators.
+	return unicode.IsSpace(r)
+}
+
+// Title treats s as UTF-8-encoded bytes and returns a copy with all Unicode letters that begin
+// words mapped to their title case.
+//
+// Deprecated: The rule Title uses for word boundaries does not handle Unicode
+// punctuation properly. Use golang.org/x/text/cases instead.
+func Title(s []byte) []byte {
+	// Use a closure here to remember state.
+	// Hackish but effective. Depends on Map scanning in order and calling
+	// the closure once per rune.
+	prev := ' '
+	return Map(
+		func(r rune) rune {
+			if isSeparator(prev) {
+				prev = r
+				return unicode.ToTitle(r)
+			}
+			prev = r
+			return r
+		},
+		s)
+}
+
+// TrimLeftFunc treats s as UTF-8-encoded bytes and returns a subslice of s by slicing off
+// all leading UTF-8-encoded code points c that satisfy f(c).
+func TrimLeftFunc(s []byte, f func(r rune) bool) []byte {
+	i := indexFunc(s, f, false)
+	if i == -1 {
+		return nil
+	}
+	return s[i:]
+}
+
+// TrimRightFunc returns a subslice of s by slicing off all trailing
+// UTF-8-encoded code points c that satisfy f(c).
+func TrimRightFunc(s []byte, f func(r rune) bool) []byte {
+	i := lastIndexFunc(s, f, false)
+	if i >= 0 && s[i] >= utf8.RuneSelf {
+		_, wid := utf8.DecodeRune(s[i:])
+		i += wid
+	} else {
+		i++
+	}
+	return s[0:i]
+}
+
+// TrimFunc returns a subslice of s by slicing off all leading and trailing
+// UTF-8-encoded code points c that satisfy f(c).
+func TrimFunc(s []byte, f func(r rune) bool) []byte {
+	return TrimRightFunc(TrimLeftFunc(s, f), f)
+}
+
+// TrimPrefix returns s without the provided leading prefix string.
+// If s doesn't start with prefix, s is returned unchanged.
+func TrimPrefix(s, prefix []byte) []byte {
+	if HasPrefix(s, prefix) {
+		return s[len(prefix):]
+	}
+	return s
+}
+
+// TrimSuffix returns s without the provided trailing suffix string.
+// If s doesn't end with suffix, s is returned unchanged.
+func TrimSuffix(s, suffix []byte) []byte {
+	if HasSuffix(s, suffix) {
+		return s[:len(s)-len(suffix)]
+	}
+	return s
+}
+
+// IndexFunc interprets s as a sequence of UTF-8-encoded code points.
+// It returns the byte index in s of the first Unicode
+// code point satisfying f(c), or -1 if none do.
+func IndexFunc(s []byte, f func(r rune) bool) int {
+	return indexFunc(s, f, true)
+}
+
+// LastIndexFunc interprets s as a sequence of UTF-8-encoded code points.
+// It returns the byte index in s of the last Unicode
+// code point satisfying f(c), or -1 if none do.
+func LastIndexFunc(s []byte, f func(r rune) bool) int {
+	return lastIndexFunc(s, f, true)
+}
+
+// indexFunc is the same as IndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted.
+func indexFunc(s []byte, f func(r rune) bool, truth bool) int {
+	start := 0
+	for start < len(s) {
+		r, wid := utf8.DecodeRune(s[start:])
+		if f(r) == truth {
+			return start
+		}
+		start += wid
+	}
+	return -1
+}
+
+// lastIndexFunc is the same as LastIndexFunc except that if
+// truth==false, the sense of the predicate function is
+// inverted.
+func lastIndexFunc(s []byte, f func(r rune) bool, truth bool) int {
+	for i := len(s); i > 0; {
+		r, size := rune(s[i-1]), 1
+		if r >= utf8.RuneSelf {
+			r, size = utf8.DecodeLastRune(s[0:i])
+		}
+		i -= size
+		if f(r) == truth {
+			return i
+		}
+	}
+	return -1
+}
+
+// asciiSet is a 32-byte value, where each bit represents the presence of a
+// given ASCII character in the set. The 128-bits of the lower 16 bytes,
+// starting with the least-significant bit of the lowest word to the
+// most-significant bit of the highest word, map to the full range of all
+// 128 ASCII characters. The 128-bits of the upper 16 bytes will be zeroed,
+// ensuring that any non-ASCII character will be reported as not in the set.
+// This allocates a total of 32 bytes even though the upper half
+// is unused to avoid bounds checks in asciiSet.contains.
+type asciiSet [8]uint32
+
+// makeASCIISet creates a set of ASCII characters and reports whether all
+// characters in chars are ASCII.
+func makeASCIISet(chars string) (as asciiSet, ok bool) {
+	for i := 0; i < len(chars); i++ {
+		c := chars[i]
+		if c >= utf8.RuneSelf {
+			return as, false
+		}
+		as[c/32] |= 1 << (c % 32)
+	}
+	return as, true
+}
+
+// contains reports whether c is inside the set.
+func (as *asciiSet) contains(c byte) bool {
+	return (as[c/32] & (1 << (c % 32))) != 0
+}
+
+// containsRune is a simplified version of strings.ContainsRune
+// to avoid importing the strings package.
+// We avoid bytes.ContainsRune to avoid allocating a temporary copy of s.
+func containsRune(s string, r rune) bool {
+	for _, c := range s {
+		if c == r {
+			return true
+		}
+	}
+	return false
+}
+
+// Trim returns a subslice of s by slicing off all leading and
+// trailing UTF-8-encoded code points contained in cutset.
+func Trim(s []byte, cutset string) []byte {
+	if len(s) == 0 {
+		// This is what we've historically done.
+		return nil
+	}
+	if cutset == "" {
+		return s
+	}
+	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
+		return trimLeftByte(trimRightByte(s, cutset[0]), cutset[0])
+	}
+	if as, ok := makeASCIISet(cutset); ok {
+		return trimLeftASCII(trimRightASCII(s, &as), &as)
+	}
+	return trimLeftUnicode(trimRightUnicode(s, cutset), cutset)
+}
+
+// TrimLeft returns a subslice of s by slicing off all leading
+// UTF-8-encoded code points contained in cutset.
+func TrimLeft(s []byte, cutset string) []byte {
+	if len(s) == 0 {
+		// This is what we've historically done.
+		return nil
+	}
+	if cutset == "" {
+		return s
+	}
+	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
+		return trimLeftByte(s, cutset[0])
+	}
+	if as, ok := makeASCIISet(cutset); ok {
+		return trimLeftASCII(s, &as)
+	}
+	return trimLeftUnicode(s, cutset)
+}
+
+func trimLeftByte(s []byte, c byte) []byte {
+	for len(s) > 0 && s[0] == c {
+		s = s[1:]
+	}
+	if len(s) == 0 {
+		// This is what we've historically done.
+		return nil
+	}
+	return s
+}
+
+func trimLeftASCII(s []byte, as *asciiSet) []byte {
+	for len(s) > 0 {
+		if !as.contains(s[0]) {
+			break
+		}
+		s = s[1:]
+	}
+	if len(s) == 0 {
+		// This is what we've historically done.
+		return nil
+	}
+	return s
+}
+
+func trimLeftUnicode(s []byte, cutset string) []byte {
+	for len(s) > 0 {
+		r, n := utf8.DecodeRune(s)
+		if !containsRune(cutset, r) {
+			break
+		}
+		s = s[n:]
+	}
+	if len(s) == 0 {
+		// This is what we've historically done.
+		return nil
+	}
+	return s
+}
+
+// TrimRight returns a subslice of s by slicing off all trailing
+// UTF-8-encoded code points that are contained in cutset.
+func TrimRight(s []byte, cutset string) []byte {
+	if len(s) == 0 || cutset == "" {
+		return s
+	}
+	if len(cutset) == 1 && cutset[0] < utf8.RuneSelf {
+		return trimRightByte(s, cutset[0])
+	}
+	if as, ok := makeASCIISet(cutset); ok {
+		return trimRightASCII(s, &as)
+	}
+	return trimRightUnicode(s, cutset)
+}
+
+func trimRightByte(s []byte, c byte) []byte {
+	for len(s) > 0 && s[len(s)-1] == c {
+		s = s[:len(s)-1]
+	}
+	return s
+}
+
+func trimRightASCII(s []byte, as *asciiSet) []byte {
+	for len(s) > 0 {
+		if !as.contains(s[len(s)-1]) {
+			break
+		}
+		s = s[:len(s)-1]
+	}
+	return s
+}
+
+func trimRightUnicode(s []byte, cutset string) []byte {
+	for len(s) > 0 {
+		r, n := rune(s[len(s)-1]), 1
+		if r >= utf8.RuneSelf {
+			r, n = utf8.DecodeLastRune(s)
+		}
+		if !containsRune(cutset, r) {
+			break
+		}
+		s = s[:len(s)-n]
+	}
+	return s
+}
+
+// TrimSpace returns a subslice of s by slicing off all leading and
+// trailing white space, as defined by Unicode.
+func TrimSpace(s []byte) []byte {
+	// Fast path for ASCII: look for the first ASCII non-space byte.
+	for lo, c := range s {
+		if c >= utf8.RuneSelf {
+			// If we run into a non-ASCII byte, fall back to the
+			// slower unicode-aware method on the remaining bytes.
+			return TrimFunc(s[lo:], unicode.IsSpace)
+		}
+		if asciiSpace[c] != 0 {
+			continue
+		}
+		s = s[lo:]
+		// Now look for the first ASCII non-space byte from the end.
+		for hi := len(s) - 1; hi >= 0; hi-- {
+			c := s[hi]
+			if c >= utf8.RuneSelf {
+				return TrimFunc(s[:hi+1], unicode.IsSpace)
+			}
+			if asciiSpace[c] == 0 {
+				// At this point, s[:hi+1] starts and ends with ASCII
+				// non-space bytes, so we're done. Non-ASCII cases have
+				// already been handled above.
+				return s[:hi+1]
+			}
+		}
+	}
+	// Special case to preserve previous TrimLeftFunc behavior,
+	// returning nil instead of empty slice if all spaces.
+	return nil
+}
+
+// Runes interprets s as a sequence of UTF-8-encoded code points.
+// It returns a slice of runes (Unicode code points) equivalent to s.
+func Runes(s []byte) []rune {
+	t := make([]rune, utf8.RuneCount(s))
+	i := 0
+	for len(s) > 0 {
+		r, l := utf8.DecodeRune(s)
+		t[i] = r
+		i++
+		s = s[l:]
+	}
+	return t
+}
+
+// Replace returns a copy of the slice s with the first n
+// non-overlapping instances of old replaced by new.
+// If old is empty, it matches at the beginning of the slice
+// and after each UTF-8 sequence, yielding up to k+1 replacements
+// for a k-rune slice.
+// If n < 0, there is no limit on the number of replacements.
+func Replace(s, old, new []byte, n int) []byte {
+	m := 0
+	if n != 0 {
+		// Compute number of replacements.
+		m = Count(s, old)
+	}
+	if m == 0 {
+		// Just return a copy.
+		return append([]byte(nil), s...)
+	}
+	if n < 0 || m < n {
+		n = m
+	}
+
+	// Apply replacements to buffer.
+	t := make([]byte, len(s)+n*(len(new)-len(old)))
+	w := 0
+	start := 0
+	if len(old) > 0 {
+		for range n {
+			j := start + Index(s[start:], old)
+			w += copy(t[w:], s[start:j])
+			w += copy(t[w:], new)
+			start = j + len(old)
+		}
+	} else { // len(old) == 0
+		w += copy(t[w:], new)
+		for range n - 1 {
+			_, wid := utf8.DecodeRune(s[start:])
+			j := start + wid
+			w += copy(t[w:], s[start:j])
+			w += copy(t[w:], new)
+			start = j
+		}
+	}
+	w += copy(t[w:], s[start:])
+	return t[0:w]
+}
+
+// ReplaceAll returns a copy of the slice s with all
+// non-overlapping instances of old replaced by new.
+// If old is empty, it matches at the beginning of the slice
+// and after each UTF-8 sequence, yielding up to k+1 replacements
+// for a k-rune slice.
+func ReplaceAll(s, old, new []byte) []byte {
+	return Replace(s, old, new, -1)
+}
+
+// EqualFold reports whether s and t, interpreted as UTF-8 strings,
+// are equal under simple Unicode case-folding, which is a more general
+// form of case-insensitivity.
+func EqualFold(s, t []byte) bool {
+	// ASCII fast path
+	i := 0
+	for n := min(len(s), len(t)); i < n; i++ {
+		sr := s[i]
+		tr := t[i]
+		if sr|tr >= utf8.RuneSelf {
+			goto hasUnicode
+		}
+
+		// Easy case.
+		if tr == sr {
+			continue
+		}
+
+		// Make sr < tr to simplify what follows.
+		if tr < sr {
+			tr, sr = sr, tr
+		}
+		// ASCII only, sr/tr must be upper/lower case
+		if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
+			continue
+		}
+		return false
+	}
+	// Check if we've exhausted both strings.
+	return len(s) == len(t)
+
+hasUnicode:
+	s = s[i:]
+	t = t[i:]
+	for len(s) != 0 && len(t) != 0 {
+		// Extract first rune from each.
+		sr, size := utf8.DecodeRune(s)
+		s = s[size:]
+		tr, size := utf8.DecodeRune(t)
+		t = t[size:]
+
+		// If they match, keep going; if not, return false.
+
+		// Easy case.
+		if tr == sr {
+			continue
+		}
+
+		// Make sr < tr to simplify what follows.
+		if tr < sr {
+			tr, sr = sr, tr
+		}
+		// Fast check for ASCII.
+		if tr < utf8.RuneSelf {
+			// ASCII only, sr/tr must be upper/lower case
+			if 'A' <= sr && sr <= 'Z' && tr == sr+'a'-'A' {
+				continue
+			}
+			return false
+		}
+
+		// General case. SimpleFold(x) returns the next equivalent rune > x
+		// or wraps around to smaller values.
+		r := unicode.SimpleFold(sr)
+		for r != sr && r < tr {
+			r = unicode.SimpleFold(r)
+		}
+		if r == tr {
+			continue
+		}
+		return false
+	}
+
+	// One string is empty. Are both?
+	return len(s) == len(t)
+}
+
+// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
+func Index(s, sep []byte) int {
+	n := len(sep)
+	switch {
+	case n == 0:
+		return 0
+	case n == 1:
+		return IndexByte(s, sep[0])
+	case n == len(s):
+		if Equal(sep, s) {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
+	case n <= bytealg.MaxLen:
+		// Use brute force when s and sep both are small
+		if len(s) <= bytealg.MaxBruteForce {
+			return bytealg.Index(s, sep)
+		}
+		c0 := sep[0]
+		c1 := sep[1]
+		i := 0
+		t := len(s) - n + 1
+		fails := 0
+		for i < t {
+			if s[i] != c0 {
+				// IndexByte is faster than bytealg.Index, so use it as long as
+				// we're not getting lots of false positives.
+				o := IndexByte(s[i+1:t], c0)
+				if o < 0 {
+					return -1
+				}
+				i += o + 1
+			}
+			if s[i+1] == c1 && Equal(s[i:i+n], sep) {
+				return i
+			}
+			fails++
+			i++
+			// Switch to bytealg.Index when IndexByte produces too many false positives.
+			if fails > bytealg.Cutover(i) {
+				r := bytealg.Index(s[i:], sep)
+				if r >= 0 {
+					return r + i
+				}
+				return -1
+			}
+		}
+		return -1
+	}
+	c0 := sep[0]
+	c1 := sep[1]
+	i := 0
+	fails := 0
+	t := len(s) - n + 1
+	for i < t {
+		if s[i] != c0 {
+			o := IndexByte(s[i+1:t], c0)
+			if o < 0 {
+				break
+			}
+			i += o + 1
+		}
+		if s[i+1] == c1 && Equal(s[i:i+n], sep) {
+			return i
+		}
+		i++
+		fails++
+		if fails >= 4+i>>4 && i < t {
+			// Give up on IndexByte, it isn't skipping ahead
+			// far enough to be better than Rabin-Karp.
+			// Experiments (using IndexPeriodic) suggest
+			// the cutover is about 16 byte skips.
+			// TODO: if large prefixes of sep are matching
+			// we should cutover at even larger average skips,
+			// because Equal becomes that much more expensive.
+			// This code does not take that effect into account.
+			j := bytealg.IndexRabinKarp(s[i:], sep)
+			if j < 0 {
+				return -1
+			}
+			return i + j
+		}
+	}
+	return -1
+}
+
+// Cut slices s around the first instance of sep,
+// returning the text before and after sep.
+// The found result reports whether sep appears in s.
+// If sep does not appear in s, cut returns s, nil, false.
+//
+// Cut returns slices of the original slice s, not copies.
+func Cut(s, sep []byte) (before, after []byte, found bool) {
+	if i := Index(s, sep); i >= 0 {
+		return s[:i], s[i+len(sep):], true
+	}
+	return s, nil, false
+}
+
+// Clone returns a copy of b[:len(b)].
+// The result may have additional unused capacity.
+// Clone(nil) returns nil.
+func Clone(b []byte) []byte {
+	if b == nil {
+		return nil
+	}
+	return append([]byte{}, b...)
+}
+
+// CutPrefix returns s without the provided leading prefix byte slice
+// and reports whether it found the prefix.
+// If s doesn't start with prefix, CutPrefix returns s, false.
+// If prefix is the empty byte slice, CutPrefix returns s, true.
+//
+// CutPrefix returns slices of the original slice s, not copies.
+func CutPrefix(s, prefix []byte) (after []byte, found bool) {
+	if !HasPrefix(s, prefix) {
+		return s, false
+	}
+	return s[len(prefix):], true
+}
+
+// CutSuffix returns s without the provided ending suffix byte slice
+// and reports whether it found the suffix.
+// If s doesn't end with suffix, CutSuffix returns s, false.
+// If suffix is the empty byte slice, CutSuffix returns s, true.
+//
+// CutSuffix returns slices of the original slice s, not copies.
+func CutSuffix(s, suffix []byte) (before []byte, found bool) {
+	if !HasSuffix(s, suffix) {
+		return s, false
+	}
+	return s[:len(s)-len(suffix)], true
+}

go/src/bytes/bytes_js_wasm_test.go

@@ -0,0 +1,21 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build js && wasm
+
+package bytes_test
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestIssue65571(t *testing.T) {
+	b := make([]byte, 1<<31+1)
+	b[1<<31] = 1
+	i := bytes.IndexByte(b, 1)
+	if i != 1<<31 {
+		t.Errorf("IndexByte(b, 1) = %d; want %d", i, 1<<31)
+	}
+}

go/src/bytes/bytes_test.go

@@ -0,0 +1,2501 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes_test
+
+import (
+	. "bytes"
+	"fmt"
+	"internal/asan"
+	"internal/testenv"
+	"iter"
+	"math"
+	"math/rand"
+	"slices"
+	"strings"
+	"testing"
+	"unicode"
+	"unicode/utf8"
+	"unsafe"
+)
+
+func sliceOfString(s [][]byte) []string {
+	result := make([]string, len(s))
+	for i, v := range s {
+		result[i] = string(v)
+	}
+	return result
+}
+
+func collect(t *testing.T, seq iter.Seq[[]byte]) [][]byte {
+	out := slices.Collect(seq)
+	out1 := slices.Collect(seq)
+	if !slices.Equal(sliceOfString(out), sliceOfString(out1)) {
+		t.Fatalf("inconsistent seq:\n%s\n%s", out, out1)
+	}
+	return out
+}
+
+type LinesTest struct {
+	a string
+	b []string
+}
+
+var linesTests = []LinesTest{
+	{a: "abc\nabc\n", b: []string{"abc\n", "abc\n"}},
+	{a: "abc\r\nabc", b: []string{"abc\r\n", "abc"}},
+	{a: "abc\r\n", b: []string{"abc\r\n"}},
+	{a: "\nabc", b: []string{"\n", "abc"}},
+	{a: "\nabc\n\n", b: []string{"\n", "abc\n", "\n"}},
+}
+
+func TestLines(t *testing.T) {
+	for _, s := range linesTests {
+		result := sliceOfString(slices.Collect(Lines([]byte(s.a))))
+		if !slices.Equal(result, s.b) {
+			t.Errorf(`slices.Collect(Lines(%q)) = %q; want %q`, s.a, result, s.b)
+		}
+	}
+}
+
+// For ease of reading, the test cases use strings that are converted to byte
+// slices before invoking the functions.
+
+var abcd = "abcd"
+var faces = "☺☻☹"
+var commas = "1,2,3,4"
+var dots = "1....2....3....4"
+
+type BinOpTest struct {
+	a string
+	b string
+	i int
+}
+
+func TestEqual(t *testing.T) {
+	// Run the tests and check for allocation at the same time.
+	allocs := testing.AllocsPerRun(10, func() {
+		for _, tt := range compareTests {
+			eql := Equal(tt.a, tt.b)
+			if eql != (tt.i == 0) {
+				t.Errorf(`Equal(%q, %q) = %v`, tt.a, tt.b, eql)
+			}
+		}
+	})
+	if allocs > 0 {
+		t.Errorf("Equal allocated %v times", allocs)
+	}
+}
+
+func TestEqualExhaustive(t *testing.T) {
+	var size = 128
+	if testing.Short() {
+		size = 32
+	}
+	a := make([]byte, size)
+	b := make([]byte, size)
+	b_init := make([]byte, size)
+	// randomish but deterministic data
+	for i := 0; i < size; i++ {
+		a[i] = byte(17 * i)
+		b_init[i] = byte(23*i + 100)
+	}
+
+	for len := 0; len <= size; len++ {
+		for x := 0; x <= size-len; x++ {
+			for y := 0; y <= size-len; y++ {
+				copy(b, b_init)
+				copy(b[y:y+len], a[x:x+len])
+				if !Equal(a[x:x+len], b[y:y+len]) || !Equal(b[y:y+len], a[x:x+len]) {
+					t.Errorf("Equal(%d, %d, %d) = false", len, x, y)
+				}
+			}
+		}
+	}
+}
+
+// make sure Equal returns false for minimally different strings. The data
+// is all zeros except for a single one in one location.
+func TestNotEqual(t *testing.T) {
+	var size = 128
+	if testing.Short() {
+		size = 32
+	}
+	a := make([]byte, size)
+	b := make([]byte, size)
+
+	for len := 0; len <= size; len++ {
+		for x := 0; x <= size-len; x++ {
+			for y := 0; y <= size-len; y++ {
+				for diffpos := x; diffpos < x+len; diffpos++ {
+					a[diffpos] = 1
+					if Equal(a[x:x+len], b[y:y+len]) || Equal(b[y:y+len], a[x:x+len]) {
+						t.Errorf("NotEqual(%d, %d, %d, %d) = true", len, x, y, diffpos)
+					}
+					a[diffpos] = 0
+				}
+			}
+		}
+	}
+}
+
+var indexTests = []BinOpTest{
+	{"", "", 0},
+	{"", "a", -1},
+	{"", "foo", -1},
+	{"fo", "foo", -1},
+	{"foo", "baz", -1},
+	{"foo", "foo", 0},
+	{"oofofoofooo", "f", 2},
+	{"oofofoofooo", "foo", 4},
+	{"barfoobarfoo", "foo", 3},
+	{"foo", "", 0},
+	{"foo", "o", 1},
+	{"abcABCabc", "A", 3},
+	// cases with one byte strings - test IndexByte and special case in Index()
+	{"", "a", -1},
+	{"x", "a", -1},
+	{"x", "x", 0},
+	{"abc", "a", 0},
+	{"abc", "b", 1},
+	{"abc", "c", 2},
+	{"abc", "x", -1},
+	{"barfoobarfooyyyzzzyyyzzzyyyzzzyyyxxxzzzyyy", "x", 33},
+	{"fofofofooofoboo", "oo", 7},
+	{"fofofofofofoboo", "ob", 11},
+	{"fofofofofofoboo", "boo", 12},
+	{"fofofofofofoboo", "oboo", 11},
+	{"fofofofofoooboo", "fooo", 8},
+	{"fofofofofofoboo", "foboo", 10},
+	{"fofofofofofoboo", "fofob", 8},
+	{"fofofofofofofoffofoobarfoo", "foffof", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffof", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofo", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofo", 13},
+	{"fofofofofoofofoffofoobarfoo", "foffofoo", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoo", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoob", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoob", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofooba", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofooba", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobar", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobar", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobarf", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobarf", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobarfo", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobarfo", 12},
+	{"fofofofofoofofoffofoobarfoo", "foffofoobarfoo", 13},
+	{"fofofofofofofoffofoobarfoo", "foffofoobarfoo", 12},
+	{"fofofofofoofofoffofoobarfoo", "ofoffofoobarfoo", 12},
+	{"fofofofofofofoffofoobarfoo", "ofoffofoobarfoo", 11},
+	{"fofofofofoofofoffofoobarfoo", "fofoffofoobarfoo", 11},
+	{"fofofofofofofoffofoobarfoo", "fofoffofoobarfoo", 10},
+	{"fofofofofoofofoffofoobarfoo", "foobars", -1},
+	{"foofyfoobarfoobar", "y", 4},
+	{"oooooooooooooooooooooo", "r", -1},
+	{"oxoxoxoxoxoxoxoxoxoxoxoy", "oy", 22},
+	{"oxoxoxoxoxoxoxoxoxoxoxox", "oy", -1},
+	// test fallback to Rabin-Karp.
+	{"000000000000000000000000000000000000000000000000000000000000000000000001", "0000000000000000000000000000000000000000000000000000000000000000001", 5},
+	// test fallback to IndexRune
+	{"oxoxoxoxoxoxoxoxoxoxox☺", "☺", 22},
+	// invalid UTF-8 byte sequence (must be longer than bytealg.MaxBruteForce to
+	// test that we don't use IndexRune)
+	{"xx0123456789012345678901234567890123456789012345678901234567890120123456789012345678901234567890123456xxx\xed\x9f\xc0", "\xed\x9f\xc0", 105},
+}
+
+var lastIndexTests = []BinOpTest{
+	{"", "", 0},
+	{"", "a", -1},
+	{"", "foo", -1},
+	{"fo", "foo", -1},
+	{"foo", "foo", 0},
+	{"foo", "f", 0},
+	{"oofofoofooo", "f", 7},
+	{"oofofoofooo", "foo", 7},
+	{"barfoobarfoo", "foo", 9},
+	{"foo", "", 3},
+	{"foo", "o", 2},
+	{"abcABCabc", "A", 3},
+	{"abcABCabc", "a", 6},
+}
+
+var indexAnyTests = []BinOpTest{
+	{"", "", -1},
+	{"", "a", -1},
+	{"", "abc", -1},
+	{"a", "", -1},
+	{"a", "a", 0},
+	{"\x80", "\xffb", 0},
+	{"aaa", "a", 0},
+	{"abc", "xyz", -1},
+	{"abc", "xcz", 2},
+	{"ab☺c", "x☺yz", 2},
+	{"a☺b☻c☹d", "cx", len("a☺b☻")},
+	{"a☺b☻c☹d", "uvw☻xyz", len("a☺b")},
+	{"aRegExp*", ".(|)*+?^$[]", 7},
+	{dots + dots + dots, " ", -1},
+	{"012abcba210", "\xffb", 4},
+	{"012\x80bcb\x80210", "\xffb", 3},
+	{"0123456\xcf\x80abc", "\xcfb\x80", 10},
+}
+
+var lastIndexAnyTests = []BinOpTest{
+	{"", "", -1},
+	{"", "a", -1},
+	{"", "abc", -1},
+	{"a", "", -1},
+	{"a", "a", 0},
+	{"\x80", "\xffb", 0},
+	{"aaa", "a", 2},
+	{"abc", "xyz", -1},
+	{"abc", "ab", 1},
+	{"ab☺c", "x☺yz", 2},
+	{"a☺b☻c☹d", "cx", len("a☺b☻")},
+	{"a☺b☻c☹d", "uvw☻xyz", len("a☺b")},
+	{"a.RegExp*", ".(|)*+?^$[]", 8},
+	{dots + dots + dots, " ", -1},
+	{"012abcba210", "\xffb", 6},
+	{"012\x80bcb\x80210", "\xffb", 7},
+	{"0123456\xcf\x80abc", "\xcfb\x80", 10},
+}
+
+// Execute f on each test case.  funcName should be the name of f; it's used
+// in failure reports.
+func runIndexTests(t *testing.T, f func(s, sep []byte) int, funcName string, testCases []BinOpTest) {
+	for _, test := range testCases {
+		a := []byte(test.a)
+		b := []byte(test.b)
+		actual := f(a, b)
+		if actual != test.i {
+			t.Errorf("%s(%q,%q) = %v; want %v", funcName, a, b, actual, test.i)
+		}
+	}
+	var allocTests = []struct {
+		a []byte
+		b []byte
+		i int
+	}{
+		// case for function Index.
+		{[]byte("000000000000000000000000000000000000000000000000000000000000000000000001"), []byte("0000000000000000000000000000000000000000000000000000000000000000001"), 5},
+		// case for function LastIndex.
+		{[]byte("000000000000000000000000000000000000000000000000000000000000000010000"), []byte("00000000000000000000000000000000000000000000000000000000000001"), 3},
+	}
+	allocs := testing.AllocsPerRun(100, func() {
+		if i := Index(allocTests[1].a, allocTests[1].b); i != allocTests[1].i {
+			t.Errorf("Index([]byte(%q), []byte(%q)) = %v; want %v", allocTests[1].a, allocTests[1].b, i, allocTests[1].i)
+		}
+		if i := LastIndex(allocTests[0].a, allocTests[0].b); i != allocTests[0].i {
+			t.Errorf("LastIndex([]byte(%q), []byte(%q)) = %v; want %v", allocTests[0].a, allocTests[0].b, i, allocTests[0].i)
+		}
+	})
+	if allocs != 0 {
+		t.Errorf("expected no allocations, got %f", allocs)
+	}
+}
+
+func runIndexAnyTests(t *testing.T, f func(s []byte, chars string) int, funcName string, testCases []BinOpTest) {
+	for _, test := range testCases {
+		a := []byte(test.a)
+		actual := f(a, test.b)
+		if actual != test.i {
+			t.Errorf("%s(%q,%q) = %v; want %v", funcName, a, test.b, actual, test.i)
+		}
+	}
+}
+
+func TestIndex(t *testing.T)     { runIndexTests(t, Index, "Index", indexTests) }
+func TestLastIndex(t *testing.T) { runIndexTests(t, LastIndex, "LastIndex", lastIndexTests) }
+func TestIndexAny(t *testing.T)  { runIndexAnyTests(t, IndexAny, "IndexAny", indexAnyTests) }
+func TestLastIndexAny(t *testing.T) {
+	runIndexAnyTests(t, LastIndexAny, "LastIndexAny", lastIndexAnyTests)
+}
+
+func TestIndexByte(t *testing.T) {
+	for _, tt := range indexTests {
+		if len(tt.b) != 1 {
+			continue
+		}
+		a := []byte(tt.a)
+		b := tt.b[0]
+		pos := IndexByte(a, b)
+		if pos != tt.i {
+			t.Errorf(`IndexByte(%q, '%c') = %v`, tt.a, b, pos)
+		}
+		posp := IndexBytePortable(a, b)
+		if posp != tt.i {
+			t.Errorf(`indexBytePortable(%q, '%c') = %v`, tt.a, b, posp)
+		}
+	}
+}
+
+func TestLastIndexByte(t *testing.T) {
+	testCases := []BinOpTest{
+		{"", "q", -1},
+		{"abcdef", "q", -1},
+		{"abcdefabcdef", "a", len("abcdef")},      // something in the middle
+		{"abcdefabcdef", "f", len("abcdefabcde")}, // last byte
+		{"zabcdefabcdef", "z", 0},                 // first byte
+		{"a☺b☻c☹d", "b", len("a☺")},               // non-ascii
+	}
+	for _, test := range testCases {
+		actual := LastIndexByte([]byte(test.a), test.b[0])
+		if actual != test.i {
+			t.Errorf("LastIndexByte(%q,%c) = %v; want %v", test.a, test.b[0], actual, test.i)
+		}
+	}
+}
+
+// test a larger buffer with different sizes and alignments
+func TestIndexByteBig(t *testing.T) {
+	var n = 1024
+	if testing.Short() {
+		n = 128
+	}
+	b := make([]byte, n)
+	for i := 0; i < n; i++ {
+		// different start alignments
+		b1 := b[i:]
+		for j := 0; j < len(b1); j++ {
+			b1[j] = 'x'
+			pos := IndexByte(b1, 'x')
+			if pos != j {
+				t.Errorf("IndexByte(%q, 'x') = %v", b1, pos)
+			}
+			b1[j] = 0
+			pos = IndexByte(b1, 'x')
+			if pos != -1 {
+				t.Errorf("IndexByte(%q, 'x') = %v", b1, pos)
+			}
+		}
+		// different end alignments
+		b1 = b[:i]
+		for j := 0; j < len(b1); j++ {
+			b1[j] = 'x'
+			pos := IndexByte(b1, 'x')
+			if pos != j {
+				t.Errorf("IndexByte(%q, 'x') = %v", b1, pos)
+			}
+			b1[j] = 0
+			pos = IndexByte(b1, 'x')
+			if pos != -1 {
+				t.Errorf("IndexByte(%q, 'x') = %v", b1, pos)
+			}
+		}
+		// different start and end alignments
+		b1 = b[i/2 : n-(i+1)/2]
+		for j := 0; j < len(b1); j++ {
+			b1[j] = 'x'
+			pos := IndexByte(b1, 'x')
+			if pos != j {
+				t.Errorf("IndexByte(%q, 'x') = %v", b1, pos)
+			}
+			b1[j] = 0
+			pos = IndexByte(b1, 'x')
+			if pos != -1 {
+				t.Errorf("IndexByte(%q, 'x') = %v", b1, pos)
+			}
+		}
+	}
+}
+
+// test a small index across all page offsets
+func TestIndexByteSmall(t *testing.T) {
+	b := make([]byte, 5015) // bigger than a page
+	// Make sure we find the correct byte even when straddling a page.
+	for i := 0; i <= len(b)-15; i++ {
+		for j := 0; j < 15; j++ {
+			b[i+j] = byte(100 + j)
+		}
+		for j := 0; j < 15; j++ {
+			p := IndexByte(b[i:i+15], byte(100+j))
+			if p != j {
+				t.Errorf("IndexByte(%q, %d) = %d", b[i:i+15], 100+j, p)
+			}
+		}
+		for j := 0; j < 15; j++ {
+			b[i+j] = 0
+		}
+	}
+	// Make sure matches outside the slice never trigger.
+	for i := 0; i <= len(b)-15; i++ {
+		for j := 0; j < 15; j++ {
+			b[i+j] = 1
+		}
+		for j := 0; j < 15; j++ {
+			p := IndexByte(b[i:i+15], byte(0))
+			if p != -1 {
+				t.Errorf("IndexByte(%q, %d) = %d", b[i:i+15], 0, p)
+			}
+		}
+		for j := 0; j < 15; j++ {
+			b[i+j] = 0
+		}
+	}
+}
+
+func TestIndexRune(t *testing.T) {
+	tests := []struct {
+		in   string
+		rune rune
+		want int
+	}{
+		{"", 'a', -1},
+		{"", '☺', -1},
+		{"foo", '☹', -1},
+		{"foo", 'o', 1},
+		{"foo☺bar", '☺', 3},
+		{"foo☺☻☹bar", '☹', 9},
+		{"a A x", 'A', 2},
+		{"some_text=some_value", '=', 9},
+		{"☺a", 'a', 3},
+		{"a☻☺b", '☺', 4},
+		{"𠀳𠀗𠀾𠁄𠀧𠁆𠁂𠀫𠀖𠀪𠀲𠀴𠁀𠀨𠀿", '𠀿', 56},
+
+		// 2 bytes
+		{"ӆ", 'ӆ', 0},
+		{"a", 'ӆ', -1},
+		{"  ӆ", 'ӆ', 2},
+		{"  a", 'ӆ', -1},
+		{strings.Repeat("ц", 64) + "ӆ", 'ӆ', 128}, // test cutover
+		{strings.Repeat("ц", 64), 'ӆ', -1},
+
+		// 3 bytes
+		{"Ꚁ", 'Ꚁ', 0},
+		{"a", 'Ꚁ', -1},
+		{"  Ꚁ", 'Ꚁ', 2},
+		{"  a", 'Ꚁ', -1},
+		{strings.Repeat("Ꙁ", 64) + "Ꚁ", 'Ꚁ', 192}, // test cutover
+		{strings.Repeat("Ꙁ", 64) + "Ꚁ", '䚀', -1},  // 'Ꚁ' and '䚀' share the same last two bytes
+
+		// 4 bytes
+		{"𡌀", '𡌀', 0},
+		{"a", '𡌀', -1},
+		{"  𡌀", '𡌀', 2},
+		{"  a", '𡌀', -1},
+		{strings.Repeat("𡋀", 64) + "𡌀", '𡌀', 256}, // test cutover
+		{strings.Repeat("𡋀", 64) + "𡌀", '𣌀', -1},  // '𡌀' and '𣌀' share the same last two bytes
+
+		// RuneError should match any invalid UTF-8 byte sequence.
+		{"�", '�', 0},
+		{"\xff", '�', 0},
+		{"☻x�", '�', len("☻x")},
+		{"☻x\xe2\x98", '�', len("☻x")},
+		{"☻x\xe2\x98�", '�', len("☻x")},
+		{"☻x\xe2\x98x", '�', len("☻x")},
+
+		// Invalid rune values should never match.
+		{"a☺b☻c☹d\xe2\x98�\xff�\xed\xa0\x80", -1, -1},
+		{"a☺b☻c☹d\xe2\x98�\xff�\xed\xa0\x80", 0xD800, -1}, // Surrogate pair
+		{"a☺b☻c☹d\xe2\x98�\xff�\xed\xa0\x80", utf8.MaxRune + 1, -1},
+
+		// Test the cutover to bytealg.Index when it is triggered in
+		// the middle of rune that contains consecutive runs of equal bytes.
+		{"aaaaaKKKK\U000bc104", '\U000bc104', 17}, // cutover: (n + 16) / 8
+		{"aaaaaKKKK鄄", '鄄', 17},
+		{"aaKKKKKa\U000bc104", '\U000bc104', 18}, // cutover: 4 + n>>4
+		{"aaKKKKKa鄄", '鄄', 18},
+	}
+	for _, tt := range tests {
+		if got := IndexRune([]byte(tt.in), tt.rune); got != tt.want {
+			t.Errorf("IndexRune(%q, %d) = %v; want %v", tt.in, tt.rune, got, tt.want)
+		}
+	}
+
+	haystack := []byte("test世界")
+	allocs := testing.AllocsPerRun(1000, func() {
+		if i := IndexRune(haystack, 's'); i != 2 {
+			t.Fatalf("'s' at %d; want 2", i)
+		}
+		if i := IndexRune(haystack, '世'); i != 4 {
+			t.Fatalf("'世' at %d; want 4", i)
+		}
+	})
+	if allocs != 0 {
+		t.Errorf("expected no allocations, got %f", allocs)
+	}
+}
+
+// test count of a single byte across page offsets
+func TestCountByte(t *testing.T) {
+	b := make([]byte, 5015) // bigger than a page
+	windows := []int{1, 2, 3, 4, 15, 16, 17, 31, 32, 33, 63, 64, 65, 128}
+	testCountWindow := func(i, window int) {
+		for j := 0; j < window; j++ {
+			b[i+j] = byte(100)
+			p := Count(b[i:i+window], []byte{100})
+			if p != j+1 {
+				t.Errorf("TestCountByte.Count(%q, 100) = %d", b[i:i+window], p)
+			}
+		}
+	}
+
+	maxWnd := windows[len(windows)-1]
+
+	for i := 0; i <= 2*maxWnd; i++ {
+		for _, window := range windows {
+			if window > len(b[i:]) {
+				window = len(b[i:])
+			}
+			testCountWindow(i, window)
+			for j := 0; j < window; j++ {
+				b[i+j] = byte(0)
+			}
+		}
+	}
+	for i := 4096 - (maxWnd + 1); i < len(b); i++ {
+		for _, window := range windows {
+			if window > len(b[i:]) {
+				window = len(b[i:])
+			}
+			testCountWindow(i, window)
+			for j := 0; j < window; j++ {
+				b[i+j] = byte(0)
+			}
+		}
+	}
+}
+
+// Make sure we don't count bytes outside our window
+func TestCountByteNoMatch(t *testing.T) {
+	b := make([]byte, 5015)
+	windows := []int{1, 2, 3, 4, 15, 16, 17, 31, 32, 33, 63, 64, 65, 128}
+	for i := 0; i <= len(b); i++ {
+		for _, window := range windows {
+			if window > len(b[i:]) {
+				window = len(b[i:])
+			}
+			// Fill the window with non-match
+			for j := 0; j < window; j++ {
+				b[i+j] = byte(100)
+			}
+			// Try to find something that doesn't exist
+			p := Count(b[i:i+window], []byte{0})
+			if p != 0 {
+				t.Errorf("TestCountByteNoMatch(%q, 0) = %d", b[i:i+window], p)
+			}
+			for j := 0; j < window; j++ {
+				b[i+j] = byte(0)
+			}
+		}
+	}
+}
+
+var bmbuf []byte
+
+func valName(x int) string {
+	if s := x >> 20; s<<20 == x {
+		return fmt.Sprintf("%dM", s)
+	}
+	if s := x >> 10; s<<10 == x {
+		return fmt.Sprintf("%dK", s)
+	}
+	return fmt.Sprint(x)
+}
+
+func benchBytes(b *testing.B, sizes []int, f func(b *testing.B, n int)) {
+	for _, n := range sizes {
+		if isRaceBuilder && n > 4<<10 {
+			continue
+		}
+		b.Run(valName(n), func(b *testing.B) {
+			if len(bmbuf) < n {
+				bmbuf = make([]byte, n)
+			}
+			b.SetBytes(int64(n))
+			f(b, n)
+		})
+	}
+}
+
+var indexSizes = []int{10, 32, 4 << 10, 4 << 20, 64 << 20}
+
+var isRaceBuilder = strings.HasSuffix(testenv.Builder(), "-race")
+
+func BenchmarkIndexByte(b *testing.B) {
+	benchBytes(b, indexSizes, bmIndexByte(IndexByte))
+}
+
+func BenchmarkIndexBytePortable(b *testing.B) {
+	benchBytes(b, indexSizes, bmIndexByte(IndexBytePortable))
+}
+
+func bmIndexByte(index func([]byte, byte) int) func(b *testing.B, n int) {
+	return func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		buf[n-1] = 'x'
+		for i := 0; i < b.N; i++ {
+			j := index(buf, 'x')
+			if j != n-1 {
+				b.Fatal("bad index", j)
+			}
+		}
+		buf[n-1] = '\x00'
+	}
+}
+
+func BenchmarkIndexRune(b *testing.B) {
+	benchBytes(b, indexSizes, bmIndexRune(IndexRune))
+}
+
+func BenchmarkIndexRuneASCII(b *testing.B) {
+	benchBytes(b, indexSizes, bmIndexRuneASCII(IndexRune))
+}
+
+func BenchmarkIndexRuneUnicode(b *testing.B) {
+	b.Run("Latin", func(b *testing.B) {
+		// Latin is mostly 1, 2, 3 byte runes.
+		benchBytes(b, indexSizes, bmIndexRuneUnicode(unicode.Latin, 'é'))
+	})
+	b.Run("Cyrillic", func(b *testing.B) {
+		// Cyrillic is mostly 2 and 3 byte runes.
+		benchBytes(b, indexSizes, bmIndexRuneUnicode(unicode.Cyrillic, 'Ꙁ'))
+	})
+	b.Run("Han", func(b *testing.B) {
+		// Han consists only of 3 and 4 byte runes.
+		benchBytes(b, indexSizes, bmIndexRuneUnicode(unicode.Han, '𠀿'))
+	})
+}
+
+func bmIndexRuneASCII(index func([]byte, rune) int) func(b *testing.B, n int) {
+	return func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		buf[n-1] = 'x'
+		for i := 0; i < b.N; i++ {
+			j := index(buf, 'x')
+			if j != n-1 {
+				b.Fatal("bad index", j)
+			}
+		}
+		buf[n-1] = '\x00'
+	}
+}
+
+func bmIndexRune(index func([]byte, rune) int) func(b *testing.B, n int) {
+	return func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		utf8.EncodeRune(buf[n-3:], '世')
+		for i := 0; i < b.N; i++ {
+			j := index(buf, '世')
+			if j != n-3 {
+				b.Fatal("bad index", j)
+			}
+		}
+		buf[n-3] = '\x00'
+		buf[n-2] = '\x00'
+		buf[n-1] = '\x00'
+	}
+}
+
+func bmIndexRuneUnicode(rt *unicode.RangeTable, needle rune) func(b *testing.B, n int) {
+	var rs []rune
+	for _, r16 := range rt.R16 {
+		for r := rune(r16.Lo); r <= rune(r16.Hi); r += rune(r16.Stride) {
+			if r != needle {
+				rs = append(rs, r)
+			}
+		}
+	}
+	for _, r32 := range rt.R32 {
+		for r := rune(r32.Lo); r <= rune(r32.Hi); r += rune(r32.Stride) {
+			if r != needle {
+				rs = append(rs, r)
+			}
+		}
+	}
+	// Shuffle the runes so that they are not in descending order.
+	// The sort is deterministic since this is used for benchmarks,
+	// which need to be repeatable.
+	rr := rand.New(rand.NewSource(1))
+	rr.Shuffle(len(rs), func(i, j int) {
+		rs[i], rs[j] = rs[j], rs[i]
+	})
+	uchars := string(rs)
+
+	return func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		o := copy(buf, uchars)
+		for o < len(buf) {
+			o += copy(buf[o:], uchars)
+		}
+
+		// Make space for the needle rune at the end of buf.
+		m := utf8.RuneLen(needle)
+		for o := m; o > 0; {
+			_, sz := utf8.DecodeLastRune(buf)
+			copy(buf[len(buf)-sz:], "\x00\x00\x00\x00")
+			buf = buf[:len(buf)-sz]
+			o -= sz
+		}
+		buf = utf8.AppendRune(buf[:n-m], needle)
+
+		n -= m // adjust for rune len
+		for i := 0; i < b.N; i++ {
+			j := IndexRune(buf, needle)
+			if j != n {
+				b.Fatal("bad index", j)
+			}
+		}
+		for i := range buf {
+			buf[i] = '\x00'
+		}
+	}
+}
+
+func BenchmarkEqual(b *testing.B) {
+	b.Run("0", func(b *testing.B) {
+		var buf [4]byte
+		buf1 := buf[0:0]
+		buf2 := buf[1:1]
+		for i := 0; i < b.N; i++ {
+			eq := Equal(buf1, buf2)
+			if !eq {
+				b.Fatal("bad equal")
+			}
+		}
+	})
+
+	sizes := []int{1, 6, 9, 15, 16, 20, 32, 4 << 10, 4 << 20, 64 << 20}
+
+	b.Run("same", func(b *testing.B) {
+		benchBytes(b, sizes, bmEqual(func(a, b []byte) bool { return Equal(a, a) }))
+	})
+
+	benchBytes(b, sizes, bmEqual(Equal))
+}
+
+func bmEqual(equal func([]byte, []byte) bool) func(b *testing.B, n int) {
+	return func(b *testing.B, n int) {
+		if len(bmbuf) < 2*n {
+			bmbuf = make([]byte, 2*n)
+		}
+		buf1 := bmbuf[0:n]
+		buf2 := bmbuf[n : 2*n]
+		buf1[n-1] = 'x'
+		buf2[n-1] = 'x'
+		for i := 0; i < b.N; i++ {
+			eq := equal(buf1, buf2)
+			if !eq {
+				b.Fatal("bad equal")
+			}
+		}
+		buf1[n-1] = '\x00'
+		buf2[n-1] = '\x00'
+	}
+}
+
+func BenchmarkEqualBothUnaligned(b *testing.B) {
+	sizes := []int{64, 4 << 10}
+	if !isRaceBuilder {
+		sizes = append(sizes, []int{4 << 20, 64 << 20}...)
+	}
+	maxSize := 2 * (sizes[len(sizes)-1] + 8)
+	if len(bmbuf) < maxSize {
+		bmbuf = make([]byte, maxSize)
+	}
+
+	for _, n := range sizes {
+		for _, off := range []int{0, 1, 4, 7} {
+			buf1 := bmbuf[off : off+n]
+			buf2Start := (len(bmbuf) / 2) + off
+			buf2 := bmbuf[buf2Start : buf2Start+n]
+			buf1[n-1] = 'x'
+			buf2[n-1] = 'x'
+			b.Run(fmt.Sprint(n, off), func(b *testing.B) {
+				b.SetBytes(int64(n))
+				for i := 0; i < b.N; i++ {
+					eq := Equal(buf1, buf2)
+					if !eq {
+						b.Fatal("bad equal")
+					}
+				}
+			})
+			buf1[n-1] = '\x00'
+			buf2[n-1] = '\x00'
+		}
+	}
+}
+
+func BenchmarkIndex(b *testing.B) {
+	benchBytes(b, indexSizes, func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		buf[n-1] = 'x'
+		for i := 0; i < b.N; i++ {
+			j := Index(buf, buf[n-7:])
+			if j != n-7 {
+				b.Fatal("bad index", j)
+			}
+		}
+		buf[n-1] = '\x00'
+	})
+}
+
+func BenchmarkIndexEasy(b *testing.B) {
+	benchBytes(b, indexSizes, func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		buf[n-1] = 'x'
+		buf[n-7] = 'x'
+		for i := 0; i < b.N; i++ {
+			j := Index(buf, buf[n-7:])
+			if j != n-7 {
+				b.Fatal("bad index", j)
+			}
+		}
+		buf[n-1] = '\x00'
+		buf[n-7] = '\x00'
+	})
+}
+
+func BenchmarkCount(b *testing.B) {
+	benchBytes(b, indexSizes, func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		buf[n-1] = 'x'
+		for i := 0; i < b.N; i++ {
+			j := Count(buf, buf[n-7:])
+			if j != 1 {
+				b.Fatal("bad count", j)
+			}
+		}
+		buf[n-1] = '\x00'
+	})
+}
+
+func BenchmarkCountEasy(b *testing.B) {
+	benchBytes(b, indexSizes, func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		buf[n-1] = 'x'
+		buf[n-7] = 'x'
+		for i := 0; i < b.N; i++ {
+			j := Count(buf, buf[n-7:])
+			if j != 1 {
+				b.Fatal("bad count", j)
+			}
+		}
+		buf[n-1] = '\x00'
+		buf[n-7] = '\x00'
+	})
+}
+
+func BenchmarkCountSingle(b *testing.B) {
+	benchBytes(b, indexSizes, func(b *testing.B, n int) {
+		buf := bmbuf[0:n]
+		step := 8
+		for i := 0; i < len(buf); i += step {
+			buf[i] = 1
+		}
+		expect := (len(buf) + (step - 1)) / step
+		for i := 0; i < b.N; i++ {
+			j := Count(buf, []byte{1})
+			if j != expect {
+				b.Fatal("bad count", j, expect)
+			}
+		}
+		clear(buf)
+	})
+}
+
+type SplitTest struct {
+	s   string
+	sep string
+	n   int
+	a   []string
+}
+
+var splittests = []SplitTest{
+	{"", "", -1, []string{}},
+	{abcd, "a", 0, nil},
+	{abcd, "", 2, []string{"a", "bcd"}},
+	{abcd, "a", -1, []string{"", "bcd"}},
+	{abcd, "z", -1, []string{"abcd"}},
+	{abcd, "", -1, []string{"a", "b", "c", "d"}},
+	{commas, ",", -1, []string{"1", "2", "3", "4"}},
+	{dots, "...", -1, []string{"1", ".2", ".3", ".4"}},
+	{faces, "☹", -1, []string{"☺☻", ""}},
+	{faces, "~", -1, []string{faces}},
+	{faces, "", -1, []string{"☺", "☻", "☹"}},
+	{"1 2 3 4", " ", 3, []string{"1", "2", "3 4"}},
+	{"1 2", " ", 3, []string{"1", "2"}},
+	{"123", "", 2, []string{"1", "23"}},
+	{"123", "", 17, []string{"1", "2", "3"}},
+	{"bT", "T", math.MaxInt / 4, []string{"b", ""}},
+	{"\xff-\xff", "", -1, []string{"\xff", "-", "\xff"}},
+	{"\xff-\xff", "-", -1, []string{"\xff", "\xff"}},
+}
+
+func TestSplit(t *testing.T) {
+	for _, tt := range splittests {
+		a := SplitN([]byte(tt.s), []byte(tt.sep), tt.n)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
+		result := sliceOfString(a)
+		if !slices.Equal(result, tt.a) {
+			t.Errorf(`Split(%q, %q, %d) = %v; want %v`, tt.s, tt.sep, tt.n, result, tt.a)
+			continue
+		}
+
+		if tt.n < 0 {
+			b := sliceOfString(slices.Collect(SplitSeq([]byte(tt.s), []byte(tt.sep))))
+			if !slices.Equal(b, tt.a) {
+				t.Errorf(`collect(SplitSeq(%q, %q)) = %v; want %v`, tt.s, tt.sep, b, tt.a)
+			}
+		}
+
+		if tt.n == 0 || len(a) == 0 {
+			continue
+		}
+
+		if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+			t.Errorf("last appended result was %s; want %s", x, want)
+		}
+
+		s := Join(a, []byte(tt.sep))
+		if string(s) != tt.s {
+			t.Errorf(`Join(Split(%q, %q, %d), %q) = %q`, tt.s, tt.sep, tt.n, tt.sep, s)
+		}
+		if tt.n < 0 {
+			b := sliceOfString(Split([]byte(tt.s), []byte(tt.sep)))
+			if !slices.Equal(result, b) {
+				t.Errorf("Split disagrees with SplitN(%q, %q, %d) = %v; want %v", tt.s, tt.sep, tt.n, b, a)
+			}
+		}
+		if len(a) > 0 {
+			in, out := a[0], s
+			if cap(in) == cap(out) && &in[:1][0] == &out[:1][0] {
+				t.Errorf("Join(%#v, %q) didn't copy", a, tt.sep)
+			}
+		}
+	}
+}
+
+var splitaftertests = []SplitTest{
+	{abcd, "a", -1, []string{"a", "bcd"}},
+	{abcd, "z", -1, []string{"abcd"}},
+	{abcd, "", -1, []string{"a", "b", "c", "d"}},
+	{commas, ",", -1, []string{"1,", "2,", "3,", "4"}},
+	{dots, "...", -1, []string{"1...", ".2...", ".3...", ".4"}},
+	{faces, "☹", -1, []string{"☺☻☹", ""}},
+	{faces, "~", -1, []string{faces}},
+	{faces, "", -1, []string{"☺", "☻", "☹"}},
+	{"1 2 3 4", " ", 3, []string{"1 ", "2 ", "3 4"}},
+	{"1 2 3", " ", 3, []string{"1 ", "2 ", "3"}},
+	{"1 2", " ", 3, []string{"1 ", "2"}},
+	{"123", "", 2, []string{"1", "23"}},
+	{"123", "", 17, []string{"1", "2", "3"}},
+}
+
+func TestSplitAfter(t *testing.T) {
+	for _, tt := range splitaftertests {
+		a := SplitAfterN([]byte(tt.s), []byte(tt.sep), tt.n)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
+		result := sliceOfString(a)
+		if !slices.Equal(result, tt.a) {
+			t.Errorf(`Split(%q, %q, %d) = %v; want %v`, tt.s, tt.sep, tt.n, result, tt.a)
+			continue
+		}
+
+		if tt.n < 0 {
+			b := sliceOfString(slices.Collect(SplitAfterSeq([]byte(tt.s), []byte(tt.sep))))
+			if !slices.Equal(b, tt.a) {
+				t.Errorf(`collect(SplitAfterSeq(%q, %q)) = %v; want %v`, tt.s, tt.sep, b, tt.a)
+			}
+		}
+
+		if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+			t.Errorf("last appended result was %s; want %s", x, want)
+		}
+
+		s := Join(a, nil)
+		if string(s) != tt.s {
+			t.Errorf(`Join(Split(%q, %q, %d), %q) = %q`, tt.s, tt.sep, tt.n, tt.sep, s)
+		}
+		if tt.n < 0 {
+			b := sliceOfString(SplitAfter([]byte(tt.s), []byte(tt.sep)))
+			if !slices.Equal(result, b) {
+				t.Errorf("SplitAfter disagrees with SplitAfterN(%q, %q, %d) = %v; want %v", tt.s, tt.sep, tt.n, b, a)
+			}
+		}
+	}
+}
+
+type FieldsTest struct {
+	s string
+	a []string
+}
+
+var fieldstests = []FieldsTest{
+	{"", []string{}},
+	{" ", []string{}},
+	{" \t ", []string{}},
+	{"  abc  ", []string{"abc"}},
+	{"1 2 3 4", []string{"1", "2", "3", "4"}},
+	{"1  2  3  4", []string{"1", "2", "3", "4"}},
+	{"1\t\t2\t\t3\t4", []string{"1", "2", "3", "4"}},
+	{"1\u20002\u20013\u20024", []string{"1", "2", "3", "4"}},
+	{"\u2000\u2001\u2002", []string{}},
+	{"\n™\t™\n", []string{"™", "™"}},
+	{faces, []string{faces}},
+}
+
+func TestFields(t *testing.T) {
+	for _, tt := range fieldstests {
+		b := []byte(tt.s)
+		a := Fields(b)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
+		result := sliceOfString(a)
+		if !slices.Equal(result, tt.a) {
+			t.Errorf("Fields(%q) = %v; want %v", tt.s, a, tt.a)
+			continue
+		}
+
+		result2 := sliceOfString(collect(t, FieldsSeq([]byte(tt.s))))
+		if !slices.Equal(result2, tt.a) {
+			t.Errorf(`collect(FieldsSeq(%q)) = %v; want %v`, tt.s, result2, tt.a)
+		}
+
+		if string(b) != tt.s {
+			t.Errorf("slice changed to %s; want %s", string(b), tt.s)
+		}
+		if len(tt.a) > 0 {
+			if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+				t.Errorf("last appended result was %s; want %s", x, want)
+			}
+		}
+	}
+}
+
+func TestFieldsFunc(t *testing.T) {
+	for _, tt := range fieldstests {
+		a := FieldsFunc([]byte(tt.s), unicode.IsSpace)
+		result := sliceOfString(a)
+		if !slices.Equal(result, tt.a) {
+			t.Errorf("FieldsFunc(%q, unicode.IsSpace) = %v; want %v", tt.s, a, tt.a)
+			continue
+		}
+	}
+	pred := func(c rune) bool { return c == 'X' }
+	var fieldsFuncTests = []FieldsTest{
+		{"", []string{}},
+		{"XX", []string{}},
+		{"XXhiXXX", []string{"hi"}},
+		{"aXXbXXXcX", []string{"a", "b", "c"}},
+	}
+	for _, tt := range fieldsFuncTests {
+		b := []byte(tt.s)
+		a := FieldsFunc(b, pred)
+
+		// Appending to the results should not change future results.
+		var x []byte
+		for _, v := range a {
+			x = append(v, 'z')
+		}
+
+		result := sliceOfString(a)
+		if !slices.Equal(result, tt.a) {
+			t.Errorf("FieldsFunc(%q) = %v, want %v", tt.s, a, tt.a)
+		}
+
+		result2 := sliceOfString(collect(t, FieldsFuncSeq([]byte(tt.s), pred)))
+		if !slices.Equal(result2, tt.a) {
+			t.Errorf(`collect(FieldsFuncSeq(%q)) = %v; want %v`, tt.s, result2, tt.a)
+		}
+
+		if string(b) != tt.s {
+			t.Errorf("slice changed to %s; want %s", b, tt.s)
+		}
+		if len(tt.a) > 0 {
+			if want := tt.a[len(tt.a)-1] + "z"; string(x) != want {
+				t.Errorf("last appended result was %s; want %s", x, want)
+			}
+		}
+	}
+}
+
+// Test case for any function which accepts and returns a byte slice.
+// For ease of creation, we write the input byte slice as a string.
+type StringTest struct {
+	in  string
+	out []byte
+}
+
+var upperTests = []StringTest{
+	{"", []byte("")},
+	{"ONLYUPPER", []byte("ONLYUPPER")},
+	{"abc", []byte("ABC")},
+	{"AbC123", []byte("ABC123")},
+	{"azAZ09_", []byte("AZAZ09_")},
+	{"longStrinGwitHmixofsmaLLandcAps", []byte("LONGSTRINGWITHMIXOFSMALLANDCAPS")},
+	{"long\u0250string\u0250with\u0250nonascii\u2C6Fchars", []byte("LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS")},
+	{"\u0250\u0250\u0250\u0250\u0250", []byte("\u2C6F\u2C6F\u2C6F\u2C6F\u2C6F")}, // grows one byte per char
+	{"a\u0080\U0010FFFF", []byte("A\u0080\U0010FFFF")},                           // test utf8.RuneSelf and utf8.MaxRune
+}
+
+var lowerTests = []StringTest{
+	{"", []byte("")},
+	{"abc", []byte("abc")},
+	{"AbC123", []byte("abc123")},
+	{"azAZ09_", []byte("azaz09_")},
+	{"longStrinGwitHmixofsmaLLandcAps", []byte("longstringwithmixofsmallandcaps")},
+	{"LONG\u2C6FSTRING\u2C6FWITH\u2C6FNONASCII\u2C6FCHARS", []byte("long\u0250string\u0250with\u0250nonascii\u0250chars")},
+	{"\u2C6D\u2C6D\u2C6D\u2C6D\u2C6D", []byte("\u0251\u0251\u0251\u0251\u0251")}, // shrinks one byte per char
+	{"A\u0080\U0010FFFF", []byte("a\u0080\U0010FFFF")},                           // test utf8.RuneSelf and utf8.MaxRune
+}
+
+const space = "\t\v\r\f\n\u0085\u00a0\u2000\u3000"
+
+var trimSpaceTests = []StringTest{
+	{"", nil},
+	{"  a", []byte("a")},
+	{"b  ", []byte("b")},
+	{"abc", []byte("abc")},
+	{space + "abc" + space, []byte("abc")},
+	{" ", nil},
+	{"\u3000 ", nil},
+	{" \u3000", nil},
+	{" \t\r\n \t\t\r\r\n\n ", nil},
+	{" \t\r\n x\t\t\r\r\n\n ", []byte("x")},
+	{" \u2000\t\r\n x\t\t\r\r\ny\n \u3000", []byte("x\t\t\r\r\ny")},
+	{"1 \t\r\n2", []byte("1 \t\r\n2")},
+	{" x\x80", []byte("x\x80")},
+	{" x\xc0", []byte("x\xc0")},
+	{"x \xc0\xc0 ", []byte("x \xc0\xc0")},
+	{"x \xc0", []byte("x \xc0")},
+	{"x \xc0 ", []byte("x \xc0")},
+	{"x \xc0\xc0 ", []byte("x \xc0\xc0")},
+	{"x ☺\xc0\xc0 ", []byte("x ☺\xc0\xc0")},
+	{"x ☺ ", []byte("x ☺")},
+}
+
+// Execute f on each test case.  funcName should be the name of f; it's used
+// in failure reports.
+func runStringTests(t *testing.T, f func([]byte) []byte, funcName string, testCases []StringTest) {
+	for _, tc := range testCases {
+		actual := f([]byte(tc.in))
+		if actual == nil && tc.out != nil {
+			t.Errorf("%s(%q) = nil; want %q", funcName, tc.in, tc.out)
+		}
+		if actual != nil && tc.out == nil {
+			t.Errorf("%s(%q) = %q; want nil", funcName, tc.in, actual)
+		}
+		if !Equal(actual, tc.out) {
+			t.Errorf("%s(%q) = %q; want %q", funcName, tc.in, actual, tc.out)
+		}
+	}
+}
+
+func tenRunes(r rune) string {
+	runes := make([]rune, 10)
+	for i := range runes {
+		runes[i] = r
+	}
+	return string(runes)
+}
+
+// User-defined self-inverse mapping function
+func rot13(r rune) rune {
+	const step = 13
+	if r >= 'a' && r <= 'z' {
+		return ((r - 'a' + step) % 26) + 'a'
+	}
+	if r >= 'A' && r <= 'Z' {
+		return ((r - 'A' + step) % 26) + 'A'
+	}
+	return r
+}
+
+func TestMap(t *testing.T) {
+	// Run a couple of awful growth/shrinkage tests
+	a := tenRunes('a')
+
+	// 1. Grow. This triggers two reallocations in Map.
+	maxRune := func(r rune) rune { return unicode.MaxRune }
+	m := Map(maxRune, []byte(a))
+	expect := tenRunes(unicode.MaxRune)
+	if string(m) != expect {
+		t.Errorf("growing: expected %q got %q", expect, m)
+	}
+
+	// 2. Shrink
+	minRune := func(r rune) rune { return 'a' }
+	m = Map(minRune, []byte(tenRunes(unicode.MaxRune)))
+	expect = a
+	if string(m) != expect {
+		t.Errorf("shrinking: expected %q got %q", expect, m)
+	}
+
+	// 3. Rot13
+	m = Map(rot13, []byte("a to zed"))
+	expect = "n gb mrq"
+	if string(m) != expect {
+		t.Errorf("rot13: expected %q got %q", expect, m)
+	}
+
+	// 4. Rot13^2
+	m = Map(rot13, Map(rot13, []byte("a to zed")))
+	expect = "a to zed"
+	if string(m) != expect {
+		t.Errorf("rot13: expected %q got %q", expect, m)
+	}
+
+	// 5. Drop
+	dropNotLatin := func(r rune) rune {
+		if unicode.Is(unicode.Latin, r) {
+			return r
+		}
+		return -1
+	}
+	m = Map(dropNotLatin, []byte("Hello, 세계"))
+	expect = "Hello"
+	if string(m) != expect {
+		t.Errorf("drop: expected %q got %q", expect, m)
+	}
+
+	// 6. Invalid rune
+	invalidRune := func(r rune) rune {
+		return utf8.MaxRune + 1
+	}
+	m = Map(invalidRune, []byte("x"))
+	expect = "\uFFFD"
+	if string(m) != expect {
+		t.Errorf("invalidRune: expected %q got %q", expect, m)
+	}
+}
+
+func TestToUpper(t *testing.T) { runStringTests(t, ToUpper, "ToUpper", upperTests) }
+
+func TestToLower(t *testing.T) { runStringTests(t, ToLower, "ToLower", lowerTests) }
+
+func BenchmarkToUpper(b *testing.B) {
+	for _, tc := range upperTests {
+		tin := []byte(tc.in)
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToUpper(tin)
+				if !Equal(actual, tc.out) {
+					b.Errorf("ToUpper(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
+func BenchmarkToLower(b *testing.B) {
+	for _, tc := range lowerTests {
+		tin := []byte(tc.in)
+		b.Run(tc.in, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				actual := ToLower(tin)
+				if !Equal(actual, tc.out) {
+					b.Errorf("ToLower(%q) = %q; want %q", tc.in, actual, tc.out)
+				}
+			}
+		})
+	}
+}
+
+var toValidUTF8Tests = []struct {
+	in   string
+	repl string
+	out  string
+}{
+	{"", "\uFFFD", ""},
+	{"abc", "\uFFFD", "abc"},
+	{"\uFDDD", "\uFFFD", "\uFDDD"},
+	{"a\xffb", "\uFFFD", "a\uFFFDb"},
+	{"a\xffb\uFFFD", "X", "aXb\uFFFD"},
+	{"a☺\xffb☺\xC0\xAFc☺\xff", "", "a☺b☺c☺"},
+	{"a☺\xffb☺\xC0\xAFc☺\xff", "日本語", "a☺日本語b☺日本語c☺日本語"},
+	{"\xC0\xAF", "\uFFFD", "\uFFFD"},
+	{"\xE0\x80\xAF", "\uFFFD", "\uFFFD"},
+	{"\xed\xa0\x80", "abc", "abc"},
+	{"\xed\xbf\xbf", "\uFFFD", "\uFFFD"},
+	{"\xF0\x80\x80\xaf", "☺", "☺"},
+	{"\xF8\x80\x80\x80\xAF", "\uFFFD", "\uFFFD"},
+	{"\xFC\x80\x80\x80\x80\xAF", "\uFFFD", "\uFFFD"},
+}
+
+func TestToValidUTF8(t *testing.T) {
+	for _, tc := range toValidUTF8Tests {
+		got := ToValidUTF8([]byte(tc.in), []byte(tc.repl))
+		if !Equal(got, []byte(tc.out)) {
+			t.Errorf("ToValidUTF8(%q, %q) = %q; want %q", tc.in, tc.repl, got, tc.out)
+		}
+	}
+}
+
+func TestTrimSpace(t *testing.T) { runStringTests(t, TrimSpace, "TrimSpace", trimSpaceTests) }
+
+type RepeatTest struct {
+	in, out string
+	count   int
+}
+
+var longString = "a" + string(make([]byte, 1<<16)) + "z"
+
+var RepeatTests = []RepeatTest{
+	{"", "", 0},
+	{"", "", 1},
+	{"", "", 2},
+	{"-", "", 0},
+	{"-", "-", 1},
+	{"-", "----------", 10},
+	{"abc ", "abc abc abc ", 3},
+	// Tests for results over the chunkLimit
+	{string(rune(0)), string(make([]byte, 1<<16)), 1 << 16},
+	{longString, longString + longString, 2},
+}
+
+func TestRepeat(t *testing.T) {
+	for _, tt := range RepeatTests {
+		tin := []byte(tt.in)
+		tout := []byte(tt.out)
+		a := Repeat(tin, tt.count)
+		if !Equal(a, tout) {
+			t.Errorf("Repeat(%q, %d) = %q; want %q", tin, tt.count, a, tout)
+			continue
+		}
+	}
+}
+
+func repeat(b []byte, count int) (err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			switch v := r.(type) {
+			case error:
+				err = v
+			default:
+				err = fmt.Errorf("%s", v)
+			}
+		}
+	}()
+
+	Repeat(b, count)
+
+	return
+}
+
+// See Issue golang.org/issue/16237
+func TestRepeatCatchesOverflow(t *testing.T) {
+	type testCase struct {
+		s      string
+		count  int
+		errStr string
+	}
+
+	runTestCases := func(prefix string, tests []testCase) {
+		for i, tt := range tests {
+			err := repeat([]byte(tt.s), tt.count)
+			if tt.errStr == "" {
+				if err != nil {
+					t.Errorf("#%d panicked %v", i, err)
+				}
+				continue
+			}
+
+			if err == nil || !strings.Contains(err.Error(), tt.errStr) {
+				t.Errorf("%s#%d got %q want %q", prefix, i, err, tt.errStr)
+			}
+		}
+	}
+
+	const maxInt = int(^uint(0) >> 1)
+
+	runTestCases("", []testCase{
+		0: {"--", -2147483647, "negative"},
+		1: {"", maxInt, ""},
+		2: {"-", 10, ""},
+		3: {"gopher", 0, ""},
+		4: {"-", -1, "negative"},
+		5: {"--", -102, "negative"},
+		6: {string(make([]byte, 255)), int((^uint(0))/255 + 1), "overflow"},
+	})
+
+	const is64Bit = 1<<(^uintptr(0)>>63)/2 != 0
+	if !is64Bit {
+		return
+	}
+
+	runTestCases("64-bit", []testCase{
+		0: {"-", maxInt, "out of range"},
+	})
+}
+
+type RunesTest struct {
+	in    string
+	out   []rune
+	lossy bool
+}
+
+var RunesTests = []RunesTest{
+	{"", []rune{}, false},
+	{" ", []rune{32}, false},
+	{"ABC", []rune{65, 66, 67}, false},
+	{"abc", []rune{97, 98, 99}, false},
+	{"\u65e5\u672c\u8a9e", []rune{26085, 26412, 35486}, false},
+	{"ab\x80c", []rune{97, 98, 0xFFFD, 99}, true},
+	{"ab\xc0c", []rune{97, 98, 0xFFFD, 99}, true},
+}
+
+func TestRunes(t *testing.T) {
+	for _, tt := range RunesTests {
+		tin := []byte(tt.in)
+		a := Runes(tin)
+		if !slices.Equal(a, tt.out) {
+			t.Errorf("Runes(%q) = %v; want %v", tin, a, tt.out)
+			continue
+		}
+		if !tt.lossy {
+			// can only test reassembly if we didn't lose information
+			s := string(a)
+			if s != tt.in {
+				t.Errorf("string(Runes(%q)) = %x; want %x", tin, s, tin)
+			}
+		}
+	}
+}
+
+type TrimTest struct {
+	f            string
+	in, arg, out string
+}
+
+var trimTests = []TrimTest{
+	{"Trim", "abba", "a", "bb"},
+	{"Trim", "abba", "ab", ""},
+	{"TrimLeft", "abba", "ab", ""},
+	{"TrimRight", "abba", "ab", ""},
+	{"TrimLeft", "abba", "a", "bba"},
+	{"TrimLeft", "abba", "b", "abba"},
+	{"TrimRight", "abba", "a", "abb"},
+	{"TrimRight", "abba", "b", "abba"},
+	{"Trim", "<tag>", "<>", "tag"},
+	{"Trim", "* listitem", " *", "listitem"},
+	{"Trim", `"quote"`, `"`, "quote"},
+	{"Trim", "\u2C6F\u2C6F\u0250\u0250\u2C6F\u2C6F", "\u2C6F", "\u0250\u0250"},
+	{"Trim", "\x80test\xff", "\xff", "test"},
+	{"Trim", " Ġ ", " ", "Ġ"},
+	{"Trim", " Ġİ0", "0 ", "Ġİ"},
+	//empty string tests
+	{"Trim", "abba", "", "abba"},
+	{"Trim", "", "123", ""},
+	{"Trim", "", "", ""},
+	{"TrimLeft", "abba", "", "abba"},
+	{"TrimLeft", "", "123", ""},
+	{"TrimLeft", "", "", ""},
+	{"TrimRight", "abba", "", "abba"},
+	{"TrimRight", "", "123", ""},
+	{"TrimRight", "", "", ""},
+	{"TrimRight", "☺\xc0", "☺", "☺\xc0"},
+	{"TrimPrefix", "aabb", "a", "abb"},
+	{"TrimPrefix", "aabb", "b", "aabb"},
+	{"TrimSuffix", "aabb", "a", "aabb"},
+	{"TrimSuffix", "aabb", "b", "aab"},
+}
+
+type TrimNilTest struct {
+	f   string
+	in  []byte
+	arg string
+	out []byte
+}
+
+var trimNilTests = []TrimNilTest{
+	{"Trim", nil, "", nil},
+	{"Trim", []byte{}, "", nil},
+	{"Trim", []byte{'a'}, "a", nil},
+	{"Trim", []byte{'a', 'a'}, "a", nil},
+	{"Trim", []byte{'a'}, "ab", nil},
+	{"Trim", []byte{'a', 'b'}, "ab", nil},
+	{"Trim", []byte("☺"), "☺", nil},
+	{"TrimLeft", nil, "", nil},
+	{"TrimLeft", []byte{}, "", nil},
+	{"TrimLeft", []byte{'a'}, "a", nil},
+	{"TrimLeft", []byte{'a', 'a'}, "a", nil},
+	{"TrimLeft", []byte{'a'}, "ab", nil},
+	{"TrimLeft", []byte{'a', 'b'}, "ab", nil},
+	{"TrimLeft", []byte("☺"), "☺", nil},
+	{"TrimRight", nil, "", nil},
+	{"TrimRight", []byte{}, "", []byte{}},
+	{"TrimRight", []byte{'a'}, "a", []byte{}},
+	{"TrimRight", []byte{'a', 'a'}, "a", []byte{}},
+	{"TrimRight", []byte{'a'}, "ab", []byte{}},
+	{"TrimRight", []byte{'a', 'b'}, "ab", []byte{}},
+	{"TrimRight", []byte("☺"), "☺", []byte{}},
+	{"TrimPrefix", nil, "", nil},
+	{"TrimPrefix", []byte{}, "", []byte{}},
+	{"TrimPrefix", []byte{'a'}, "a", []byte{}},
+	{"TrimPrefix", []byte("☺"), "☺", []byte{}},
+	{"TrimSuffix", nil, "", nil},
+	{"TrimSuffix", []byte{}, "", []byte{}},
+	{"TrimSuffix", []byte{'a'}, "a", []byte{}},
+	{"TrimSuffix", []byte("☺"), "☺", []byte{}},
+}
+
+func TestTrim(t *testing.T) {
+	toFn := func(name string) (func([]byte, string) []byte, func([]byte, []byte) []byte) {
+		switch name {
+		case "Trim":
+			return Trim, nil
+		case "TrimLeft":
+			return TrimLeft, nil
+		case "TrimRight":
+			return TrimRight, nil
+		case "TrimPrefix":
+			return nil, TrimPrefix
+		case "TrimSuffix":
+			return nil, TrimSuffix
+		default:
+			t.Errorf("Undefined trim function %s", name)
+			return nil, nil
+		}
+	}
+
+	for _, tc := range trimTests {
+		name := tc.f
+		f, fb := toFn(name)
+		if f == nil && fb == nil {
+			continue
+		}
+		var actual string
+		if f != nil {
+			actual = string(f([]byte(tc.in), tc.arg))
+		} else {
+			actual = string(fb([]byte(tc.in), []byte(tc.arg)))
+		}
+		if actual != tc.out {
+			t.Errorf("%s(%q, %q) = %q; want %q", name, tc.in, tc.arg, actual, tc.out)
+		}
+	}
+
+	for _, tc := range trimNilTests {
+		name := tc.f
+		f, fb := toFn(name)
+		if f == nil && fb == nil {
+			continue
+		}
+		var actual []byte
+		if f != nil {
+			actual = f(tc.in, tc.arg)
+		} else {
+			actual = fb(tc.in, []byte(tc.arg))
+		}
+		report := func(s []byte) string {
+			if s == nil {
+				return "nil"
+			} else {
+				return fmt.Sprintf("%q", s)
+			}
+		}
+		if len(actual) != 0 {
+			t.Errorf("%s(%s, %q) returned non-empty value", name, report(tc.in), tc.arg)
+		} else {
+			actualNil := actual == nil
+			outNil := tc.out == nil
+			if actualNil != outNil {
+				t.Errorf("%s(%s, %q) got nil %t; want nil %t", name, report(tc.in), tc.arg, actualNil, outNil)
+			}
+		}
+	}
+}
+
+type predicate struct {
+	f    func(r rune) bool
+	name string
+}
+
+var isSpace = predicate{unicode.IsSpace, "IsSpace"}
+var isDigit = predicate{unicode.IsDigit, "IsDigit"}
+var isUpper = predicate{unicode.IsUpper, "IsUpper"}
+var isValidRune = predicate{
+	func(r rune) bool {
+		return r != utf8.RuneError
+	},
+	"IsValidRune",
+}
+
+type TrimFuncTest struct {
+	f        predicate
+	in       string
+	trimOut  []byte
+	leftOut  []byte
+	rightOut []byte
+}
+
+func not(p predicate) predicate {
+	return predicate{
+		func(r rune) bool {
+			return !p.f(r)
+		},
+		"not " + p.name,
+	}
+}
+
+var trimFuncTests = []TrimFuncTest{
+	{isSpace, space + " hello " + space,
+		[]byte("hello"),
+		[]byte("hello " + space),
+		[]byte(space + " hello")},
+	{isDigit, "\u0e50\u0e5212hello34\u0e50\u0e51",
+		[]byte("hello"),
+		[]byte("hello34\u0e50\u0e51"),
+		[]byte("\u0e50\u0e5212hello")},
+	{isUpper, "\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F",
+		[]byte("hello"),
+		[]byte("helloEF\u2C6F\u2C6FGH\u2C6F\u2C6F"),
+		[]byte("\u2C6F\u2C6F\u2C6F\u2C6FABCDhello")},
+	{not(isSpace), "hello" + space + "hello",
+		[]byte(space),
+		[]byte(space + "hello"),
+		[]byte("hello" + space)},
+	{not(isDigit), "hello\u0e50\u0e521234\u0e50\u0e51helo",
+		[]byte("\u0e50\u0e521234\u0e50\u0e51"),
+		[]byte("\u0e50\u0e521234\u0e50\u0e51helo"),
+		[]byte("hello\u0e50\u0e521234\u0e50\u0e51")},
+	{isValidRune, "ab\xc0a\xc0cd",
+		[]byte("\xc0a\xc0"),
+		[]byte("\xc0a\xc0cd"),
+		[]byte("ab\xc0a\xc0")},
+	{not(isValidRune), "\xc0a\xc0",
+		[]byte("a"),
+		[]byte("a\xc0"),
+		[]byte("\xc0a")},
+	// The nils returned by TrimLeftFunc are odd behavior, but we need
+	// to preserve backwards compatibility.
+	{isSpace, "",
+		nil,
+		nil,
+		[]byte("")},
+	{isSpace, " ",
+		nil,
+		nil,
+		[]byte("")},
+}
+
+func TestTrimFunc(t *testing.T) {
+	for _, tc := range trimFuncTests {
+		trimmers := []struct {
+			name string
+			trim func(s []byte, f func(r rune) bool) []byte
+			out  []byte
+		}{
+			{"TrimFunc", TrimFunc, tc.trimOut},
+			{"TrimLeftFunc", TrimLeftFunc, tc.leftOut},
+			{"TrimRightFunc", TrimRightFunc, tc.rightOut},
+		}
+		for _, trimmer := range trimmers {
+			actual := trimmer.trim([]byte(tc.in), tc.f.f)
+			if actual == nil && trimmer.out != nil {
+				t.Errorf("%s(%q, %q) = nil; want %q", trimmer.name, tc.in, tc.f.name, trimmer.out)
+			}
+			if actual != nil && trimmer.out == nil {
+				t.Errorf("%s(%q, %q) = %q; want nil", trimmer.name, tc.in, tc.f.name, actual)
+			}
+			if !Equal(actual, trimmer.out) {
+				t.Errorf("%s(%q, %q) = %q; want %q", trimmer.name, tc.in, tc.f.name, actual, trimmer.out)
+			}
+		}
+	}
+}
+
+type IndexFuncTest struct {
+	in          string
+	f           predicate
+	first, last int
+}
+
+var indexFuncTests = []IndexFuncTest{
+	{"", isValidRune, -1, -1},
+	{"abc", isDigit, -1, -1},
+	{"0123", isDigit, 0, 3},
+	{"a1b", isDigit, 1, 1},
+	{space, isSpace, 0, len(space) - 3}, // last rune in space is 3 bytes
+	{"\u0e50\u0e5212hello34\u0e50\u0e51", isDigit, 0, 18},
+	{"\u2C6F\u2C6F\u2C6F\u2C6FABCDhelloEF\u2C6F\u2C6FGH\u2C6F\u2C6F", isUpper, 0, 34},
+	{"12\u0e50\u0e52hello34\u0e50\u0e51", not(isDigit), 8, 12},
+
+	// tests of invalid UTF-8
+	{"\x801", isDigit, 1, 1},
+	{"\x80abc", isDigit, -1, -1},
+	{"\xc0a\xc0", isValidRune, 1, 1},
+	{"\xc0a\xc0", not(isValidRune), 0, 2},
+	{"\xc0☺\xc0", not(isValidRune), 0, 4},
+	{"\xc0☺\xc0\xc0", not(isValidRune), 0, 5},
+	{"ab\xc0a\xc0cd", not(isValidRune), 2, 4},
+	{"a\xe0\x80cd", not(isValidRune), 1, 2},
+}
+
+func TestIndexFunc(t *testing.T) {
+	for _, tc := range indexFuncTests {
+		first := IndexFunc([]byte(tc.in), tc.f.f)
+		if first != tc.first {
+			t.Errorf("IndexFunc(%q, %s) = %d; want %d", tc.in, tc.f.name, first, tc.first)
+		}
+		last := LastIndexFunc([]byte(tc.in), tc.f.f)
+		if last != tc.last {
+			t.Errorf("LastIndexFunc(%q, %s) = %d; want %d", tc.in, tc.f.name, last, tc.last)
+		}
+	}
+}
+
+type ReplaceTest struct {
+	in       string
+	old, new string
+	n        int
+	out      string
+}
+
+var ReplaceTests = []ReplaceTest{
+	{"hello", "l", "L", 0, "hello"},
+	{"hello", "l", "L", -1, "heLLo"},
+	{"hello", "x", "X", -1, "hello"},
+	{"", "x", "X", -1, ""},
+	{"radar", "r", "<r>", -1, "<r>ada<r>"},
+	{"", "", "<>", -1, "<>"},
+	{"banana", "a", "<>", -1, "b<>n<>n<>"},
+	{"banana", "a", "<>", 1, "b<>nana"},
+	{"banana", "a", "<>", 1000, "b<>n<>n<>"},
+	{"banana", "an", "<>", -1, "b<><>a"},
+	{"banana", "ana", "<>", -1, "b<>na"},
+	{"banana", "", "<>", -1, "<>b<>a<>n<>a<>n<>a<>"},
+	{"banana", "", "<>", 10, "<>b<>a<>n<>a<>n<>a<>"},
+	{"banana", "", "<>", 6, "<>b<>a<>n<>a<>n<>a"},
+	{"banana", "", "<>", 5, "<>b<>a<>n<>a<>na"},
+	{"banana", "", "<>", 1, "<>banana"},
+	{"banana", "a", "a", -1, "banana"},
+	{"banana", "a", "a", 1, "banana"},
+	{"☺☻☹", "", "<>", -1, "<>☺<>☻<>☹<>"},
+}
+
+func TestReplace(t *testing.T) {
+	for _, tt := range ReplaceTests {
+		var (
+			in  = []byte(tt.in)
+			old = []byte(tt.old)
+			new = []byte(tt.new)
+		)
+		if !asan.Enabled {
+			allocs := testing.AllocsPerRun(10, func() { Replace(in, old, new, tt.n) })
+			if allocs > 1 {
+				t.Errorf("Replace(%q, %q, %q, %d) allocates %.2f objects", tt.in, tt.old, tt.new, tt.n, allocs)
+			}
+		}
+		in = append(in, "<spare>"...)
+		in = in[:len(tt.in)]
+		out := Replace(in, old, new, tt.n)
+		if s := string(out); s != tt.out {
+			t.Errorf("Replace(%q, %q, %q, %d) = %q, want %q", tt.in, tt.old, tt.new, tt.n, s, tt.out)
+		}
+		if cap(in) == cap(out) && &in[:1][0] == &out[:1][0] {
+			t.Errorf("Replace(%q, %q, %q, %d) didn't copy", tt.in, tt.old, tt.new, tt.n)
+		}
+		if tt.n == -1 {
+			out := ReplaceAll(in, old, new)
+			if s := string(out); s != tt.out {
+				t.Errorf("ReplaceAll(%q, %q, %q) = %q, want %q", tt.in, tt.old, tt.new, s, tt.out)
+			}
+		}
+	}
+}
+
+func FuzzReplace(f *testing.F) {
+	for _, tt := range ReplaceTests {
+		f.Add([]byte(tt.in), []byte(tt.old), []byte(tt.new), tt.n)
+	}
+	f.Fuzz(func(t *testing.T, in, old, new []byte, n int) {
+		differentImpl := func(in, old, new []byte, n int) []byte {
+			var out Buffer
+			if n < 0 {
+				n = math.MaxInt
+			}
+			for i := 0; i < len(in); {
+				if n == 0 {
+					out.Write(in[i:])
+					break
+				}
+				if HasPrefix(in[i:], old) {
+					out.Write(new)
+					i += len(old)
+					n--
+					if len(old) != 0 {
+						continue
+					}
+					if i == len(in) {
+						break
+					}
+				}
+				if len(old) == 0 {
+					_, length := utf8.DecodeRune(in[i:])
+					out.Write(in[i : i+length])
+					i += length
+				} else {
+					out.WriteByte(in[i])
+					i++
+				}
+			}
+			if len(old) == 0 && n != 0 {
+				out.Write(new)
+			}
+			return out.Bytes()
+		}
+		if simple, replace := differentImpl(in, old, new, n), Replace(in, old, new, n); !slices.Equal(simple, replace) {
+			t.Errorf("The two implementations do not match %q != %q for Replace(%q, %q, %q, %d)", simple, replace, in, old, new, n)
+		}
+	})
+}
+
+func BenchmarkReplace(b *testing.B) {
+	for _, tt := range ReplaceTests {
+		desc := fmt.Sprintf("%q %q %q %d", tt.in, tt.old, tt.new, tt.n)
+		var (
+			in  = []byte(tt.in)
+			old = []byte(tt.old)
+			new = []byte(tt.new)
+		)
+		b.Run(desc, func(b *testing.B) {
+			b.ReportAllocs()
+			for b.Loop() {
+				Replace(in, old, new, tt.n)
+			}
+		})
+	}
+}
+
+type TitleTest struct {
+	in, out string
+}
+
+var TitleTests = []TitleTest{
+	{"", ""},
+	{"a", "A"},
+	{" aaa aaa aaa ", " Aaa Aaa Aaa "},
+	{" Aaa Aaa Aaa ", " Aaa Aaa Aaa "},
+	{"123a456", "123a456"},
+	{"double-blind", "Double-Blind"},
+	{"ÿøû", "Ÿøû"},
+	{"with_underscore", "With_underscore"},
+	{"unicode \xe2\x80\xa8 line separator", "Unicode \xe2\x80\xa8 Line Separator"},
+}
+
+func TestTitle(t *testing.T) {
+	for _, tt := range TitleTests {
+		if s := string(Title([]byte(tt.in))); s != tt.out {
+			t.Errorf("Title(%q) = %q, want %q", tt.in, s, tt.out)
+		}
+	}
+}
+
+var ToTitleTests = []TitleTest{
+	{"", ""},
+	{"a", "A"},
+	{" aaa aaa aaa ", " AAA AAA AAA "},
+	{" Aaa Aaa Aaa ", " AAA AAA AAA "},
+	{"123a456", "123A456"},
+	{"double-blind", "DOUBLE-BLIND"},
+	{"ÿøû", "ŸØÛ"},
+}
+
+func TestToTitle(t *testing.T) {
+	for _, tt := range ToTitleTests {
+		if s := string(ToTitle([]byte(tt.in))); s != tt.out {
+			t.Errorf("ToTitle(%q) = %q, want %q", tt.in, s, tt.out)
+		}
+	}
+}
+
+var EqualFoldTests = []struct {
+	s, t string
+	out  bool
+}{
+	{"abc", "abc", true},
+	{"ABcd", "ABcd", true},
+	{"123abc", "123ABC", true},
+	{"αβδ", "ΑΒΔ", true},
+	{"abc", "xyz", false},
+	{"abc", "XYZ", false},
+	{"abcdefghijk", "abcdefghijX", false},
+	{"abcdefghijk", "abcdefghij\u212A", true},
+	{"abcdefghijK", "abcdefghij\u212A", true},
+	{"abcdefghijkz", "abcdefghij\u212Ay", false},
+	{"abcdefghijKz", "abcdefghij\u212Ay", false},
+}
+
+func TestEqualFold(t *testing.T) {
+	for _, tt := range EqualFoldTests {
+		if out := EqualFold([]byte(tt.s), []byte(tt.t)); out != tt.out {
+			t.Errorf("EqualFold(%#q, %#q) = %v, want %v", tt.s, tt.t, out, tt.out)
+		}
+		if out := EqualFold([]byte(tt.t), []byte(tt.s)); out != tt.out {
+			t.Errorf("EqualFold(%#q, %#q) = %v, want %v", tt.t, tt.s, out, tt.out)
+		}
+	}
+}
+
+var cutTests = []struct {
+	s, sep        string
+	before, after string
+	found         bool
+}{
+	{"abc", "b", "a", "c", true},
+	{"abc", "a", "", "bc", true},
+	{"abc", "c", "ab", "", true},
+	{"abc", "abc", "", "", true},
+	{"abc", "", "", "abc", true},
+	{"abc", "d", "abc", "", false},
+	{"", "d", "", "", false},
+	{"", "", "", "", true},
+}
+
+func TestCut(t *testing.T) {
+	for _, tt := range cutTests {
+		if before, after, found := Cut([]byte(tt.s), []byte(tt.sep)); string(before) != tt.before || string(after) != tt.after || found != tt.found {
+			t.Errorf("Cut(%q, %q) = %q, %q, %v, want %q, %q, %v", tt.s, tt.sep, before, after, found, tt.before, tt.after, tt.found)
+		}
+	}
+}
+
+var cutPrefixTests = []struct {
+	s, sep string
+	after  string
+	found  bool
+}{
+	{"abc", "a", "bc", true},
+	{"abc", "abc", "", true},
+	{"abc", "", "abc", true},
+	{"abc", "d", "abc", false},
+	{"", "d", "", false},
+	{"", "", "", true},
+}
+
+func TestCutPrefix(t *testing.T) {
+	for _, tt := range cutPrefixTests {
+		if after, found := CutPrefix([]byte(tt.s), []byte(tt.sep)); string(after) != tt.after || found != tt.found {
+			t.Errorf("CutPrefix(%q, %q) = %q, %v, want %q, %v", tt.s, tt.sep, after, found, tt.after, tt.found)
+		}
+	}
+}
+
+var cutSuffixTests = []struct {
+	s, sep string
+	before string
+	found  bool
+}{
+	{"abc", "bc", "a", true},
+	{"abc", "abc", "", true},
+	{"abc", "", "abc", true},
+	{"abc", "d", "abc", false},
+	{"", "d", "", false},
+	{"", "", "", true},
+}
+
+func TestCutSuffix(t *testing.T) {
+	for _, tt := range cutSuffixTests {
+		if before, found := CutSuffix([]byte(tt.s), []byte(tt.sep)); string(before) != tt.before || found != tt.found {
+			t.Errorf("CutSuffix(%q, %q) = %q, %v, want %q, %v", tt.s, tt.sep, before, found, tt.before, tt.found)
+		}
+	}
+}
+
+func TestBufferGrowNegative(t *testing.T) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Fatal("Grow(-1) should have panicked")
+		}
+	}()
+	var b Buffer
+	b.Grow(-1)
+}
+
+func TestBufferTruncateNegative(t *testing.T) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Fatal("Truncate(-1) should have panicked")
+		}
+	}()
+	var b Buffer
+	b.Truncate(-1)
+}
+
+func TestBufferTruncateOutOfRange(t *testing.T) {
+	defer func() {
+		if err := recover(); err == nil {
+			t.Fatal("Truncate(20) should have panicked")
+		}
+	}()
+	var b Buffer
+	b.Write(make([]byte, 10))
+	b.Truncate(20)
+}
+
+var containsTests = []struct {
+	b, subslice []byte
+	want        bool
+}{
+	{[]byte("hello"), []byte("hel"), true},
+	{[]byte("日本語"), []byte("日本"), true},
+	{[]byte("hello"), []byte("Hello, world"), false},
+	{[]byte("東京"), []byte("京東"), false},
+}
+
+func TestContains(t *testing.T) {
+	for _, tt := range containsTests {
+		if got := Contains(tt.b, tt.subslice); got != tt.want {
+			t.Errorf("Contains(%q, %q) = %v, want %v", tt.b, tt.subslice, got, tt.want)
+		}
+	}
+}
+
+var ContainsAnyTests = []struct {
+	b        []byte
+	substr   string
+	expected bool
+}{
+	{[]byte(""), "", false},
+	{[]byte(""), "a", false},
+	{[]byte(""), "abc", false},
+	{[]byte("a"), "", false},
+	{[]byte("a"), "a", true},
+	{[]byte("aaa"), "a", true},
+	{[]byte("abc"), "xyz", false},
+	{[]byte("abc"), "xcz", true},
+	{[]byte("a☺b☻c☹d"), "uvw☻xyz", true},
+	{[]byte("aRegExp*"), ".(|)*+?^$[]", true},
+	{[]byte(dots + dots + dots), " ", false},
+}
+
+func TestContainsAny(t *testing.T) {
+	for _, ct := range ContainsAnyTests {
+		if ContainsAny(ct.b, ct.substr) != ct.expected {
+			t.Errorf("ContainsAny(%s, %s) = %v, want %v",
+				ct.b, ct.substr, !ct.expected, ct.expected)
+		}
+	}
+}
+
+var ContainsRuneTests = []struct {
+	b        []byte
+	r        rune
+	expected bool
+}{
+	{[]byte(""), 'a', false},
+	{[]byte("a"), 'a', true},
+	{[]byte("aaa"), 'a', true},
+	{[]byte("abc"), 'y', false},
+	{[]byte("abc"), 'c', true},
+	{[]byte("a☺b☻c☹d"), 'x', false},
+	{[]byte("a☺b☻c☹d"), '☻', true},
+	{[]byte("aRegExp*"), '*', true},
+}
+
+func TestContainsRune(t *testing.T) {
+	for _, ct := range ContainsRuneTests {
+		if ContainsRune(ct.b, ct.r) != ct.expected {
+			t.Errorf("ContainsRune(%q, %q) = %v, want %v",
+				ct.b, ct.r, !ct.expected, ct.expected)
+		}
+	}
+}
+
+func TestContainsFunc(t *testing.T) {
+	for _, ct := range ContainsRuneTests {
+		if ContainsFunc(ct.b, func(r rune) bool {
+			return ct.r == r
+		}) != ct.expected {
+			t.Errorf("ContainsFunc(%q, func(%q)) = %v, want %v",
+				ct.b, ct.r, !ct.expected, ct.expected)
+		}
+	}
+}
+
+var makeFieldsInput = func() []byte {
+	x := make([]byte, 1<<20)
+	// Input is ~10% space, ~10% 2-byte UTF-8, rest ASCII non-space.
+	r := rand.New(rand.NewSource(99))
+	for i := range x {
+		switch r.Intn(10) {
+		case 0:
+			x[i] = ' '
+		case 1:
+			if i > 0 && x[i-1] == 'x' {
+				copy(x[i-1:], "χ")
+				break
+			}
+			fallthrough
+		default:
+			x[i] = 'x'
+		}
+	}
+	return x
+}
+
+var makeFieldsInputASCII = func() []byte {
+	x := make([]byte, 1<<20)
+	// Input is ~10% space, rest ASCII non-space.
+	r := rand.New(rand.NewSource(99))
+	for i := range x {
+		if r.Intn(10) == 0 {
+			x[i] = ' '
+		} else {
+			x[i] = 'x'
+		}
+	}
+	return x
+}
+
+var bytesdata = []struct {
+	name string
+	data []byte
+}{
+	{"ASCII", makeFieldsInputASCII()},
+	{"Mixed", makeFieldsInput()},
+}
+
+func BenchmarkFields(b *testing.B) {
+	for _, sd := range bytesdata {
+		b.Run(sd.name, func(b *testing.B) {
+			for j := 1 << 4; j <= 1<<20; j <<= 4 {
+				b.Run(fmt.Sprintf("%d", j), func(b *testing.B) {
+					b.ReportAllocs()
+					b.SetBytes(int64(j))
+					data := sd.data[:j]
+					for i := 0; i < b.N; i++ {
+						Fields(data)
+					}
+				})
+			}
+		})
+	}
+}
+
+func BenchmarkFieldsFunc(b *testing.B) {
+	for _, sd := range bytesdata {
+		b.Run(sd.name, func(b *testing.B) {
+			for j := 1 << 4; j <= 1<<20; j <<= 4 {
+				b.Run(fmt.Sprintf("%d", j), func(b *testing.B) {
+					b.ReportAllocs()
+					b.SetBytes(int64(j))
+					data := sd.data[:j]
+					for i := 0; i < b.N; i++ {
+						FieldsFunc(data, unicode.IsSpace)
+					}
+				})
+			}
+		})
+	}
+}
+
+func BenchmarkTrimSpace(b *testing.B) {
+	tests := []struct {
+		name  string
+		input []byte
+	}{
+		{"NoTrim", []byte("typical")},
+		{"ASCII", []byte("  foo bar  ")},
+		{"SomeNonASCII", []byte("    \u2000\t\r\n x\t\t\r\r\ny\n \u3000    ")},
+		{"JustNonASCII", []byte("\u2000\u2000\u2000☺☺☺☺\u3000\u3000\u3000")},
+	}
+	for _, test := range tests {
+		b.Run(test.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				TrimSpace(test.input)
+			}
+		})
+	}
+}
+
+func BenchmarkToValidUTF8(b *testing.B) {
+	tests := []struct {
+		name  string
+		input []byte
+	}{
+		{"Valid", []byte("typical")},
+		{"InvalidASCII", []byte("foo\xffbar")},
+		{"InvalidNonASCII", []byte("日本語\xff日本語")},
+	}
+	replacement := []byte("\uFFFD")
+	b.ResetTimer()
+	for _, test := range tests {
+		b.Run(test.name, func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				ToValidUTF8(test.input, replacement)
+			}
+		})
+	}
+}
+
+func makeBenchInputHard() []byte {
+	tokens := [...]string{
+		"<a>", "<p>", "<b>", "<strong>",
+		"</a>", "</p>", "</b>", "</strong>",
+		"hello", "world",
+	}
+	x := make([]byte, 0, 1<<20)
+	r := rand.New(rand.NewSource(99))
+	for {
+		i := r.Intn(len(tokens))
+		if len(x)+len(tokens[i]) >= 1<<20 {
+			break
+		}
+		x = append(x, tokens[i]...)
+	}
+	return x
+}
+
+var benchInputHard = makeBenchInputHard()
+
+func benchmarkIndexHard(b *testing.B, sep []byte) {
+	n := Index(benchInputHard, sep)
+	if n < 0 {
+		n = len(benchInputHard)
+	}
+	b.SetBytes(int64(n))
+	for i := 0; i < b.N; i++ {
+		Index(benchInputHard, sep)
+	}
+}
+
+func benchmarkLastIndexHard(b *testing.B, sep []byte) {
+	for i := 0; i < b.N; i++ {
+		LastIndex(benchInputHard, sep)
+	}
+}
+
+func benchmarkCountHard(b *testing.B, sep []byte) {
+	for i := 0; i < b.N; i++ {
+		Count(benchInputHard, sep)
+	}
+}
+
+func BenchmarkIndexHard1(b *testing.B) { benchmarkIndexHard(b, []byte("<>")) }
+func BenchmarkIndexHard2(b *testing.B) { benchmarkIndexHard(b, []byte("</pre>")) }
+func BenchmarkIndexHard3(b *testing.B) { benchmarkIndexHard(b, []byte("<b>hello world</b>")) }
+func BenchmarkIndexHard4(b *testing.B) {
+	benchmarkIndexHard(b, []byte("<pre><b>hello</b><strong>world</strong></pre>"))
+}
+
+func BenchmarkLastIndexHard1(b *testing.B) { benchmarkLastIndexHard(b, []byte("<>")) }
+func BenchmarkLastIndexHard2(b *testing.B) { benchmarkLastIndexHard(b, []byte("</pre>")) }
+func BenchmarkLastIndexHard3(b *testing.B) { benchmarkLastIndexHard(b, []byte("<b>hello world</b>")) }
+
+func BenchmarkCountHard1(b *testing.B) { benchmarkCountHard(b, []byte("<>")) }
+func BenchmarkCountHard2(b *testing.B) { benchmarkCountHard(b, []byte("</pre>")) }
+func BenchmarkCountHard3(b *testing.B) { benchmarkCountHard(b, []byte("<b>hello world</b>")) }
+
+func BenchmarkSplitEmptySeparator(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, nil)
+	}
+}
+
+func BenchmarkSplitSingleByteSeparator(b *testing.B) {
+	sep := []byte("/")
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, sep)
+	}
+}
+
+func BenchmarkSplitMultiByteSeparator(b *testing.B) {
+	sep := []byte("hello")
+	for i := 0; i < b.N; i++ {
+		Split(benchInputHard, sep)
+	}
+}
+
+func BenchmarkSplitNSingleByteSeparator(b *testing.B) {
+	sep := []byte("/")
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, sep, 10)
+	}
+}
+
+func BenchmarkSplitNMultiByteSeparator(b *testing.B) {
+	sep := []byte("hello")
+	for i := 0; i < b.N; i++ {
+		SplitN(benchInputHard, sep, 10)
+	}
+}
+
+func BenchmarkRepeat(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		Repeat([]byte("-"), 80)
+	}
+}
+
+func BenchmarkRepeatLarge(b *testing.B) {
+	s := Repeat([]byte("@"), 8*1024)
+	for j := 8; j <= 30; j++ {
+		for _, k := range []int{1, 16, 4097} {
+			s := s[:k]
+			n := (1 << j) / k
+			if n == 0 {
+				continue
+			}
+			b.Run(fmt.Sprintf("%d/%d", 1<<j, k), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					Repeat(s, n)
+				}
+				b.SetBytes(int64(n * len(s)))
+			})
+		}
+	}
+}
+
+func BenchmarkBytesCompare(b *testing.B) {
+	for n := 1; n <= 2048; n <<= 1 {
+		b.Run(fmt.Sprint(n), func(b *testing.B) {
+			var x = make([]byte, n)
+			var y = make([]byte, n)
+
+			for i := 0; i < n; i++ {
+				x[i] = 'a'
+			}
+
+			for i := 0; i < n; i++ {
+				y[i] = 'a'
+			}
+
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				Compare(x, y)
+			}
+		})
+	}
+}
+
+func BenchmarkIndexAnyASCII(b *testing.B) {
+	x := Repeat([]byte{'#'}, 2048) // Never matches set
+	cs := "0123456789abcdefghijklmnopqrstuvwxyz0123456789abcdefghijklmnopqrstuvwxyz"
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					IndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkIndexAnyUTF8(b *testing.B) {
+	x := Repeat([]byte{'#'}, 2048) // Never matches set
+	cs := "你好世界, hello world. 你好世界, hello world. 你好世界, hello world."
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					IndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkLastIndexAnyASCII(b *testing.B) {
+	x := Repeat([]byte{'#'}, 2048) // Never matches set
+	cs := "0123456789abcdefghijklmnopqrstuvwxyz0123456789abcdefghijklmnopqrstuvwxyz"
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					LastIndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkLastIndexAnyUTF8(b *testing.B) {
+	x := Repeat([]byte{'#'}, 2048) // Never matches set
+	cs := "你好世界, hello world. 你好世界, hello world. 你好世界, hello world."
+	for k := 1; k <= 2048; k <<= 4 {
+		for j := 1; j <= 64; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					LastIndexAny(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkTrimASCII(b *testing.B) {
+	cs := "0123456789abcdef"
+	for k := 1; k <= 4096; k <<= 4 {
+		for j := 1; j <= 16; j <<= 1 {
+			b.Run(fmt.Sprintf("%d:%d", k, j), func(b *testing.B) {
+				x := Repeat([]byte(cs[:j]), k) // Always matches set
+				for i := 0; i < b.N; i++ {
+					Trim(x[:k], cs[:j])
+				}
+			})
+		}
+	}
+}
+
+func BenchmarkTrimByte(b *testing.B) {
+	x := []byte("  the quick brown fox   ")
+	for i := 0; i < b.N; i++ {
+		Trim(x, " ")
+	}
+}
+
+func BenchmarkIndexPeriodic(b *testing.B) {
+	key := []byte{1, 1}
+	for _, skip := range [...]int{2, 4, 8, 16, 32, 64} {
+		b.Run(fmt.Sprintf("IndexPeriodic%d", skip), func(b *testing.B) {
+			buf := make([]byte, 1<<16)
+			for i := 0; i < len(buf); i += skip {
+				buf[i] = 1
+			}
+			for i := 0; i < b.N; i++ {
+				Index(buf, key)
+			}
+		})
+	}
+}
+
+func TestClone(t *testing.T) {
+	var cloneTests = [][]byte{
+		[]byte(nil),
+		[]byte{},
+		Clone([]byte{}),
+		[]byte(strings.Repeat("a", 42))[:0],
+		[]byte(strings.Repeat("a", 42))[:0:0],
+		[]byte("short"),
+		[]byte(strings.Repeat("a", 42)),
+	}
+	for _, input := range cloneTests {
+		clone := Clone(input)
+		if !Equal(clone, input) {
+			t.Errorf("Clone(%q) = %q; want %q", input, clone, input)
+		}
+
+		if input == nil && clone != nil {
+			t.Errorf("Clone(%#v) return value should be equal to nil slice.", input)
+		}
+
+		if input != nil && clone == nil {
+			t.Errorf("Clone(%#v) return value should not be equal to nil slice.", input)
+		}
+
+		if cap(input) != 0 && unsafe.SliceData(input) == unsafe.SliceData(clone) {
+			t.Errorf("Clone(%q) return value should not reference inputs backing memory.", input)
+		}
+	}
+}

go/src/bytes/compare_test.go

@@ -0,0 +1,296 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes_test
+
+import (
+	. "bytes"
+	"fmt"
+	"testing"
+)
+
+var compareTests = []struct {
+	a, b []byte
+	i    int
+}{
+	{[]byte(""), []byte(""), 0},
+	{[]byte("a"), []byte(""), 1},
+	{[]byte(""), []byte("a"), -1},
+	{[]byte("abc"), []byte("abc"), 0},
+	{[]byte("abd"), []byte("abc"), 1},
+	{[]byte("abc"), []byte("abd"), -1},
+	{[]byte("ab"), []byte("abc"), -1},
+	{[]byte("abc"), []byte("ab"), 1},
+	{[]byte("x"), []byte("ab"), 1},
+	{[]byte("ab"), []byte("x"), -1},
+	{[]byte("x"), []byte("a"), 1},
+	{[]byte("b"), []byte("x"), -1},
+	// test runtime·memeq's chunked implementation
+	{[]byte("abcdefgh"), []byte("abcdefgh"), 0},
+	{[]byte("abcdefghi"), []byte("abcdefghi"), 0},
+	{[]byte("abcdefghi"), []byte("abcdefghj"), -1},
+	{[]byte("abcdefghj"), []byte("abcdefghi"), 1},
+	// nil tests
+	{nil, nil, 0},
+	{[]byte(""), nil, 0},
+	{nil, []byte(""), 0},
+	{[]byte("a"), nil, 1},
+	{nil, []byte("a"), -1},
+}
+
+func TestCompare(t *testing.T) {
+	for _, tt := range compareTests {
+		numShifts := 16
+		buffer := make([]byte, len(tt.b)+numShifts)
+		// vary the input alignment of tt.b
+		for offset := 0; offset <= numShifts; offset++ {
+			shiftedB := buffer[offset : len(tt.b)+offset]
+			copy(shiftedB, tt.b)
+			cmp := Compare(tt.a, shiftedB)
+			if cmp != tt.i {
+				t.Errorf(`Compare(%q, %q), offset %d = %v; want %v`, tt.a, tt.b, offset, cmp, tt.i)
+			}
+		}
+	}
+}
+
+func TestCompareIdenticalSlice(t *testing.T) {
+	var b = []byte("Hello Gophers!")
+	if Compare(b, b) != 0 {
+		t.Error("b != b")
+	}
+	if Compare(b, b[:1]) != 1 {
+		t.Error("b > b[:1] failed")
+	}
+}
+
+func TestCompareBytes(t *testing.T) {
+	lengths := make([]int, 0) // lengths to test in ascending order
+	for i := 0; i <= 128; i++ {
+		lengths = append(lengths, i)
+	}
+	lengths = append(lengths, 256, 512, 1024, 1333, 4095, 4096, 4097)
+
+	if !testing.Short() {
+		lengths = append(lengths, 65535, 65536, 65537, 99999)
+	}
+
+	n := lengths[len(lengths)-1]
+	a := make([]byte, n+1)
+	b := make([]byte, n+1)
+	for _, len := range lengths {
+		// randomish but deterministic data. No 0 or 255.
+		for i := 0; i < len; i++ {
+			a[i] = byte(1 + 31*i%254)
+			b[i] = byte(1 + 31*i%254)
+		}
+		// data past the end is different
+		for i := len; i <= n; i++ {
+			a[i] = 8
+			b[i] = 9
+		}
+		cmp := Compare(a[:len], b[:len])
+		if cmp != 0 {
+			t.Errorf(`CompareIdentical(%d) = %d`, len, cmp)
+		}
+		if len > 0 {
+			cmp = Compare(a[:len-1], b[:len])
+			if cmp != -1 {
+				t.Errorf(`CompareAshorter(%d) = %d`, len, cmp)
+			}
+			cmp = Compare(a[:len], b[:len-1])
+			if cmp != 1 {
+				t.Errorf(`CompareBshorter(%d) = %d`, len, cmp)
+			}
+		}
+		for k := 0; k < len; k++ {
+			b[k] = a[k] - 1
+			cmp = Compare(a[:len], b[:len])
+			if cmp != 1 {
+				t.Errorf(`CompareAbigger(%d,%d) = %d`, len, k, cmp)
+			}
+			b[k] = a[k] + 1
+			cmp = Compare(a[:len], b[:len])
+			if cmp != -1 {
+				t.Errorf(`CompareBbigger(%d,%d) = %d`, len, k, cmp)
+			}
+			b[k] = a[k]
+		}
+	}
+}
+
+func TestEndianBaseCompare(t *testing.T) {
+	// This test compares byte slices that are almost identical, except one
+	// difference that for some j, a[j]>b[j] and a[j+1]<b[j+1]. If the implementation
+	// compares large chunks with wrong endianness, it gets wrong result.
+	// no vector register is larger than 512 bytes for now
+	const maxLength = 512
+	a := make([]byte, maxLength)
+	b := make([]byte, maxLength)
+	// randomish but deterministic data. No 0 or 255.
+	for i := 0; i < maxLength; i++ {
+		a[i] = byte(1 + 31*i%254)
+		b[i] = byte(1 + 31*i%254)
+	}
+	for i := 2; i <= maxLength; i <<= 1 {
+		for j := 0; j < i-1; j++ {
+			a[j] = b[j] - 1
+			a[j+1] = b[j+1] + 1
+			cmp := Compare(a[:i], b[:i])
+			if cmp != -1 {
+				t.Errorf(`CompareBbigger(%d,%d) = %d`, i, j, cmp)
+			}
+			a[j] = b[j] + 1
+			a[j+1] = b[j+1] - 1
+			cmp = Compare(a[:i], b[:i])
+			if cmp != 1 {
+				t.Errorf(`CompareAbigger(%d,%d) = %d`, i, j, cmp)
+			}
+			a[j] = b[j]
+			a[j+1] = b[j+1]
+		}
+	}
+}
+
+func BenchmarkCompareBytesEqual(b *testing.B) {
+	b1 := []byte("Hello Gophers!")
+	b2 := []byte("Hello Gophers!")
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+}
+
+func BenchmarkCompareBytesToNil(b *testing.B) {
+	b1 := []byte("Hello Gophers!")
+	var b2 []byte
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != 1 {
+			b.Fatal("b1 > b2 failed")
+		}
+	}
+}
+
+func BenchmarkCompareBytesEmpty(b *testing.B) {
+	b1 := []byte("")
+	b2 := b1
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+}
+
+func BenchmarkCompareBytesIdentical(b *testing.B) {
+	b1 := []byte("Hello Gophers!")
+	b2 := b1
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+}
+
+func BenchmarkCompareBytesSameLength(b *testing.B) {
+	b1 := []byte("Hello Gophers!")
+	b2 := []byte("Hello, Gophers")
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != -1 {
+			b.Fatal("b1 < b2 failed")
+		}
+	}
+}
+
+func BenchmarkCompareBytesDifferentLength(b *testing.B) {
+	b1 := []byte("Hello Gophers!")
+	b2 := []byte("Hello, Gophers!")
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != -1 {
+			b.Fatal("b1 < b2 failed")
+		}
+	}
+}
+
+func benchmarkCompareBytesBigUnaligned(b *testing.B, offset int) {
+	b.StopTimer()
+	b1 := make([]byte, 0, 1<<20)
+	for len(b1) < 1<<20 {
+		b1 = append(b1, "Hello Gophers!"...)
+	}
+	b2 := append([]byte("12345678")[:offset], b1...)
+	b.StartTimer()
+	for j := 0; j < b.N; j++ {
+		if Compare(b1, b2[offset:]) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+	b.SetBytes(int64(len(b1)))
+}
+
+func BenchmarkCompareBytesBigUnaligned(b *testing.B) {
+	for i := 1; i < 8; i++ {
+		b.Run(fmt.Sprintf("offset=%d", i), func(b *testing.B) {
+			benchmarkCompareBytesBigUnaligned(b, i)
+		})
+	}
+}
+
+func benchmarkCompareBytesBigBothUnaligned(b *testing.B, offset int) {
+	b.StopTimer()
+	pattern := []byte("Hello Gophers!")
+	b1 := make([]byte, 0, 1<<20+len(pattern))
+	for len(b1) < 1<<20 {
+		b1 = append(b1, pattern...)
+	}
+	b2 := make([]byte, len(b1))
+	copy(b2, b1)
+	b.StartTimer()
+	for j := 0; j < b.N; j++ {
+		if Compare(b1[offset:], b2[offset:]) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+	b.SetBytes(int64(len(b1[offset:])))
+}
+
+func BenchmarkCompareBytesBigBothUnaligned(b *testing.B) {
+	for i := 0; i < 8; i++ {
+		b.Run(fmt.Sprintf("offset=%d", i), func(b *testing.B) {
+			benchmarkCompareBytesBigBothUnaligned(b, i)
+		})
+	}
+}
+
+func BenchmarkCompareBytesBig(b *testing.B) {
+	b.StopTimer()
+	b1 := make([]byte, 0, 1<<20)
+	for len(b1) < 1<<20 {
+		b1 = append(b1, "Hello Gophers!"...)
+	}
+	b2 := append([]byte{}, b1...)
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+	b.SetBytes(int64(len(b1)))
+}
+
+func BenchmarkCompareBytesBigIdentical(b *testing.B) {
+	b.StopTimer()
+	b1 := make([]byte, 0, 1<<20)
+	for len(b1) < 1<<20 {
+		b1 = append(b1, "Hello Gophers!"...)
+	}
+	b2 := b1
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		if Compare(b1, b2) != 0 {
+			b.Fatal("b1 != b2")
+		}
+	}
+	b.SetBytes(int64(len(b1)))
+}

go/src/bytes/example_test.go

@@ -0,0 +1,720 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes_test
+
+import (
+	"bytes"
+	"encoding/base64"
+	"fmt"
+	"io"
+	"os"
+	"slices"
+	"strconv"
+	"unicode"
+)
+
+func ExampleBuffer() {
+	var b bytes.Buffer // A Buffer needs no initialization.
+	b.Write([]byte("Hello "))
+	fmt.Fprintf(&b, "world!")
+	b.WriteTo(os.Stdout)
+	// Output: Hello world!
+}
+
+func ExampleBuffer_reader() {
+	// A Buffer can turn a string or a []byte into an io.Reader.
+	buf := bytes.NewBufferString("R29waGVycyBydWxlIQ==")
+	dec := base64.NewDecoder(base64.StdEncoding, buf)
+	io.Copy(os.Stdout, dec)
+	// Output: Gophers rule!
+}
+
+func ExampleBuffer_Bytes() {
+	buf := bytes.Buffer{}
+	buf.Write([]byte{'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'})
+	os.Stdout.Write(buf.Bytes())
+	// Output: hello world
+}
+
+func ExampleBuffer_AvailableBuffer() {
+	var buf bytes.Buffer
+	for i := 0; i < 4; i++ {
+		b := buf.AvailableBuffer()
+		b = strconv.AppendInt(b, int64(i), 10)
+		b = append(b, ' ')
+		buf.Write(b)
+	}
+	os.Stdout.Write(buf.Bytes())
+	// Output: 0 1 2 3
+}
+
+func ExampleBuffer_Cap() {
+	buf1 := bytes.NewBuffer(make([]byte, 10))
+	buf2 := bytes.NewBuffer(make([]byte, 0, 10))
+	fmt.Println(buf1.Cap())
+	fmt.Println(buf2.Cap())
+	// Output:
+	// 10
+	// 10
+}
+
+func ExampleBuffer_Grow() {
+	var b bytes.Buffer
+	b.Grow(64)
+	bb := b.Bytes()
+	b.Write([]byte("64 bytes or fewer"))
+	fmt.Printf("%q", bb[:b.Len()])
+	// Output: "64 bytes or fewer"
+}
+
+func ExampleBuffer_Len() {
+	var b bytes.Buffer
+	b.Grow(64)
+	b.Write([]byte("abcde"))
+	fmt.Printf("%d", b.Len())
+	// Output: 5
+}
+
+func ExampleBuffer_Next() {
+	var b bytes.Buffer
+	b.Grow(64)
+	b.Write([]byte("abcde"))
+	fmt.Printf("%s\n", b.Next(2))
+	fmt.Printf("%s\n", b.Next(2))
+	fmt.Printf("%s", b.Next(2))
+	// Output:
+	// ab
+	// cd
+	// e
+}
+
+func ExampleBuffer_Read() {
+	var b bytes.Buffer
+	b.Grow(64)
+	b.Write([]byte("abcde"))
+	rdbuf := make([]byte, 1)
+	n, err := b.Read(rdbuf)
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(n)
+	fmt.Println(b.String())
+	fmt.Println(string(rdbuf))
+	// Output:
+	// 1
+	// bcde
+	// a
+}
+
+func ExampleBuffer_ReadByte() {
+	var b bytes.Buffer
+	b.Grow(64)
+	b.Write([]byte("abcde"))
+	c, err := b.ReadByte()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(c)
+	fmt.Println(b.String())
+	// Output:
+	// 97
+	// bcde
+}
+
+func ExampleClone() {
+	b := []byte("abc")
+	clone := bytes.Clone(b)
+	fmt.Printf("%s\n", clone)
+	clone[0] = 'd'
+	fmt.Printf("%s\n", b)
+	fmt.Printf("%s\n", clone)
+	// Output:
+	// abc
+	// abc
+	// dbc
+}
+
+func ExampleCompare() {
+	// Interpret Compare's result by comparing it to zero.
+	var a, b []byte
+	if bytes.Compare(a, b) < 0 {
+		// a less b
+	}
+	if bytes.Compare(a, b) <= 0 {
+		// a less or equal b
+	}
+	if bytes.Compare(a, b) > 0 {
+		// a greater b
+	}
+	if bytes.Compare(a, b) >= 0 {
+		// a greater or equal b
+	}
+
+	// Prefer Equal to Compare for equality comparisons.
+	if bytes.Equal(a, b) {
+		// a equal b
+	}
+	if !bytes.Equal(a, b) {
+		// a not equal b
+	}
+}
+
+func ExampleCompare_search() {
+	// Binary search to find a matching byte slice.
+	var needle []byte
+	var haystack [][]byte // Assume sorted
+	_, found := slices.BinarySearchFunc(haystack, needle, bytes.Compare)
+	if found {
+		// Found it!
+	}
+}
+
+func ExampleContains() {
+	fmt.Println(bytes.Contains([]byte("seafood"), []byte("foo")))
+	fmt.Println(bytes.Contains([]byte("seafood"), []byte("bar")))
+	fmt.Println(bytes.Contains([]byte("seafood"), []byte("")))
+	fmt.Println(bytes.Contains([]byte(""), []byte("")))
+	// Output:
+	// true
+	// false
+	// true
+	// true
+}
+
+func ExampleContainsAny() {
+	fmt.Println(bytes.ContainsAny([]byte("I like seafood."), "fÄo!"))
+	fmt.Println(bytes.ContainsAny([]byte("I like seafood."), "去是伟大的."))
+	fmt.Println(bytes.ContainsAny([]byte("I like seafood."), ""))
+	fmt.Println(bytes.ContainsAny([]byte(""), ""))
+	// Output:
+	// true
+	// true
+	// false
+	// false
+}
+
+func ExampleContainsRune() {
+	fmt.Println(bytes.ContainsRune([]byte("I like seafood."), 'f'))
+	fmt.Println(bytes.ContainsRune([]byte("I like seafood."), 'ö'))
+	fmt.Println(bytes.ContainsRune([]byte("去是伟大的!"), '大'))
+	fmt.Println(bytes.ContainsRune([]byte("去是伟大的!"), '!'))
+	fmt.Println(bytes.ContainsRune([]byte(""), '@'))
+	// Output:
+	// true
+	// false
+	// true
+	// true
+	// false
+}
+
+func ExampleContainsFunc() {
+	f := func(r rune) bool {
+		return r >= 'a' && r <= 'z'
+	}
+	fmt.Println(bytes.ContainsFunc([]byte("HELLO"), f))
+	fmt.Println(bytes.ContainsFunc([]byte("World"), f))
+	// Output:
+	// false
+	// true
+}
+
+func ExampleCount() {
+	fmt.Println(bytes.Count([]byte("cheese"), []byte("e")))
+	fmt.Println(bytes.Count([]byte("five"), []byte(""))) // before & after each rune
+	// Output:
+	// 3
+	// 5
+}
+
+func ExampleCut() {
+	show := func(s, sep string) {
+		before, after, found := bytes.Cut([]byte(s), []byte(sep))
+		fmt.Printf("Cut(%q, %q) = %q, %q, %v\n", s, sep, before, after, found)
+	}
+	show("Gopher", "Go")
+	show("Gopher", "ph")
+	show("Gopher", "er")
+	show("Gopher", "Badger")
+	// Output:
+	// Cut("Gopher", "Go") = "", "pher", true
+	// Cut("Gopher", "ph") = "Go", "er", true
+	// Cut("Gopher", "er") = "Goph", "", true
+	// Cut("Gopher", "Badger") = "Gopher", "", false
+}
+
+func ExampleCutPrefix() {
+	show := func(s, prefix string) {
+		after, found := bytes.CutPrefix([]byte(s), []byte(prefix))
+		fmt.Printf("CutPrefix(%q, %q) = %q, %v\n", s, prefix, after, found)
+	}
+	show("Gopher", "Go")
+	show("Gopher", "ph")
+	// Output:
+	// CutPrefix("Gopher", "Go") = "pher", true
+	// CutPrefix("Gopher", "ph") = "Gopher", false
+}
+
+func ExampleCutSuffix() {
+	show := func(s, suffix string) {
+		before, found := bytes.CutSuffix([]byte(s), []byte(suffix))
+		fmt.Printf("CutSuffix(%q, %q) = %q, %v\n", s, suffix, before, found)
+	}
+	show("Gopher", "Go")
+	show("Gopher", "er")
+	// Output:
+	// CutSuffix("Gopher", "Go") = "Gopher", false
+	// CutSuffix("Gopher", "er") = "Goph", true
+}
+
+func ExampleEqual() {
+	fmt.Println(bytes.Equal([]byte("Go"), []byte("Go")))
+	fmt.Println(bytes.Equal([]byte("Go"), []byte("C++")))
+	// Output:
+	// true
+	// false
+}
+
+func ExampleEqualFold() {
+	fmt.Println(bytes.EqualFold([]byte("Go"), []byte("go")))
+	// Output: true
+}
+
+func ExampleFields() {
+	fmt.Printf("Fields are: %q", bytes.Fields([]byte("  foo bar  baz   ")))
+	// Output: Fields are: ["foo" "bar" "baz"]
+}
+
+func ExampleFieldsFunc() {
+	f := func(c rune) bool {
+		return !unicode.IsLetter(c) && !unicode.IsNumber(c)
+	}
+	fmt.Printf("Fields are: %q", bytes.FieldsFunc([]byte("  foo1;bar2,baz3..."), f))
+	// Output: Fields are: ["foo1" "bar2" "baz3"]
+}
+
+func ExampleHasPrefix() {
+	fmt.Println(bytes.HasPrefix([]byte("Gopher"), []byte("Go")))
+	fmt.Println(bytes.HasPrefix([]byte("Gopher"), []byte("C")))
+	fmt.Println(bytes.HasPrefix([]byte("Gopher"), []byte("")))
+	// Output:
+	// true
+	// false
+	// true
+}
+
+func ExampleHasSuffix() {
+	fmt.Println(bytes.HasSuffix([]byte("Amigo"), []byte("go")))
+	fmt.Println(bytes.HasSuffix([]byte("Amigo"), []byte("O")))
+	fmt.Println(bytes.HasSuffix([]byte("Amigo"), []byte("Ami")))
+	fmt.Println(bytes.HasSuffix([]byte("Amigo"), []byte("")))
+	// Output:
+	// true
+	// false
+	// false
+	// true
+}
+
+func ExampleIndex() {
+	fmt.Println(bytes.Index([]byte("chicken"), []byte("ken")))
+	fmt.Println(bytes.Index([]byte("chicken"), []byte("dmr")))
+	// Output:
+	// 4
+	// -1
+}
+
+func ExampleIndexByte() {
+	fmt.Println(bytes.IndexByte([]byte("chicken"), byte('k')))
+	fmt.Println(bytes.IndexByte([]byte("chicken"), byte('g')))
+	// Output:
+	// 4
+	// -1
+}
+
+func ExampleIndexFunc() {
+	f := func(c rune) bool {
+		return unicode.Is(unicode.Han, c)
+	}
+	fmt.Println(bytes.IndexFunc([]byte("Hello, 世界"), f))
+	fmt.Println(bytes.IndexFunc([]byte("Hello, world"), f))
+	// Output:
+	// 7
+	// -1
+}
+
+func ExampleIndexAny() {
+	fmt.Println(bytes.IndexAny([]byte("chicken"), "aeiouy"))
+	fmt.Println(bytes.IndexAny([]byte("crwth"), "aeiouy"))
+	// Output:
+	// 2
+	// -1
+}
+
+func ExampleIndexRune() {
+	fmt.Println(bytes.IndexRune([]byte("chicken"), 'k'))
+	fmt.Println(bytes.IndexRune([]byte("chicken"), 'd'))
+	// Output:
+	// 4
+	// -1
+}
+
+func ExampleJoin() {
+	s := [][]byte{[]byte("foo"), []byte("bar"), []byte("baz")}
+	fmt.Printf("%s", bytes.Join(s, []byte(", ")))
+	// Output: foo, bar, baz
+}
+
+func ExampleLastIndex() {
+	fmt.Println(bytes.Index([]byte("go gopher"), []byte("go")))
+	fmt.Println(bytes.LastIndex([]byte("go gopher"), []byte("go")))
+	fmt.Println(bytes.LastIndex([]byte("go gopher"), []byte("rodent")))
+	// Output:
+	// 0
+	// 3
+	// -1
+}
+
+func ExampleLastIndexAny() {
+	fmt.Println(bytes.LastIndexAny([]byte("go gopher"), "MüQp"))
+	fmt.Println(bytes.LastIndexAny([]byte("go 地鼠"), "地大"))
+	fmt.Println(bytes.LastIndexAny([]byte("go gopher"), "z,!."))
+	// Output:
+	// 5
+	// 3
+	// -1
+}
+
+func ExampleLastIndexByte() {
+	fmt.Println(bytes.LastIndexByte([]byte("go gopher"), byte('g')))
+	fmt.Println(bytes.LastIndexByte([]byte("go gopher"), byte('r')))
+	fmt.Println(bytes.LastIndexByte([]byte("go gopher"), byte('z')))
+	// Output:
+	// 3
+	// 8
+	// -1
+}
+
+func ExampleLastIndexFunc() {
+	fmt.Println(bytes.LastIndexFunc([]byte("go gopher!"), unicode.IsLetter))
+	fmt.Println(bytes.LastIndexFunc([]byte("go gopher!"), unicode.IsPunct))
+	fmt.Println(bytes.LastIndexFunc([]byte("go gopher!"), unicode.IsNumber))
+	// Output:
+	// 8
+	// 9
+	// -1
+}
+
+func ExampleMap() {
+	rot13 := func(r rune) rune {
+		switch {
+		case r >= 'A' && r <= 'Z':
+			return 'A' + (r-'A'+13)%26
+		case r >= 'a' && r <= 'z':
+			return 'a' + (r-'a'+13)%26
+		}
+		return r
+	}
+	fmt.Printf("%s\n", bytes.Map(rot13, []byte("'Twas brillig and the slithy gopher...")))
+	// Output:
+	// 'Gjnf oevyyvt naq gur fyvgul tbcure...
+}
+
+func ExampleReader_Len() {
+	fmt.Println(bytes.NewReader([]byte("Hi!")).Len())
+	fmt.Println(bytes.NewReader([]byte("こんにちは!")).Len())
+	// Output:
+	// 3
+	// 16
+}
+
+func ExampleRepeat() {
+	fmt.Printf("ba%s", bytes.Repeat([]byte("na"), 2))
+	// Output: banana
+}
+
+func ExampleReplace() {
+	fmt.Printf("%s\n", bytes.Replace([]byte("oink oink oink"), []byte("k"), []byte("ky"), 2))
+	fmt.Printf("%s\n", bytes.Replace([]byte("oink oink oink"), []byte("oink"), []byte("moo"), -1))
+	// Output:
+	// oinky oinky oink
+	// moo moo moo
+}
+
+func ExampleReplaceAll() {
+	fmt.Printf("%s\n", bytes.ReplaceAll([]byte("oink oink oink"), []byte("oink"), []byte("moo")))
+	// Output:
+	// moo moo moo
+}
+
+func ExampleRunes() {
+	rs := bytes.Runes([]byte("go gopher"))
+	for _, r := range rs {
+		fmt.Printf("%#U\n", r)
+	}
+	// Output:
+	// U+0067 'g'
+	// U+006F 'o'
+	// U+0020 ' '
+	// U+0067 'g'
+	// U+006F 'o'
+	// U+0070 'p'
+	// U+0068 'h'
+	// U+0065 'e'
+	// U+0072 'r'
+}
+
+func ExampleSplit() {
+	fmt.Printf("%q\n", bytes.Split([]byte("a,b,c"), []byte(",")))
+	fmt.Printf("%q\n", bytes.Split([]byte("a man a plan a canal panama"), []byte("a ")))
+	fmt.Printf("%q\n", bytes.Split([]byte(" xyz "), []byte("")))
+	fmt.Printf("%q\n", bytes.Split([]byte(""), []byte("Bernardo O'Higgins")))
+	// Output:
+	// ["a" "b" "c"]
+	// ["" "man " "plan " "canal panama"]
+	// [" " "x" "y" "z" " "]
+	// [""]
+}
+
+func ExampleSplitN() {
+	fmt.Printf("%q\n", bytes.SplitN([]byte("a,b,c"), []byte(","), 2))
+	z := bytes.SplitN([]byte("a,b,c"), []byte(","), 0)
+	fmt.Printf("%q (nil = %v)\n", z, z == nil)
+	// Output:
+	// ["a" "b,c"]
+	// [] (nil = true)
+}
+
+func ExampleSplitAfter() {
+	fmt.Printf("%q\n", bytes.SplitAfter([]byte("a,b,c"), []byte(",")))
+	// Output: ["a," "b," "c"]
+}
+
+func ExampleSplitAfterN() {
+	fmt.Printf("%q\n", bytes.SplitAfterN([]byte("a,b,c"), []byte(","), 2))
+	// Output: ["a," "b,c"]
+}
+
+func ExampleTitle() {
+	fmt.Printf("%s", bytes.Title([]byte("her royal highness")))
+	// Output: Her Royal Highness
+}
+
+func ExampleToTitle() {
+	fmt.Printf("%s\n", bytes.ToTitle([]byte("loud noises")))
+	fmt.Printf("%s\n", bytes.ToTitle([]byte("брат")))
+	// Output:
+	// LOUD NOISES
+	// БРАТ
+}
+
+func ExampleToTitleSpecial() {
+	str := []byte("ahoj vývojári golang")
+	totitle := bytes.ToTitleSpecial(unicode.AzeriCase, str)
+	fmt.Println("Original : " + string(str))
+	fmt.Println("ToTitle : " + string(totitle))
+	// Output:
+	// Original : ahoj vývojári golang
+	// ToTitle : AHOJ VÝVOJÁRİ GOLANG
+}
+
+func ExampleToValidUTF8() {
+	fmt.Printf("%s\n", bytes.ToValidUTF8([]byte("abc"), []byte("\uFFFD")))
+	fmt.Printf("%s\n", bytes.ToValidUTF8([]byte("a\xffb\xC0\xAFc\xff"), []byte("")))
+	fmt.Printf("%s\n", bytes.ToValidUTF8([]byte("\xed\xa0\x80"), []byte("abc")))
+	// Output:
+	// abc
+	// abc
+	// abc
+}
+
+func ExampleTrim() {
+	fmt.Printf("[%q]", bytes.Trim([]byte(" !!! Achtung! Achtung! !!! "), "! "))
+	// Output: ["Achtung! Achtung"]
+}
+
+func ExampleTrimFunc() {
+	fmt.Println(string(bytes.TrimFunc([]byte("go-gopher!"), unicode.IsLetter)))
+	fmt.Println(string(bytes.TrimFunc([]byte("\"go-gopher!\""), unicode.IsLetter)))
+	fmt.Println(string(bytes.TrimFunc([]byte("go-gopher!"), unicode.IsPunct)))
+	fmt.Println(string(bytes.TrimFunc([]byte("1234go-gopher!567"), unicode.IsNumber)))
+	// Output:
+	// -gopher!
+	// "go-gopher!"
+	// go-gopher
+	// go-gopher!
+}
+
+func ExampleTrimLeft() {
+	fmt.Print(string(bytes.TrimLeft([]byte("453gopher8257"), "0123456789")))
+	// Output:
+	// gopher8257
+}
+
+func ExampleTrimLeftFunc() {
+	fmt.Println(string(bytes.TrimLeftFunc([]byte("go-gopher"), unicode.IsLetter)))
+	fmt.Println(string(bytes.TrimLeftFunc([]byte("go-gopher!"), unicode.IsPunct)))
+	fmt.Println(string(bytes.TrimLeftFunc([]byte("1234go-gopher!567"), unicode.IsNumber)))
+	// Output:
+	// -gopher
+	// go-gopher!
+	// go-gopher!567
+}
+
+func ExampleTrimPrefix() {
+	var b = []byte("Goodbye,, world!")
+	b = bytes.TrimPrefix(b, []byte("Goodbye,"))
+	b = bytes.TrimPrefix(b, []byte("See ya,"))
+	fmt.Printf("Hello%s", b)
+	// Output: Hello, world!
+}
+
+func ExampleTrimSpace() {
+	fmt.Printf("%s", bytes.TrimSpace([]byte(" \t\n a lone gopher \n\t\r\n")))
+	// Output: a lone gopher
+}
+
+func ExampleTrimSuffix() {
+	var b = []byte("Hello, goodbye, etc!")
+	b = bytes.TrimSuffix(b, []byte("goodbye, etc!"))
+	b = bytes.TrimSuffix(b, []byte("gopher"))
+	b = append(b, bytes.TrimSuffix([]byte("world!"), []byte("x!"))...)
+	os.Stdout.Write(b)
+	// Output: Hello, world!
+}
+
+func ExampleTrimRight() {
+	fmt.Print(string(bytes.TrimRight([]byte("453gopher8257"), "0123456789")))
+	// Output:
+	// 453gopher
+}
+
+func ExampleTrimRightFunc() {
+	fmt.Println(string(bytes.TrimRightFunc([]byte("go-gopher"), unicode.IsLetter)))
+	fmt.Println(string(bytes.TrimRightFunc([]byte("go-gopher!"), unicode.IsPunct)))
+	fmt.Println(string(bytes.TrimRightFunc([]byte("1234go-gopher!567"), unicode.IsNumber)))
+	// Output:
+	// go-
+	// go-gopher
+	// 1234go-gopher!
+}
+
+func ExampleToLower() {
+	fmt.Printf("%s", bytes.ToLower([]byte("Gopher")))
+	// Output: gopher
+}
+
+func ExampleToLowerSpecial() {
+	str := []byte("AHOJ VÝVOJÁRİ GOLANG")
+	totitle := bytes.ToLowerSpecial(unicode.AzeriCase, str)
+	fmt.Println("Original : " + string(str))
+	fmt.Println("ToLower : " + string(totitle))
+	// Output:
+	// Original : AHOJ VÝVOJÁRİ GOLANG
+	// ToLower : ahoj vývojári golang
+}
+
+func ExampleToUpper() {
+	fmt.Printf("%s", bytes.ToUpper([]byte("Gopher")))
+	// Output: GOPHER
+}
+
+func ExampleToUpperSpecial() {
+	str := []byte("ahoj vývojári golang")
+	totitle := bytes.ToUpperSpecial(unicode.AzeriCase, str)
+	fmt.Println("Original : " + string(str))
+	fmt.Println("ToUpper : " + string(totitle))
+	// Output:
+	// Original : ahoj vývojári golang
+	// ToUpper : AHOJ VÝVOJÁRİ GOLANG
+}
+
+func ExampleLines() {
+	text := []byte("Hello\nWorld\nGo Programming\n")
+	for line := range bytes.Lines(text) {
+		fmt.Printf("%q\n", line)
+	}
+
+	// Output:
+	// "Hello\n"
+	// "World\n"
+	// "Go Programming\n"
+}
+
+func ExampleSplitSeq() {
+	s := []byte("a,b,c,d")
+	for part := range bytes.SplitSeq(s, []byte(",")) {
+		fmt.Printf("%q\n", part)
+	}
+
+	// Output:
+	// "a"
+	// "b"
+	// "c"
+	// "d"
+}
+
+func ExampleSplitAfterSeq() {
+	s := []byte("a,b,c,d")
+	for part := range bytes.SplitAfterSeq(s, []byte(",")) {
+		fmt.Printf("%q\n", part)
+	}
+
+	// Output:
+	// "a,"
+	// "b,"
+	// "c,"
+	// "d"
+}
+
+func ExampleFieldsSeq() {
+	text := []byte("The quick brown fox")
+	fmt.Println("Split byte slice into fields:")
+	for word := range bytes.FieldsSeq(text) {
+		fmt.Printf("%q\n", word)
+	}
+
+	textWithSpaces := []byte("  lots   of   spaces  ")
+	fmt.Println("\nSplit byte slice with multiple spaces:")
+	for word := range bytes.FieldsSeq(textWithSpaces) {
+		fmt.Printf("%q\n", word)
+	}
+
+	// Output:
+	// Split byte slice into fields:
+	// "The"
+	// "quick"
+	// "brown"
+	// "fox"
+	//
+	// Split byte slice with multiple spaces:
+	// "lots"
+	// "of"
+	// "spaces"
+}
+
+func ExampleFieldsFuncSeq() {
+	text := []byte("The quick brown fox")
+	fmt.Println("Split on whitespace(similar to FieldsSeq):")
+	for word := range bytes.FieldsFuncSeq(text, unicode.IsSpace) {
+		fmt.Printf("%q\n", word)
+	}
+
+	mixedText := []byte("abc123def456ghi")
+	fmt.Println("\nSplit on digits:")
+	for word := range bytes.FieldsFuncSeq(mixedText, unicode.IsDigit) {
+		fmt.Printf("%q\n", word)
+	}
+
+	// Output:
+	// Split on whitespace(similar to FieldsSeq):
+	// "The"
+	// "quick"
+	// "brown"
+	// "fox"
+	//
+	// Split on digits:
+	// "abc"
+	// "def"
+	// "ghi"
+}

go/src/bytes/export_test.go

@@ -0,0 +1,8 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes
+
+// Export func for testing
+var IndexBytePortable = indexBytePortable

go/src/bytes/iter.go

@@ -0,0 +1,137 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes
+
+import (
+	"iter"
+	"unicode"
+	"unicode/utf8"
+)
+
+// Lines returns an iterator over the newline-terminated lines in the byte slice s.
+// The lines yielded by the iterator include their terminating newlines.
+// If s is empty, the iterator yields no lines at all.
+// If s does not end in a newline, the final yielded line will not end in a newline.
+// It returns a single-use iterator.
+func Lines(s []byte) iter.Seq[[]byte] {
+	return func(yield func([]byte) bool) {
+		for len(s) > 0 {
+			var line []byte
+			if i := IndexByte(s, '\n'); i >= 0 {
+				line, s = s[:i+1], s[i+1:]
+			} else {
+				line, s = s, nil
+			}
+			if !yield(line[:len(line):len(line)]) {
+				return
+			}
+		}
+	}
+}
+
+// splitSeq is SplitSeq or SplitAfterSeq, configured by how many
+// bytes of sep to include in the results (none or all).
+func splitSeq(s, sep []byte, sepSave int) iter.Seq[[]byte] {
+	return func(yield func([]byte) bool) {
+		if len(sep) == 0 {
+			for len(s) > 0 {
+				_, size := utf8.DecodeRune(s)
+				if !yield(s[:size:size]) {
+					return
+				}
+				s = s[size:]
+			}
+			return
+		}
+		for {
+			i := Index(s, sep)
+			if i < 0 {
+				break
+			}
+			frag := s[:i+sepSave]
+			if !yield(frag[:len(frag):len(frag)]) {
+				return
+			}
+			s = s[i+len(sep):]
+		}
+		yield(s[:len(s):len(s)])
+	}
+}
+
+// SplitSeq returns an iterator over all subslices of s separated by sep.
+// The iterator yields the same subslices that would be returned by [Split](s, sep),
+// but without constructing a new slice containing the subslices.
+// It returns a single-use iterator.
+func SplitSeq(s, sep []byte) iter.Seq[[]byte] {
+	return splitSeq(s, sep, 0)
+}
+
+// SplitAfterSeq returns an iterator over subslices of s split after each instance of sep.
+// The iterator yields the same subslices that would be returned by [SplitAfter](s, sep),
+// but without constructing a new slice containing the subslices.
+// It returns a single-use iterator.
+func SplitAfterSeq(s, sep []byte) iter.Seq[[]byte] {
+	return splitSeq(s, sep, len(sep))
+}
+
+// FieldsSeq returns an iterator over subslices of s split around runs of
+// whitespace characters, as defined by [unicode.IsSpace].
+// The iterator yields the same subslices that would be returned by [Fields](s),
+// but without constructing a new slice containing the subslices.
+func FieldsSeq(s []byte) iter.Seq[[]byte] {
+	return func(yield func([]byte) bool) {
+		start := -1
+		for i := 0; i < len(s); {
+			size := 1
+			r := rune(s[i])
+			isSpace := asciiSpace[s[i]] != 0
+			if r >= utf8.RuneSelf {
+				r, size = utf8.DecodeRune(s[i:])
+				isSpace = unicode.IsSpace(r)
+			}
+			if isSpace {
+				if start >= 0 {
+					if !yield(s[start:i:i]) {
+						return
+					}
+					start = -1
+				}
+			} else if start < 0 {
+				start = i
+			}
+			i += size
+		}
+		if start >= 0 {
+			yield(s[start:len(s):len(s)])
+		}
+	}
+}
+
+// FieldsFuncSeq returns an iterator over subslices of s split around runs of
+// Unicode code points satisfying f(c).
+// The iterator yields the same subslices that would be returned by [FieldsFunc](s),
+// but without constructing a new slice containing the subslices.
+func FieldsFuncSeq(s []byte, f func(rune) bool) iter.Seq[[]byte] {
+	return func(yield func([]byte) bool) {
+		start := -1
+		for i := 0; i < len(s); {
+			r, size := utf8.DecodeRune(s[i:])
+			if f(r) {
+				if start >= 0 {
+					if !yield(s[start:i:i]) {
+						return
+					}
+					start = -1
+				}
+			} else if start < 0 {
+				start = i
+			}
+			i += size
+		}
+		if start >= 0 {
+			yield(s[start:len(s):len(s)])
+		}
+	}
+}

go/src/bytes/iter_test.go

@@ -0,0 +1,56 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes_test
+
+import (
+	. "bytes"
+	"testing"
+)
+
+func BenchmarkSplitSeqEmptySeparator(b *testing.B) {
+	for range b.N {
+		for range SplitSeq(benchInputHard, nil) {
+		}
+	}
+}
+
+func BenchmarkSplitSeqSingleByteSeparator(b *testing.B) {
+	sep := []byte("/")
+	for range b.N {
+		for range SplitSeq(benchInputHard, sep) {
+		}
+	}
+}
+
+func BenchmarkSplitSeqMultiByteSeparator(b *testing.B) {
+	sep := []byte("hello")
+	for range b.N {
+		for range SplitSeq(benchInputHard, sep) {
+		}
+	}
+}
+
+func BenchmarkSplitAfterSeqEmptySeparator(b *testing.B) {
+	for range b.N {
+		for range SplitAfterSeq(benchInputHard, nil) {
+		}
+	}
+}
+
+func BenchmarkSplitAfterSeqSingleByteSeparator(b *testing.B) {
+	sep := []byte("/")
+	for range b.N {
+		for range SplitAfterSeq(benchInputHard, sep) {
+		}
+	}
+}
+
+func BenchmarkSplitAfterSeqMultiByteSeparator(b *testing.B) {
+	sep := []byte("hello")
+	for range b.N {
+		for range SplitAfterSeq(benchInputHard, sep) {
+		}
+	}
+}

go/src/bytes/reader.go

@@ -0,0 +1,159 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes
+
+import (
+	"errors"
+	"io"
+	"unicode/utf8"
+)
+
+// A Reader implements the [io.Reader], [io.ReaderAt], [io.WriterTo], [io.Seeker],
+// [io.ByteScanner], and [io.RuneScanner] interfaces by reading from
+// a byte slice.
+// Unlike a [Buffer], a Reader is read-only and supports seeking.
+// The zero value for Reader operates like a Reader of an empty slice.
+type Reader struct {
+	s        []byte
+	i        int64 // current reading index
+	prevRune int   // index of previous rune; or < 0
+}
+
+// Len returns the number of bytes of the unread portion of the
+// slice.
+func (r *Reader) Len() int {
+	if r.i >= int64(len(r.s)) {
+		return 0
+	}
+	return int(int64(len(r.s)) - r.i)
+}
+
+// Size returns the original length of the underlying byte slice.
+// Size is the number of bytes available for reading via [Reader.ReadAt].
+// The result is unaffected by any method calls except [Reader.Reset].
+func (r *Reader) Size() int64 { return int64(len(r.s)) }
+
+// Read implements the [io.Reader] interface.
+func (r *Reader) Read(b []byte) (n int, err error) {
+	if r.i >= int64(len(r.s)) {
+		return 0, io.EOF
+	}
+	r.prevRune = -1
+	n = copy(b, r.s[r.i:])
+	r.i += int64(n)
+	return
+}
+
+// ReadAt implements the [io.ReaderAt] interface.
+func (r *Reader) ReadAt(b []byte, off int64) (n int, err error) {
+	// cannot modify state - see io.ReaderAt
+	if off < 0 {
+		return 0, errors.New("bytes.Reader.ReadAt: negative offset")
+	}
+	if off >= int64(len(r.s)) {
+		return 0, io.EOF
+	}
+	n = copy(b, r.s[off:])
+	if n < len(b) {
+		err = io.EOF
+	}
+	return
+}
+
+// ReadByte implements the [io.ByteReader] interface.
+func (r *Reader) ReadByte() (byte, error) {
+	r.prevRune = -1
+	if r.i >= int64(len(r.s)) {
+		return 0, io.EOF
+	}
+	b := r.s[r.i]
+	r.i++
+	return b, nil
+}
+
+// UnreadByte complements [Reader.ReadByte] in implementing the [io.ByteScanner] interface.
+func (r *Reader) UnreadByte() error {
+	if r.i <= 0 {
+		return errors.New("bytes.Reader.UnreadByte: at beginning of slice")
+	}
+	r.prevRune = -1
+	r.i--
+	return nil
+}
+
+// ReadRune implements the [io.RuneReader] interface.
+func (r *Reader) ReadRune() (ch rune, size int, err error) {
+	if r.i >= int64(len(r.s)) {
+		r.prevRune = -1
+		return 0, 0, io.EOF
+	}
+	r.prevRune = int(r.i)
+	if c := r.s[r.i]; c < utf8.RuneSelf {
+		r.i++
+		return rune(c), 1, nil
+	}
+	ch, size = utf8.DecodeRune(r.s[r.i:])
+	r.i += int64(size)
+	return
+}
+
+// UnreadRune complements [Reader.ReadRune] in implementing the [io.RuneScanner] interface.
+func (r *Reader) UnreadRune() error {
+	if r.i <= 0 {
+		return errors.New("bytes.Reader.UnreadRune: at beginning of slice")
+	}
+	if r.prevRune < 0 {
+		return errors.New("bytes.Reader.UnreadRune: previous operation was not ReadRune")
+	}
+	r.i = int64(r.prevRune)
+	r.prevRune = -1
+	return nil
+}
+
+// Seek implements the [io.Seeker] interface.
+func (r *Reader) Seek(offset int64, whence int) (int64, error) {
+	r.prevRune = -1
+	var abs int64
+	switch whence {
+	case io.SeekStart:
+		abs = offset
+	case io.SeekCurrent:
+		abs = r.i + offset
+	case io.SeekEnd:
+		abs = int64(len(r.s)) + offset
+	default:
+		return 0, errors.New("bytes.Reader.Seek: invalid whence")
+	}
+	if abs < 0 {
+		return 0, errors.New("bytes.Reader.Seek: negative position")
+	}
+	r.i = abs
+	return abs, nil
+}
+
+// WriteTo implements the [io.WriterTo] interface.
+func (r *Reader) WriteTo(w io.Writer) (n int64, err error) {
+	r.prevRune = -1
+	if r.i >= int64(len(r.s)) {
+		return 0, nil
+	}
+	b := r.s[r.i:]
+	m, err := w.Write(b)
+	if m > len(b) {
+		panic("bytes.Reader.WriteTo: invalid Write count")
+	}
+	r.i += int64(m)
+	n = int64(m)
+	if m != len(b) && err == nil {
+		err = io.ErrShortWrite
+	}
+	return
+}
+
+// Reset resets the [Reader] to be reading from b.
+func (r *Reader) Reset(b []byte) { *r = Reader{b, 0, -1} }
+
+// NewReader returns a new [Reader] reading from b.
+func NewReader(b []byte) *Reader { return &Reader{b, 0, -1} }

go/src/bytes/reader_test.go

@@ -0,0 +1,319 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package bytes_test
+
+import (
+	. "bytes"
+	"fmt"
+	"io"
+	"sync"
+	"testing"
+)
+
+func TestReader(t *testing.T) {
+	r := NewReader([]byte("0123456789"))
+	tests := []struct {
+		off     int64
+		seek    int
+		n       int
+		want    string
+		wantpos int64
+		readerr error
+		seekerr string
+	}{
+		{seek: io.SeekStart, off: 0, n: 20, want: "0123456789"},
+		{seek: io.SeekStart, off: 1, n: 1, want: "1"},
+		{seek: io.SeekCurrent, off: 1, wantpos: 3, n: 2, want: "34"},
+		{seek: io.SeekStart, off: -1, seekerr: "bytes.Reader.Seek: negative position"},
+		{seek: io.SeekStart, off: 1 << 33, wantpos: 1 << 33, readerr: io.EOF},
+		{seek: io.SeekCurrent, off: 1, wantpos: 1<<33 + 1, readerr: io.EOF},
+		{seek: io.SeekStart, n: 5, want: "01234"},
+		{seek: io.SeekCurrent, n: 5, want: "56789"},
+		{seek: io.SeekEnd, off: -1, n: 1, wantpos: 9, want: "9"},
+	}
+
+	for i, tt := range tests {
+		pos, err := r.Seek(tt.off, tt.seek)
+		if err == nil && tt.seekerr != "" {
+			t.Errorf("%d. want seek error %q", i, tt.seekerr)
+			continue
+		}
+		if err != nil && err.Error() != tt.seekerr {
+			t.Errorf("%d. seek error = %q; want %q", i, err.Error(), tt.seekerr)
+			continue
+		}
+		if tt.wantpos != 0 && tt.wantpos != pos {
+			t.Errorf("%d. pos = %d, want %d", i, pos, tt.wantpos)
+		}
+		buf := make([]byte, tt.n)
+		n, err := r.Read(buf)
+		if err != tt.readerr {
+			t.Errorf("%d. read = %v; want %v", i, err, tt.readerr)
+			continue
+		}
+		got := string(buf[:n])
+		if got != tt.want {
+			t.Errorf("%d. got %q; want %q", i, got, tt.want)
+		}
+	}
+}
+
+func TestReadAfterBigSeek(t *testing.T) {
+	r := NewReader([]byte("0123456789"))
+	if _, err := r.Seek(1<<31+5, io.SeekStart); err != nil {
+		t.Fatal(err)
+	}
+	if n, err := r.Read(make([]byte, 10)); n != 0 || err != io.EOF {
+		t.Errorf("Read = %d, %v; want 0, EOF", n, err)
+	}
+}
+
+func TestReaderAt(t *testing.T) {
+	r := NewReader([]byte("0123456789"))
+	tests := []struct {
+		off     int64
+		n       int
+		want    string
+		wanterr any
+	}{
+		{0, 10, "0123456789", nil},
+		{1, 10, "123456789", io.EOF},
+		{1, 9, "123456789", nil},
+		{11, 10, "", io.EOF},
+		{0, 0, "", nil},
+		{-1, 0, "", "bytes.Reader.ReadAt: negative offset"},
+	}
+	for i, tt := range tests {
+		b := make([]byte, tt.n)
+		rn, err := r.ReadAt(b, tt.off)
+		got := string(b[:rn])
+		if got != tt.want {
+			t.Errorf("%d. got %q; want %q", i, got, tt.want)
+		}
+		if fmt.Sprintf("%v", err) != fmt.Sprintf("%v", tt.wanterr) {
+			t.Errorf("%d. got error = %v; want %v", i, err, tt.wanterr)
+		}
+	}
+}
+
+func TestReaderAtConcurrent(t *testing.T) {
+	// Test for the race detector, to verify ReadAt doesn't mutate
+	// any state.
+	r := NewReader([]byte("0123456789"))
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			var buf [1]byte
+			r.ReadAt(buf[:], int64(i))
+		}(i)
+	}
+	wg.Wait()
+}
+
+func TestEmptyReaderConcurrent(t *testing.T) {
+	// Test for the race detector, to verify a Read that doesn't yield any bytes
+	// is okay to use from multiple goroutines. This was our historic behavior.
+	// See golang.org/issue/7856
+	r := NewReader([]byte{})
+	var wg sync.WaitGroup
+	for i := 0; i < 5; i++ {
+		wg.Add(2)
+		go func() {
+			defer wg.Done()
+			var buf [1]byte
+			r.Read(buf[:])
+		}()
+		go func() {
+			defer wg.Done()
+			r.Read(nil)
+		}()
+	}
+	wg.Wait()
+}
+
+func TestReaderWriteTo(t *testing.T) {
+	for i := 0; i < 30; i += 3 {
+		var l int
+		if i > 0 {
+			l = len(testString) / i
+		}
+		s := testString[:l]
+		r := NewReader(testBytes[:l])
+		var b Buffer
+		n, err := r.WriteTo(&b)
+		if expect := int64(len(s)); n != expect {
+			t.Errorf("got %v; want %v", n, expect)
+		}
+		if err != nil {
+			t.Errorf("for length %d: got error = %v; want nil", l, err)
+		}
+		if b.String() != s {
+			t.Errorf("got string %q; want %q", b.String(), s)
+		}
+		if r.Len() != 0 {
+			t.Errorf("reader contains %v bytes; want 0", r.Len())
+		}
+	}
+}
+
+func TestReaderLen(t *testing.T) {
+	const data = "hello world"
+	r := NewReader([]byte(data))
+	if got, want := r.Len(), 11; got != want {
+		t.Errorf("r.Len(): got %d, want %d", got, want)
+	}
+	if n, err := r.Read(make([]byte, 10)); err != nil || n != 10 {
+		t.Errorf("Read failed: read %d %v", n, err)
+	}
+	if got, want := r.Len(), 1; got != want {
+		t.Errorf("r.Len(): got %d, want %d", got, want)
+	}
+	if n, err := r.Read(make([]byte, 1)); err != nil || n != 1 {
+		t.Errorf("Read failed: read %d %v; want 1, nil", n, err)
+	}
+	if got, want := r.Len(), 0; got != want {
+		t.Errorf("r.Len(): got %d, want %d", got, want)
+	}
+}
+
+var UnreadRuneErrorTests = []struct {
+	name string
+	f    func(*Reader)
+}{
+	{"Read", func(r *Reader) { r.Read([]byte{0}) }},
+	{"ReadByte", func(r *Reader) { r.ReadByte() }},
+	{"UnreadRune", func(r *Reader) { r.UnreadRune() }},
+	{"Seek", func(r *Reader) { r.Seek(0, io.SeekCurrent) }},
+	{"WriteTo", func(r *Reader) { r.WriteTo(&Buffer{}) }},
+}
+
+func TestUnreadRuneError(t *testing.T) {
+	for _, tt := range UnreadRuneErrorTests {
+		reader := NewReader([]byte("0123456789"))
+		if _, _, err := reader.ReadRune(); err != nil {
+			// should not happen
+			t.Fatal(err)
+		}
+		tt.f(reader)
+		err := reader.UnreadRune()
+		if err == nil {
+			t.Errorf("Unreading after %s: expected error", tt.name)
+		}
+	}
+}
+
+func TestReaderDoubleUnreadRune(t *testing.T) {
+	buf := NewBuffer([]byte("groucho"))
+	if _, _, err := buf.ReadRune(); err != nil {
+		// should not happen
+		t.Fatal(err)
+	}
+	if err := buf.UnreadByte(); err != nil {
+		// should not happen
+		t.Fatal(err)
+	}
+	if err := buf.UnreadByte(); err == nil {
+		t.Fatal("UnreadByte: expected error, got nil")
+	}
+}
+
+// verify that copying from an empty reader always has the same results,
+// regardless of the presence of a WriteTo method.
+func TestReaderCopyNothing(t *testing.T) {
+	type nErr struct {
+		n   int64
+		err error
+	}
+	type justReader struct {
+		io.Reader
+	}
+	type justWriter struct {
+		io.Writer
+	}
+	discard := justWriter{io.Discard} // hide ReadFrom
+
+	var with, withOut nErr
+	with.n, with.err = io.Copy(discard, NewReader(nil))
+	withOut.n, withOut.err = io.Copy(discard, justReader{NewReader(nil)})
+	if with != withOut {
+		t.Errorf("behavior differs: with = %#v; without: %#v", with, withOut)
+	}
+}
+
+// tests that Len is affected by reads, but Size is not.
+func TestReaderLenSize(t *testing.T) {
+	r := NewReader([]byte("abc"))
+	io.CopyN(io.Discard, r, 1)
+	if r.Len() != 2 {
+		t.Errorf("Len = %d; want 2", r.Len())
+	}
+	if r.Size() != 3 {
+		t.Errorf("Size = %d; want 3", r.Size())
+	}
+}
+
+func TestReaderReset(t *testing.T) {
+	r := NewReader([]byte("世界"))
+	if _, _, err := r.ReadRune(); err != nil {
+		t.Errorf("ReadRune: unexpected error: %v", err)
+	}
+
+	const want = "abcdef"
+	r.Reset([]byte(want))
+	if err := r.UnreadRune(); err == nil {
+		t.Errorf("UnreadRune: expected error, got nil")
+	}
+	buf, err := io.ReadAll(r)
+	if err != nil {
+		t.Errorf("ReadAll: unexpected error: %v", err)
+	}
+	if got := string(buf); got != want {
+		t.Errorf("ReadAll: got %q, want %q", got, want)
+	}
+}
+
+func TestReaderZero(t *testing.T) {
+	if l := (&Reader{}).Len(); l != 0 {
+		t.Errorf("Len: got %d, want 0", l)
+	}
+
+	if n, err := (&Reader{}).Read(nil); n != 0 || err != io.EOF {
+		t.Errorf("Read: got %d, %v; want 0, io.EOF", n, err)
+	}
+
+	if n, err := (&Reader{}).ReadAt(nil, 11); n != 0 || err != io.EOF {
+		t.Errorf("ReadAt: got %d, %v; want 0, io.EOF", n, err)
+	}
+
+	if b, err := (&Reader{}).ReadByte(); b != 0 || err != io.EOF {
+		t.Errorf("ReadByte: got %d, %v; want 0, io.EOF", b, err)
+	}
+
+	if ch, size, err := (&Reader{}).ReadRune(); ch != 0 || size != 0 || err != io.EOF {
+		t.Errorf("ReadRune: got %d, %d, %v; want 0, 0, io.EOF", ch, size, err)
+	}
+
+	if offset, err := (&Reader{}).Seek(11, io.SeekStart); offset != 11 || err != nil {
+		t.Errorf("Seek: got %d, %v; want 11, nil", offset, err)
+	}
+
+	if s := (&Reader{}).Size(); s != 0 {
+		t.Errorf("Size: got %d, want 0", s)
+	}
+
+	if (&Reader{}).UnreadByte() == nil {
+		t.Errorf("UnreadByte: got nil, want error")
+	}
+
+	if (&Reader{}).UnreadRune() == nil {
+		t.Errorf("UnreadRune: got nil, want error")
+	}
+
+	if n, err := (&Reader{}).WriteTo(io.Discard); n != 0 || err != nil {
+		t.Errorf("WriteTo: got %d, %v; want 0, nil", n, err)
+	}
+}

go/src/cmd/README.vendor

@@ -0,0 +1,2 @@
+See src/README.vendor for information on loading vendored packages
+and updating the vendor directory.

go/src/cmd/go.mod

@@ -0,0 +1,21 @@
+module cmd
+
+go 1.26
+
+require (
+	github.com/google/pprof v0.0.0-20251114195745-4902fdda35c8
+	golang.org/x/arch v0.23.0
+	golang.org/x/build v0.0.0-20251128064159-b9bfd88b30e8
+	golang.org/x/mod v0.30.1-0.20251115032019-269c237cf350
+	golang.org/x/sync v0.19.0
+	golang.org/x/sys v0.39.0
+	golang.org/x/telemetry v0.0.0-20251128220624-abf20d0e57ec
+	golang.org/x/term v0.38.0
+	golang.org/x/tools v0.39.1-0.20260323181443-4f499ecaa91d
+)
+
+require (
+	github.com/ianlancetaylor/demangle v0.0.0-20250417193237-f615e6bd150b // indirect
+	golang.org/x/text v0.32.0 // indirect
+	rsc.io/markdown v0.0.0-20240306144322-0bf8f97ee8ef // indirect
+)

go/src/cmd/go.sum

@@ -0,0 +1,28 @@
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/google/pprof v0.0.0-20251114195745-4902fdda35c8 h1:3DsUAV+VNEQa2CUVLxCY3f87278uWfIDhJnbdvDjvmE=
+github.com/google/pprof v0.0.0-20251114195745-4902fdda35c8/go.mod h1:I6V7YzU0XDpsHqbsyrghnFZLO1gwK6NPTNvmetQIk9U=
+github.com/ianlancetaylor/demangle v0.0.0-20250417193237-f615e6bd150b h1:ogbOPx86mIhFy764gGkqnkFC8m5PJA7sPzlk9ppLVQA=
+github.com/ianlancetaylor/demangle v0.0.0-20250417193237-f615e6bd150b/go.mod h1:gx7rwoVhcfuVKG5uya9Hs3Sxj7EIvldVofAWIUtGouw=
+github.com/yuin/goldmark v1.6.0 h1:boZcn2GTjpsynOsC0iJHnBWa4Bi0qzfJjthwauItG68=
+github.com/yuin/goldmark v1.6.0/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
+golang.org/x/arch v0.23.0 h1:lKF64A2jF6Zd8L0knGltUnegD62JMFBiCPBmQpToHhg=
+golang.org/x/arch v0.23.0/go.mod h1:dNHoOeKiyja7GTvF9NJS1l3Z2yntpQNzgrjh1cU103A=
+golang.org/x/build v0.0.0-20251128064159-b9bfd88b30e8 h1:Mp+uRtHbKFW85lGBTOkOOfkPBz7AUKmZGcflkavmGRM=
+golang.org/x/build v0.0.0-20251128064159-b9bfd88b30e8/go.mod h1:Jx2RBBeTWGRSCwfSZ+w2Hg1f7LjWycsSkx+EciLAmPE=
+golang.org/x/mod v0.30.1-0.20251115032019-269c237cf350 h1:JGDMsCp8NahDR9HSvwrF6V8tzEf87m7Bo4oZ07vRxdU=
+golang.org/x/mod v0.30.1-0.20251115032019-269c237cf350/go.mod h1:lAsf5O2EvJeSFMiBxXDki7sCgAxEUcZHXoXMKT4GJKc=
+golang.org/x/sync v0.19.0 h1:vV+1eWNmZ5geRlYjzm2adRgW2/mcpevXNg50YZtPCE4=
+golang.org/x/sync v0.19.0/go.mod h1:9KTHXmSnoGruLpwFjVSX0lNNA75CykiMECbovNTZqGI=
+golang.org/x/sys v0.39.0 h1:CvCKL8MeisomCi6qNZ+wbb0DN9E5AATixKsvNtMoMFk=
+golang.org/x/sys v0.39.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/telemetry v0.0.0-20251128220624-abf20d0e57ec h1:dRVkWZl6bUOp+oxnOe4BuyhWSIPmt29N4ooHarm7Ic8=
+golang.org/x/telemetry v0.0.0-20251128220624-abf20d0e57ec/go.mod h1:hKdjCMrbv9skySur+Nek8Hd0uJ0GuxJIoIX2payrIdQ=
+golang.org/x/term v0.38.0 h1:PQ5pkm/rLO6HnxFR7N2lJHOZX6Kez5Y1gDSJla6jo7Q=
+golang.org/x/term v0.38.0/go.mod h1:bSEAKrOT1W+VSu9TSCMtoGEOUcKxOKgl3LE5QEF/xVg=
+golang.org/x/text v0.32.0 h1:ZD01bjUt1FQ9WJ0ClOL5vxgxOI/sVCNgX1YtKwcY0mU=
+golang.org/x/text v0.32.0/go.mod h1:o/rUWzghvpD5TXrTIBuJU77MTaN0ljMWE47kxGJQ7jY=
+golang.org/x/tools v0.39.1-0.20260323181443-4f499ecaa91d h1:d9RYG/Z8xQk+tFy5IyhSwUrt4HhSsqHw/uhwmn1KaJg=
+golang.org/x/tools v0.39.1-0.20260323181443-4f499ecaa91d/go.mod h1:JnefbkDPyD8UU2kI5fuf8ZX4/yUeh9W877ZeBONxUqQ=
+rsc.io/markdown v0.0.0-20240306144322-0bf8f97ee8ef h1:mqLYrXCXYEZOop9/Dbo6RPX11539nwiCNBb1icVPmw8=
+rsc.io/markdown v0.0.0-20240306144322-0bf8f97ee8ef/go.mod h1:8xcPgWmwlZONN1D9bjxtHEjrUtSEa3fakVF8iaewYKQ=

go/src/cmp/cmp.go

@@ -0,0 +1,77 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package cmp provides types and functions related to comparing
+// ordered values.
+package cmp
+
+// Ordered is a constraint that permits any ordered type: any type
+// that supports the operators < <= >= >.
+// If future releases of Go add new ordered types,
+// this constraint will be modified to include them.
+//
+// Note that floating-point types may contain NaN ("not-a-number") values.
+// An operator such as == or < will always report false when
+// comparing a NaN value with any other value, NaN or not.
+// See the [Compare] function for a consistent way to compare NaN values.
+type Ordered interface {
+	~int | ~int8 | ~int16 | ~int32 | ~int64 |
+		~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 | ~uintptr |
+		~float32 | ~float64 |
+		~string
+}
+
+// Less reports whether x is less than y.
+// For floating-point types, a NaN is considered less than any non-NaN,
+// and -0.0 is not less than (is equal to) 0.0.
+func Less[T Ordered](x, y T) bool {
+	return (isNaN(x) && !isNaN(y)) || x < y
+}
+
+// Compare returns
+//
+//	-1 if x is less than y,
+//	 0 if x equals y,
+//	+1 if x is greater than y.
+//
+// For floating-point types, a NaN is considered less than any non-NaN,
+// a NaN is considered equal to a NaN, and -0.0 is equal to 0.0.
+func Compare[T Ordered](x, y T) int {
+	xNaN := isNaN(x)
+	yNaN := isNaN(y)
+	if xNaN {
+		if yNaN {
+			return 0
+		}
+		return -1
+	}
+	if yNaN {
+		return +1
+	}
+	if x < y {
+		return -1
+	}
+	if x > y {
+		return +1
+	}
+	return 0
+}
+
+// isNaN reports whether x is a NaN without requiring the math package.
+// This will always return false if T is not floating-point.
+func isNaN[T Ordered](x T) bool {
+	return x != x
+}
+
+// Or returns the first of its arguments that is not equal to the zero value.
+// If no argument is non-zero, it returns the zero value.
+func Or[T comparable](vals ...T) T {
+	var zero T
+	for _, val := range vals {
+		if val != zero {
+			return val
+		}
+	}
+	return zero
+}

go/src/cmp/cmp_test.go

@@ -0,0 +1,202 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cmp_test
+
+import (
+	"cmp"
+	"fmt"
+	"math"
+	"slices"
+	"sort"
+	"strings"
+	"testing"
+	"unsafe"
+)
+
+var negzero = math.Copysign(0, -1)
+var nonnilptr uintptr = uintptr(unsafe.Pointer(&negzero))
+var nilptr uintptr = uintptr(unsafe.Pointer(nil))
+
+var tests = []struct {
+	x, y    any
+	compare int
+}{
+	{1, 2, -1},
+	{1, 1, 0},
+	{2, 1, +1},
+	{"a", "aa", -1},
+	{"a", "a", 0},
+	{"aa", "a", +1},
+	{1.0, 1.1, -1},
+	{1.1, 1.1, 0},
+	{1.1, 1.0, +1},
+	{math.Inf(1), math.Inf(1), 0},
+	{math.Inf(-1), math.Inf(-1), 0},
+	{math.Inf(-1), 1.0, -1},
+	{1.0, math.Inf(-1), +1},
+	{math.Inf(1), 1.0, +1},
+	{1.0, math.Inf(1), -1},
+	{math.NaN(), math.NaN(), 0},
+	{0.0, math.NaN(), +1},
+	{math.NaN(), 0.0, -1},
+	{math.NaN(), math.Inf(-1), -1},
+	{math.Inf(-1), math.NaN(), +1},
+	{0.0, 0.0, 0},
+	{negzero, negzero, 0},
+	{negzero, 0.0, 0},
+	{0.0, negzero, 0},
+	{negzero, 1.0, -1},
+	{negzero, -1.0, +1},
+	{nilptr, nonnilptr, -1},
+	{nonnilptr, nilptr, 1},
+	{nonnilptr, nonnilptr, 0},
+}
+
+func TestLess(t *testing.T) {
+	for _, test := range tests {
+		var b bool
+		switch test.x.(type) {
+		case int:
+			b = cmp.Less(test.x.(int), test.y.(int))
+		case string:
+			b = cmp.Less(test.x.(string), test.y.(string))
+		case float64:
+			b = cmp.Less(test.x.(float64), test.y.(float64))
+		case uintptr:
+			b = cmp.Less(test.x.(uintptr), test.y.(uintptr))
+		}
+		if b != (test.compare < 0) {
+			t.Errorf("Less(%v, %v) == %t, want %t", test.x, test.y, b, test.compare < 0)
+		}
+	}
+}
+
+func TestCompare(t *testing.T) {
+	for _, test := range tests {
+		var c int
+		switch test.x.(type) {
+		case int:
+			c = cmp.Compare(test.x.(int), test.y.(int))
+		case string:
+			c = cmp.Compare(test.x.(string), test.y.(string))
+		case float64:
+			c = cmp.Compare(test.x.(float64), test.y.(float64))
+		case uintptr:
+			c = cmp.Compare(test.x.(uintptr), test.y.(uintptr))
+		}
+		if c != test.compare {
+			t.Errorf("Compare(%v, %v) == %d, want %d", test.x, test.y, c, test.compare)
+		}
+	}
+}
+
+func TestSort(t *testing.T) {
+	// Test that our comparison function is consistent with
+	// sort.Float64s.
+	input := []float64{1.0, 0.0, negzero, math.Inf(1), math.Inf(-1), math.NaN()}
+	sort.Float64s(input)
+	for i := 0; i < len(input)-1; i++ {
+		if cmp.Less(input[i+1], input[i]) {
+			t.Errorf("Less sort mismatch at %d in %v", i, input)
+		}
+		if cmp.Compare(input[i], input[i+1]) > 0 {
+			t.Errorf("Compare sort mismatch at %d in %v", i, input)
+		}
+	}
+}
+
+func TestOr(t *testing.T) {
+	cases := []struct {
+		in   []int
+		want int
+	}{
+		{nil, 0},
+		{[]int{0}, 0},
+		{[]int{1}, 1},
+		{[]int{0, 2}, 2},
+		{[]int{3, 0}, 3},
+		{[]int{4, 5}, 4},
+		{[]int{0, 6, 7}, 6},
+	}
+	for _, tc := range cases {
+		if got := cmp.Or(tc.in...); got != tc.want {
+			t.Errorf("cmp.Or(%v) = %v; want %v", tc.in, got, tc.want)
+		}
+	}
+}
+
+func ExampleOr() {
+	// Suppose we have some user input
+	// that may or may not be an empty string
+	userInput1 := ""
+	userInput2 := "some text"
+
+	fmt.Println(cmp.Or(userInput1, "default"))
+	fmt.Println(cmp.Or(userInput2, "default"))
+	fmt.Println(cmp.Or(userInput1, userInput2, "default"))
+	// Output:
+	// default
+	// some text
+	// some text
+}
+
+func ExampleOr_sort() {
+	type Order struct {
+		Product  string
+		Customer string
+		Price    float64
+	}
+	orders := []Order{
+		{"foo", "alice", 1.00},
+		{"bar", "bob", 3.00},
+		{"baz", "carol", 4.00},
+		{"foo", "alice", 2.00},
+		{"bar", "carol", 1.00},
+		{"foo", "bob", 4.00},
+	}
+	// Sort by customer first, product second, and last by higher price
+	slices.SortFunc(orders, func(a, b Order) int {
+		return cmp.Or(
+			strings.Compare(a.Customer, b.Customer),
+			strings.Compare(a.Product, b.Product),
+			cmp.Compare(b.Price, a.Price),
+		)
+	})
+	for _, order := range orders {
+		fmt.Printf("%s %s %.2f\n", order.Product, order.Customer, order.Price)
+	}
+
+	// Output:
+	// foo alice 2.00
+	// foo alice 1.00
+	// bar bob 3.00
+	// foo bob 4.00
+	// bar carol 1.00
+	// baz carol 4.00
+}
+
+func ExampleLess() {
+	fmt.Println(cmp.Less(1, 2))
+	fmt.Println(cmp.Less("a", "aa"))
+	fmt.Println(cmp.Less(1.0, math.NaN()))
+	fmt.Println(cmp.Less(math.NaN(), 1.0))
+	// Output:
+	// true
+	// true
+	// false
+	// true
+}
+
+func ExampleCompare() {
+	fmt.Println(cmp.Compare(1, 2))
+	fmt.Println(cmp.Compare("a", "aa"))
+	fmt.Println(cmp.Compare(1.5, 1.5))
+	fmt.Println(cmp.Compare(math.NaN(), 1.0))
+	// Output:
+	// -1
+	// -1
+	// 0
+	// -1
+}

go/src/context/afterfunc_test.go

@@ -0,0 +1,141 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"sync"
+	"testing"
+	"time"
+)
+
+// afterFuncContext is a context that's not one of the types
+// defined in context.go, that supports registering AfterFuncs.
+type afterFuncContext struct {
+	mu         sync.Mutex
+	afterFuncs map[*byte]func()
+	done       chan struct{}
+	err        error
+}
+
+var _ context.Context = (*afterFuncContext)(nil)
+
+func (c *afterFuncContext) Deadline() (time.Time, bool) {
+	return time.Time{}, false
+}
+
+func (c *afterFuncContext) Done() <-chan struct{} {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.done == nil {
+		c.done = make(chan struct{})
+	}
+	return c.done
+}
+
+func (c *afterFuncContext) Err() error {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.err
+}
+
+func (c *afterFuncContext) Value(key any) any {
+	return nil
+}
+
+func (c *afterFuncContext) AfterFunc(f func()) func() bool {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	k := new(byte)
+	if c.afterFuncs == nil {
+		c.afterFuncs = make(map[*byte]func())
+	}
+	c.afterFuncs[k] = f
+	return func() bool {
+		c.mu.Lock()
+		defer c.mu.Unlock()
+		_, ok := c.afterFuncs[k]
+		delete(c.afterFuncs, k)
+		return ok
+	}
+}
+
+func (c *afterFuncContext) cancel(err error) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.err != nil {
+		return
+	}
+	c.err = err
+	for _, f := range c.afterFuncs {
+		go f()
+	}
+	c.afterFuncs = nil
+}
+
+func TestCustomContextAfterFuncCancel(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	ctx1, cancel := context.WithCancel(ctx0)
+	defer cancel()
+	ctx0.cancel(context.Canceled)
+	<-ctx1.Done()
+}
+
+func TestCustomContextAfterFuncTimeout(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	ctx1, cancel := context.WithTimeout(ctx0, veryLongDuration)
+	defer cancel()
+	ctx0.cancel(context.Canceled)
+	<-ctx1.Done()
+}
+
+func TestCustomContextAfterFuncAfterFunc(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	donec := make(chan struct{})
+	stop := context.AfterFunc(ctx0, func() {
+		close(donec)
+	})
+	defer stop()
+	ctx0.cancel(context.Canceled)
+	<-donec
+}
+
+func TestCustomContextAfterFuncUnregisterCancel(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	_, cancel1 := context.WithCancel(ctx0)
+	_, cancel2 := context.WithCancel(ctx0)
+	if got, want := len(ctx0.afterFuncs), 2; got != want {
+		t.Errorf("after WithCancel(ctx0): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+	cancel1()
+	cancel2()
+	if got, want := len(ctx0.afterFuncs), 0; got != want {
+		t.Errorf("after canceling WithCancel(ctx0): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+}
+
+func TestCustomContextAfterFuncUnregisterTimeout(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	_, cancel := context.WithTimeout(ctx0, veryLongDuration)
+	if got, want := len(ctx0.afterFuncs), 1; got != want {
+		t.Errorf("after WithTimeout(ctx0, d): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+	cancel()
+	if got, want := len(ctx0.afterFuncs), 0; got != want {
+		t.Errorf("after canceling WithTimeout(ctx0, d): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+}
+
+func TestCustomContextAfterFuncUnregisterAfterFunc(t *testing.T) {
+	ctx0 := &afterFuncContext{}
+	stop := context.AfterFunc(ctx0, func() {})
+	if got, want := len(ctx0.afterFuncs), 1; got != want {
+		t.Errorf("after AfterFunc(ctx0, f): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+	stop()
+	if got, want := len(ctx0.afterFuncs), 0; got != want {
+		t.Errorf("after stopping AfterFunc(ctx0, f): ctx0 has %v afterFuncs, want %v", got, want)
+	}
+}

go/src/context/benchmark_test.go

@@ -0,0 +1,210 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	. "context"
+	"fmt"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+func BenchmarkCommonParentCancel(b *testing.B) {
+	root := WithValue(Background(), "key", "value")
+	shared, sharedcancel := WithCancel(root)
+	defer sharedcancel()
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		x := 0
+		for pb.Next() {
+			ctx, cancel := WithCancel(shared)
+			if ctx.Value("key").(string) != "value" {
+				b.Fatal("should not be reached")
+			}
+			for i := 0; i < 100; i++ {
+				x /= x + 1
+			}
+			cancel()
+			for i := 0; i < 100; i++ {
+				x /= x + 1
+			}
+		}
+	})
+}
+
+func BenchmarkWithTimeout(b *testing.B) {
+	for concurrency := 40; concurrency <= 4e5; concurrency *= 100 {
+		name := fmt.Sprintf("concurrency=%d", concurrency)
+		b.Run(name, func(b *testing.B) {
+			benchmarkWithTimeout(b, concurrency)
+		})
+	}
+}
+
+func benchmarkWithTimeout(b *testing.B, concurrentContexts int) {
+	gomaxprocs := runtime.GOMAXPROCS(0)
+	perPContexts := concurrentContexts / gomaxprocs
+	root := Background()
+
+	// Generate concurrent contexts.
+	var wg sync.WaitGroup
+	ccf := make([][]CancelFunc, gomaxprocs)
+	for i := range ccf {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+			cf := make([]CancelFunc, perPContexts)
+			for j := range cf {
+				_, cf[j] = WithTimeout(root, time.Hour)
+			}
+			ccf[i] = cf
+		}(i)
+	}
+	wg.Wait()
+
+	b.ResetTimer()
+	b.RunParallel(func(pb *testing.PB) {
+		wcf := make([]CancelFunc, 10)
+		for pb.Next() {
+			for i := range wcf {
+				_, wcf[i] = WithTimeout(root, time.Hour)
+			}
+			for _, f := range wcf {
+				f()
+			}
+		}
+	})
+	b.StopTimer()
+
+	for _, cf := range ccf {
+		for _, f := range cf {
+			f()
+		}
+	}
+}
+
+func BenchmarkCancelTree(b *testing.B) {
+	depths := []int{1, 10, 100, 1000}
+	for _, d := range depths {
+		b.Run(fmt.Sprintf("depth=%d", d), func(b *testing.B) {
+			b.Run("Root=Background", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					buildContextTree(Background(), d)
+				}
+			})
+			b.Run("Root=OpenCanceler", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					ctx, cancel := WithCancel(Background())
+					buildContextTree(ctx, d)
+					cancel()
+				}
+			})
+			b.Run("Root=ClosedCanceler", func(b *testing.B) {
+				for i := 0; i < b.N; i++ {
+					ctx, cancel := WithCancel(Background())
+					cancel()
+					buildContextTree(ctx, d)
+				}
+			})
+		})
+	}
+}
+
+func buildContextTree(root Context, depth int) {
+	for d := 0; d < depth; d++ {
+		root, _ = WithCancel(root)
+	}
+}
+
+func BenchmarkCheckCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	b.Run("Err", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			ctx.Err()
+		}
+	})
+	b.Run("Done", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}
+
+func BenchmarkContextCancelDone(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	defer cancel()
+
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			select {
+			case <-ctx.Done():
+			default:
+			}
+		}
+	})
+}
+
+func BenchmarkDeepValueNewGoRoutine(b *testing.B) {
+	for _, depth := range []int{10, 20, 30, 50, 100} {
+		ctx := Background()
+		for i := 0; i < depth; i++ {
+			ctx = WithValue(ctx, i, i)
+		}
+
+		b.Run(fmt.Sprintf("depth=%d", depth), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				var wg sync.WaitGroup
+				wg.Add(1)
+				go func() {
+					defer wg.Done()
+					ctx.Value(-1)
+				}()
+				wg.Wait()
+			}
+		})
+	}
+}
+
+func BenchmarkDeepValueSameGoRoutine(b *testing.B) {
+	for _, depth := range []int{10, 20, 30, 50, 100} {
+		ctx := Background()
+		for i := 0; i < depth; i++ {
+			ctx = WithValue(ctx, i, i)
+		}
+
+		b.Run(fmt.Sprintf("depth=%d", depth), func(b *testing.B) {
+			for i := 0; i < b.N; i++ {
+				ctx.Value(-1)
+			}
+		})
+	}
+}
+
+func BenchmarkErrOK(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	defer cancel()
+	for b.Loop() {
+		if err := ctx.Err(); err != nil {
+			b.Fatalf("ctx.Err() = %v", err)
+		}
+	}
+}
+
+func BenchmarkErrCanceled(b *testing.B) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	for b.Loop() {
+		if err := ctx.Err(); err == nil {
+			b.Fatalf("ctx.Err() = %v", err)
+		}
+	}
+}

go/src/context/context.go

@@ -0,0 +1,806 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package context defines the Context type, which carries deadlines,
+// cancellation signals, and other request-scoped values across API boundaries
+// and between processes.
+//
+// Incoming requests to a server should create a [Context], and outgoing
+// calls to servers should accept a Context. The chain of function
+// calls between them must propagate the Context, optionally replacing
+// it with a derived Context created using [WithCancel], [WithDeadline],
+// [WithTimeout], or [WithValue].
+//
+// A Context may be canceled to indicate that work done on its behalf should stop.
+// A Context with a deadline is canceled after the deadline passes.
+// When a Context is canceled, all Contexts derived from it are also canceled.
+//
+// The [WithCancel], [WithDeadline], and [WithTimeout] functions take a
+// Context (the parent) and return a derived Context (the child) and a
+// [CancelFunc]. Calling the CancelFunc directly cancels the child and its
+// children, removes the parent's reference to the child, and stops
+// any associated timers. Failing to call the CancelFunc leaks the
+// child and its children until the parent is canceled. The go vet tool
+// checks that CancelFuncs are used on all control-flow paths.
+//
+// The [WithCancelCause], [WithDeadlineCause], and [WithTimeoutCause] functions
+// return a [CancelCauseFunc], which takes an error and records it as
+// the cancellation cause. Calling [Cause] on the canceled context
+// or any of its children retrieves the cause. If no cause is specified,
+// Cause(ctx) returns the same value as ctx.Err().
+//
+// Programs that use Contexts should follow these rules to keep interfaces
+// consistent across packages and enable static analysis tools to check context
+// propagation:
+//
+// Do not store Contexts inside a struct type; instead, pass a Context
+// explicitly to each function that needs it. This is discussed further in
+// https://go.dev/blog/context-and-structs. The Context should be the first
+// parameter, typically named ctx:
+//
+//	func DoSomething(ctx context.Context, arg Arg) error {
+//		// ... use ctx ...
+//	}
+//
+// Do not pass a nil [Context], even if a function permits it. Pass [context.TODO]
+// if you are unsure about which Context to use.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The same Context may be passed to functions running in different goroutines;
+// Contexts are safe for simultaneous use by multiple goroutines.
+//
+// See https://go.dev/blog/context for example code for a server that uses
+// Contexts.
+package context
+
+import (
+	"errors"
+	"internal/reflectlite"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// A Context carries a deadline, a cancellation signal, and other values across
+// API boundaries.
+//
+// Context's methods may be called by multiple goroutines simultaneously.
+type Context interface {
+	// Deadline returns the time when work done on behalf of this context
+	// should be canceled. Deadline returns ok==false when no deadline is
+	// set. Successive calls to Deadline return the same results.
+	Deadline() (deadline time.Time, ok bool)
+
+	// Done returns a channel that's closed when work done on behalf of this
+	// context should be canceled. Done may return nil if this context can
+	// never be canceled. Successive calls to Done return the same value.
+	// The close of the Done channel may happen asynchronously,
+	// after the cancel function returns.
+	//
+	// WithCancel arranges for Done to be closed when cancel is called;
+	// WithDeadline arranges for Done to be closed when the deadline
+	// expires; WithTimeout arranges for Done to be closed when the timeout
+	// elapses.
+	//
+	// Done is provided for use in select statements:
+	//
+	//  // Stream generates values with DoSomething and sends them to out
+	//  // until DoSomething returns an error or ctx.Done is closed.
+	//  func Stream(ctx context.Context, out chan<- Value) error {
+	//  	for {
+	//  		v, err := DoSomething(ctx)
+	//  		if err != nil {
+	//  			return err
+	//  		}
+	//  		select {
+	//  		case <-ctx.Done():
+	//  			return ctx.Err()
+	//  		case out <- v:
+	//  		}
+	//  	}
+	//  }
+	//
+	// See https://go.dev/blog/pipelines for more examples of how to use
+	// a Done channel for cancellation.
+	Done() <-chan struct{}
+
+	// If Done is not yet closed, Err returns nil.
+	// If Done is closed, Err returns a non-nil error explaining why:
+	// DeadlineExceeded if the context's deadline passed,
+	// or Canceled if the context was canceled for some other reason.
+	// After Err returns a non-nil error, successive calls to Err return the same error.
+	Err() error
+
+	// Value returns the value associated with this context for key, or nil
+	// if no value is associated with key. Successive calls to Value with
+	// the same key returns the same result.
+	//
+	// Use context values only for request-scoped data that transits
+	// processes and API boundaries, not for passing optional parameters to
+	// functions.
+	//
+	// A key identifies a specific value in a Context. Functions that wish
+	// to store values in Context typically allocate a key in a global
+	// variable then use that key as the argument to context.WithValue and
+	// Context.Value. A key can be any type that supports equality;
+	// packages should define keys as an unexported type to avoid
+	// collisions.
+	//
+	// Packages that define a Context key should provide type-safe accessors
+	// for the values stored using that key:
+	//
+	// 	// Package user defines a User type that's stored in Contexts.
+	// 	package user
+	//
+	// 	import "context"
+	//
+	// 	// User is the type of value stored in the Contexts.
+	// 	type User struct {...}
+	//
+	// 	// key is an unexported type for keys defined in this package.
+	// 	// This prevents collisions with keys defined in other packages.
+	// 	type key int
+	//
+	// 	// userKey is the key for user.User values in Contexts. It is
+	// 	// unexported; clients use user.NewContext and user.FromContext
+	// 	// instead of using this key directly.
+	// 	var userKey key
+	//
+	// 	// NewContext returns a new Context that carries value u.
+	// 	func NewContext(ctx context.Context, u *User) context.Context {
+	// 		return context.WithValue(ctx, userKey, u)
+	// 	}
+	//
+	// 	// FromContext returns the User value stored in ctx, if any.
+	// 	func FromContext(ctx context.Context) (*User, bool) {
+	// 		u, ok := ctx.Value(userKey).(*User)
+	// 		return u, ok
+	// 	}
+	Value(key any) any
+}
+
+// Canceled is the error returned by [Context.Err] when the context is canceled
+// for some reason other than its deadline passing.
+var Canceled = errors.New("context canceled")
+
+// DeadlineExceeded is the error returned by [Context.Err] when the context is canceled
+// due to its deadline passing.
+var DeadlineExceeded error = deadlineExceededError{}
+
+type deadlineExceededError struct{}
+
+func (deadlineExceededError) Error() string   { return "context deadline exceeded" }
+func (deadlineExceededError) Timeout() bool   { return true }
+func (deadlineExceededError) Temporary() bool { return true }
+
+// An emptyCtx is never canceled, has no values, and has no deadline.
+// It is the common base of backgroundCtx and todoCtx.
+type emptyCtx struct{}
+
+func (emptyCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (emptyCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (emptyCtx) Err() error {
+	return nil
+}
+
+func (emptyCtx) Value(key any) any {
+	return nil
+}
+
+type backgroundCtx struct{ emptyCtx }
+
+func (backgroundCtx) String() string {
+	return "context.Background"
+}
+
+type todoCtx struct{ emptyCtx }
+
+func (todoCtx) String() string {
+	return "context.TODO"
+}
+
+// Background returns a non-nil, empty [Context]. It is never canceled, has no
+// values, and has no deadline. It is typically used by the main function,
+// initialization, and tests, and as the top-level Context for incoming
+// requests.
+func Background() Context {
+	return backgroundCtx{}
+}
+
+// TODO returns a non-nil, empty [Context]. Code should use context.TODO when
+// it's unclear which Context to use or it is not yet available (because the
+// surrounding function has not yet been extended to accept a Context
+// parameter).
+func TODO() Context {
+	return todoCtx{}
+}
+
+// A CancelFunc tells an operation to abandon its work.
+// A CancelFunc does not wait for the work to stop.
+// A CancelFunc may be called by multiple goroutines simultaneously.
+// After the first call, subsequent calls to a CancelFunc do nothing.
+type CancelFunc func()
+
+// WithCancel returns a derived context that points to the parent context
+// but has a new Done channel. The returned context's Done channel is closed
+// when the returned cancel function is called or when the parent context's
+// Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this [Context] complete.
+func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
+	c := withCancel(parent)
+	return c, func() { c.cancel(true, Canceled, nil) }
+}
+
+// A CancelCauseFunc behaves like a [CancelFunc] but additionally sets the cancellation cause.
+// This cause can be retrieved by calling [Cause] on the canceled Context or on
+// any of its derived Contexts.
+//
+// If the context has already been canceled, CancelCauseFunc does not set the cause.
+// For example, if childContext is derived from parentContext:
+//   - if parentContext is canceled with cause1 before childContext is canceled with cause2,
+//     then Cause(parentContext) == Cause(childContext) == cause1
+//   - if childContext is canceled with cause2 before parentContext is canceled with cause1,
+//     then Cause(parentContext) == cause1 and Cause(childContext) == cause2
+type CancelCauseFunc func(cause error)
+
+// WithCancelCause behaves like [WithCancel] but returns a [CancelCauseFunc] instead of a [CancelFunc].
+// Calling cancel with a non-nil error (the "cause") records that error in ctx;
+// it can then be retrieved using Cause(ctx).
+// Calling cancel with nil sets the cause to Canceled.
+//
+// Example use:
+//
+//	ctx, cancel := context.WithCancelCause(parent)
+//	cancel(myError)
+//	ctx.Err() // returns context.Canceled
+//	context.Cause(ctx) // returns myError
+func WithCancelCause(parent Context) (ctx Context, cancel CancelCauseFunc) {
+	c := withCancel(parent)
+	return c, func(cause error) { c.cancel(true, Canceled, cause) }
+}
+
+func withCancel(parent Context) *cancelCtx {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	c := &cancelCtx{}
+	c.propagateCancel(parent, c)
+	return c
+}
+
+// Cause returns a non-nil error explaining why c was canceled.
+// The first cancellation of c or one of its parents sets the cause.
+// If that cancellation happened via a call to CancelCauseFunc(err),
+// then [Cause] returns err.
+// Otherwise Cause(c) returns the same value as c.Err().
+// Cause returns nil if c has not been canceled yet.
+func Cause(c Context) error {
+	err := c.Err()
+	if err == nil {
+		return nil
+	}
+	if cc, ok := c.Value(&cancelCtxKey).(*cancelCtx); ok {
+		cc.mu.Lock()
+		cause := cc.cause
+		cc.mu.Unlock()
+		if cause != nil {
+			return cause
+		}
+		// The parent cancelCtx doesn't have a cause,
+		// so c must have been canceled in some custom context implementation.
+	}
+	// We don't have a cause to return from a parent cancelCtx,
+	// so return the context's error.
+	return err
+}
+
+// AfterFunc arranges to call f in its own goroutine after ctx is canceled.
+// If ctx is already canceled, AfterFunc calls f immediately in its own goroutine.
+//
+// Multiple calls to AfterFunc on a context operate independently;
+// one does not replace another.
+//
+// Calling the returned stop function stops the association of ctx with f.
+// It returns true if the call stopped f from being run.
+// If stop returns false,
+// either the context is canceled and f has been started in its own goroutine;
+// or f was already stopped.
+// The stop function does not wait for f to complete before returning.
+// If the caller needs to know whether f is completed,
+// it must coordinate with f explicitly.
+//
+// If ctx has a "AfterFunc(func()) func() bool" method,
+// AfterFunc will use it to schedule the call.
+func AfterFunc(ctx Context, f func()) (stop func() bool) {
+	a := &afterFuncCtx{
+		f: f,
+	}
+	a.cancelCtx.propagateCancel(ctx, a)
+	return func() bool {
+		stopped := false
+		a.once.Do(func() {
+			stopped = true
+		})
+		if stopped {
+			a.cancel(true, Canceled, nil)
+		}
+		return stopped
+	}
+}
+
+type afterFuncer interface {
+	AfterFunc(func()) func() bool
+}
+
+type afterFuncCtx struct {
+	cancelCtx
+	once sync.Once // either starts running f or stops f from running
+	f    func()
+}
+
+func (a *afterFuncCtx) cancel(removeFromParent bool, err, cause error) {
+	a.cancelCtx.cancel(false, err, cause)
+	if removeFromParent {
+		removeChild(a.Context, a)
+	}
+	a.once.Do(func() {
+		go a.f()
+	})
+}
+
+// A stopCtx is used as the parent context of a cancelCtx when
+// an AfterFunc has been registered with the parent.
+// It holds the stop function used to unregister the AfterFunc.
+type stopCtx struct {
+	Context
+	stop func() bool
+}
+
+// goroutines counts the number of goroutines ever created; for testing.
+var goroutines atomic.Int32
+
+// &cancelCtxKey is the key that a cancelCtx returns itself for.
+var cancelCtxKey int
+
+// parentCancelCtx returns the underlying *cancelCtx for parent.
+// It does this by looking up parent.Value(&cancelCtxKey) to find
+// the innermost enclosing *cancelCtx and then checking whether
+// parent.Done() matches that *cancelCtx. (If not, the *cancelCtx
+// has been wrapped in a custom implementation providing a
+// different done channel, in which case we should not bypass it.)
+func parentCancelCtx(parent Context) (*cancelCtx, bool) {
+	done := parent.Done()
+	if done == closedchan || done == nil {
+		return nil, false
+	}
+	p, ok := parent.Value(&cancelCtxKey).(*cancelCtx)
+	if !ok {
+		return nil, false
+	}
+	pdone, _ := p.done.Load().(chan struct{})
+	if pdone != done {
+		return nil, false
+	}
+	return p, true
+}
+
+// removeChild removes a context from its parent.
+func removeChild(parent Context, child canceler) {
+	if s, ok := parent.(stopCtx); ok {
+		s.stop()
+		return
+	}
+	p, ok := parentCancelCtx(parent)
+	if !ok {
+		return
+	}
+	p.mu.Lock()
+	if p.children != nil {
+		delete(p.children, child)
+	}
+	p.mu.Unlock()
+}
+
+// A canceler is a context type that can be canceled directly. The
+// implementations are *cancelCtx and *timerCtx.
+type canceler interface {
+	cancel(removeFromParent bool, err, cause error)
+	Done() <-chan struct{}
+}
+
+// closedchan is a reusable closed channel.
+var closedchan = make(chan struct{})
+
+func init() {
+	close(closedchan)
+}
+
+// A cancelCtx can be canceled. When canceled, it also cancels any children
+// that implement canceler.
+type cancelCtx struct {
+	Context
+
+	mu       sync.Mutex            // protects following fields
+	done     atomic.Value          // of chan struct{}, created lazily, closed by first cancel call
+	children map[canceler]struct{} // set to nil by the first cancel call
+	err      atomic.Value          // set to non-nil by the first cancel call
+	cause    error                 // set to non-nil by the first cancel call
+}
+
+func (c *cancelCtx) Value(key any) any {
+	if key == &cancelCtxKey {
+		return c
+	}
+	return value(c.Context, key)
+}
+
+func (c *cancelCtx) Done() <-chan struct{} {
+	d := c.done.Load()
+	if d != nil {
+		return d.(chan struct{})
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	d = c.done.Load()
+	if d == nil {
+		d = make(chan struct{})
+		c.done.Store(d)
+	}
+	return d.(chan struct{})
+}
+
+func (c *cancelCtx) Err() error {
+	// An atomic load is ~5x faster than a mutex, which can matter in tight loops.
+	if err := c.err.Load(); err != nil {
+		// Ensure the done channel has been closed before returning a non-nil error.
+		<-c.Done()
+		return err.(error)
+	}
+	return nil
+}
+
+// propagateCancel arranges for child to be canceled when parent is.
+// It sets the parent context of cancelCtx.
+func (c *cancelCtx) propagateCancel(parent Context, child canceler) {
+	c.Context = parent
+
+	done := parent.Done()
+	if done == nil {
+		return // parent is never canceled
+	}
+
+	select {
+	case <-done:
+		// parent is already canceled
+		child.cancel(false, parent.Err(), Cause(parent))
+		return
+	default:
+	}
+
+	if p, ok := parentCancelCtx(parent); ok {
+		// parent is a *cancelCtx, or derives from one.
+		p.mu.Lock()
+		if err := p.err.Load(); err != nil {
+			// parent has already been canceled
+			child.cancel(false, err.(error), p.cause)
+		} else {
+			if p.children == nil {
+				p.children = make(map[canceler]struct{})
+			}
+			p.children[child] = struct{}{}
+		}
+		p.mu.Unlock()
+		return
+	}
+
+	if a, ok := parent.(afterFuncer); ok {
+		// parent implements an AfterFunc method.
+		c.mu.Lock()
+		stop := a.AfterFunc(func() {
+			child.cancel(false, parent.Err(), Cause(parent))
+		})
+		c.Context = stopCtx{
+			Context: parent,
+			stop:    stop,
+		}
+		c.mu.Unlock()
+		return
+	}
+
+	goroutines.Add(1)
+	go func() {
+		select {
+		case <-parent.Done():
+			child.cancel(false, parent.Err(), Cause(parent))
+		case <-child.Done():
+		}
+	}()
+}
+
+type stringer interface {
+	String() string
+}
+
+func contextName(c Context) string {
+	if s, ok := c.(stringer); ok {
+		return s.String()
+	}
+	return reflectlite.TypeOf(c).String()
+}
+
+func (c *cancelCtx) String() string {
+	return contextName(c.Context) + ".WithCancel"
+}
+
+// cancel closes c.done, cancels each of c's children, and, if
+// removeFromParent is true, removes c from its parent's children.
+// cancel sets c.cause to cause if this is the first time c is canceled.
+func (c *cancelCtx) cancel(removeFromParent bool, err, cause error) {
+	if err == nil {
+		panic("context: internal error: missing cancel error")
+	}
+	if cause == nil {
+		cause = err
+	}
+	c.mu.Lock()
+	if c.err.Load() != nil {
+		c.mu.Unlock()
+		return // already canceled
+	}
+	c.err.Store(err)
+	c.cause = cause
+	d, _ := c.done.Load().(chan struct{})
+	if d == nil {
+		c.done.Store(closedchan)
+	} else {
+		close(d)
+	}
+	for child := range c.children {
+		// NOTE: acquiring the child's lock while holding parent's lock.
+		child.cancel(false, err, cause)
+	}
+	c.children = nil
+	c.mu.Unlock()
+
+	if removeFromParent {
+		removeChild(c.Context, c)
+	}
+}
+
+// WithoutCancel returns a derived context that points to the parent context
+// and is not canceled when parent is canceled.
+// The returned context returns no Deadline or Err, and its Done channel is nil.
+// Calling [Cause] on the returned context returns nil.
+func WithoutCancel(parent Context) Context {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	return withoutCancelCtx{parent}
+}
+
+type withoutCancelCtx struct {
+	c Context
+}
+
+func (withoutCancelCtx) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (withoutCancelCtx) Done() <-chan struct{} {
+	return nil
+}
+
+func (withoutCancelCtx) Err() error {
+	return nil
+}
+
+func (c withoutCancelCtx) Value(key any) any {
+	return value(c, key)
+}
+
+func (c withoutCancelCtx) String() string {
+	return contextName(c.c) + ".WithoutCancel"
+}
+
+// WithDeadline returns a derived context that points to the parent context
+// but has the deadline adjusted to be no later than d. If the parent's
+// deadline is already earlier than d, WithDeadline(parent, d) is semantically
+// equivalent to parent. The returned [Context.Done] channel is closed when
+// the deadline expires, when the returned cancel function is called,
+// or when the parent context's Done channel is closed, whichever happens first.
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this [Context] complete.
+func WithDeadline(parent Context, d time.Time) (Context, CancelFunc) {
+	return WithDeadlineCause(parent, d, nil)
+}
+
+// WithDeadlineCause behaves like [WithDeadline] but also sets the cause of the
+// returned Context when the deadline is exceeded. The returned [CancelFunc] does
+// not set the cause.
+func WithDeadlineCause(parent Context, d time.Time, cause error) (Context, CancelFunc) {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	if cur, ok := parent.Deadline(); ok && cur.Before(d) {
+		// The current deadline is already sooner than the new one.
+		return WithCancel(parent)
+	}
+	c := &timerCtx{
+		deadline: d,
+	}
+	c.cancelCtx.propagateCancel(parent, c)
+	dur := time.Until(d)
+	if dur <= 0 {
+		c.cancel(true, DeadlineExceeded, cause) // deadline has already passed
+		return c, func() { c.cancel(false, Canceled, nil) }
+	}
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	if c.err.Load() == nil {
+		c.timer = time.AfterFunc(dur, func() {
+			c.cancel(true, DeadlineExceeded, cause)
+		})
+	}
+	return c, func() { c.cancel(true, Canceled, nil) }
+}
+
+// A timerCtx carries a timer and a deadline. It embeds a cancelCtx to
+// implement Done and Err. It implements cancel by stopping its timer then
+// delegating to cancelCtx.cancel.
+type timerCtx struct {
+	cancelCtx
+	timer *time.Timer // Under cancelCtx.mu.
+
+	deadline time.Time
+}
+
+func (c *timerCtx) Deadline() (deadline time.Time, ok bool) {
+	return c.deadline, true
+}
+
+func (c *timerCtx) String() string {
+	return contextName(c.cancelCtx.Context) + ".WithDeadline(" +
+		c.deadline.String() + " [" +
+		time.Until(c.deadline).String() + "])"
+}
+
+func (c *timerCtx) cancel(removeFromParent bool, err, cause error) {
+	c.cancelCtx.cancel(false, err, cause)
+	if removeFromParent {
+		// Remove this timerCtx from its parent cancelCtx's children.
+		removeChild(c.cancelCtx.Context, c)
+	}
+	c.mu.Lock()
+	if c.timer != nil {
+		c.timer.Stop()
+		c.timer = nil
+	}
+	c.mu.Unlock()
+}
+
+// WithTimeout returns WithDeadline(parent, time.Now().Add(timeout)).
+//
+// Canceling this context releases resources associated with it, so code should
+// call cancel as soon as the operations running in this [Context] complete:
+//
+//	func slowOperationWithTimeout(ctx context.Context) (Result, error) {
+//		ctx, cancel := context.WithTimeout(ctx, 100*time.Millisecond)
+//		defer cancel()  // releases resources if slowOperation completes before timeout elapses
+//		return slowOperation(ctx)
+//	}
+func WithTimeout(parent Context, timeout time.Duration) (Context, CancelFunc) {
+	return WithDeadline(parent, time.Now().Add(timeout))
+}
+
+// WithTimeoutCause behaves like [WithTimeout] but also sets the cause of the
+// returned Context when the timeout expires. The returned [CancelFunc] does
+// not set the cause.
+func WithTimeoutCause(parent Context, timeout time.Duration, cause error) (Context, CancelFunc) {
+	return WithDeadlineCause(parent, time.Now().Add(timeout), cause)
+}
+
+// WithValue returns a derived context that points to the parent Context.
+// In the derived context, the value associated with key is val.
+//
+// Use context Values only for request-scoped data that transits processes and
+// APIs, not for passing optional parameters to functions.
+//
+// The provided key must be comparable and should not be of type
+// string or any other built-in type to avoid collisions between
+// packages using context. Users of WithValue should define their own
+// types for keys. To avoid allocating when assigning to an
+// interface{}, context keys often have concrete type
+// struct{}. Alternatively, exported context key variables' static
+// type should be a pointer or interface.
+func WithValue(parent Context, key, val any) Context {
+	if parent == nil {
+		panic("cannot create context from nil parent")
+	}
+	if key == nil {
+		panic("nil key")
+	}
+	if !reflectlite.TypeOf(key).Comparable() {
+		panic("key is not comparable")
+	}
+	return &valueCtx{parent, key, val}
+}
+
+// A valueCtx carries a key-value pair. It implements Value for that key and
+// delegates all other calls to the embedded Context.
+type valueCtx struct {
+	Context
+	key, val any
+}
+
+// stringify tries a bit to stringify v, without using fmt, since we don't
+// want context depending on the unicode tables. This is only used by
+// *valueCtx.String().
+func stringify(v any) string {
+	switch s := v.(type) {
+	case stringer:
+		return s.String()
+	case string:
+		return s
+	case nil:
+		return "<nil>"
+	}
+	return reflectlite.TypeOf(v).String()
+}
+
+func (c *valueCtx) String() string {
+	return contextName(c.Context) + ".WithValue(" +
+		stringify(c.key) + ", " +
+		stringify(c.val) + ")"
+}
+
+func (c *valueCtx) Value(key any) any {
+	if c.key == key {
+		return c.val
+	}
+	return value(c.Context, key)
+}
+
+func value(c Context, key any) any {
+	for {
+		switch ctx := c.(type) {
+		case *valueCtx:
+			if key == ctx.key {
+				return ctx.val
+			}
+			c = ctx.Context
+		case *cancelCtx:
+			if key == &cancelCtxKey {
+				return c
+			}
+			c = ctx.Context
+		case withoutCancelCtx:
+			if key == &cancelCtxKey {
+				// This implements Cause(ctx) == nil
+				// when ctx is created using WithoutCancel.
+				return nil
+			}
+			c = ctx.c
+		case *timerCtx:
+			if key == &cancelCtxKey {
+				return &ctx.cancelCtx
+			}
+			c = ctx.Context
+		case backgroundCtx, todoCtx:
+			return nil
+		default:
+			return c.Value(key)
+		}
+	}
+}

go/src/context/context_test.go

@@ -0,0 +1,297 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context
+
+// Tests in package context cannot depend directly on package testing due to an import cycle.
+// If your test requires access to unexported members of the context package,
+// add your test below as `func XTestFoo(t testingT)` and add a `TestFoo` to x_test.go
+// that calls it. Otherwise, write a regular test in a test.go file in package context_test.
+
+import (
+	"time"
+)
+
+type testingT interface {
+	Deadline() (time.Time, bool)
+	Error(args ...any)
+	Errorf(format string, args ...any)
+	Fail()
+	FailNow()
+	Failed() bool
+	Fatal(args ...any)
+	Fatalf(format string, args ...any)
+	Helper()
+	Log(args ...any)
+	Logf(format string, args ...any)
+	Name() string
+	Parallel()
+	Skip(args ...any)
+	SkipNow()
+	Skipf(format string, args ...any)
+	Skipped() bool
+}
+
+const veryLongDuration = 1000 * time.Hour // an arbitrary upper bound on the test's running time
+
+func contains(m map[canceler]struct{}, key canceler) bool {
+	_, ret := m[key]
+	return ret
+}
+
+func XTestParentFinishesChild(t testingT) {
+	// Context tree:
+	// parent -> cancelChild
+	// parent -> valueChild -> timerChild
+	// parent -> afterChild
+	parent, cancel := WithCancel(Background())
+	cancelChild, stop := WithCancel(parent)
+	defer stop()
+	valueChild := WithValue(parent, "key", "value")
+	timerChild, stop := WithTimeout(valueChild, veryLongDuration)
+	defer stop()
+	afterStop := AfterFunc(parent, func() {})
+	defer afterStop()
+
+	select {
+	case x := <-parent.Done():
+		t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
+	case x := <-cancelChild.Done():
+		t.Errorf("<-cancelChild.Done() == %v want nothing (it should block)", x)
+	case x := <-timerChild.Done():
+		t.Errorf("<-timerChild.Done() == %v want nothing (it should block)", x)
+	case x := <-valueChild.Done():
+		t.Errorf("<-valueChild.Done() == %v want nothing (it should block)", x)
+	default:
+	}
+
+	// The parent's children should contain the three cancelable children.
+	pc := parent.(*cancelCtx)
+	cc := cancelChild.(*cancelCtx)
+	tc := timerChild.(*timerCtx)
+	pc.mu.Lock()
+	var ac *afterFuncCtx
+	for c := range pc.children {
+		if a, ok := c.(*afterFuncCtx); ok {
+			ac = a
+			break
+		}
+	}
+	if len(pc.children) != 3 || !contains(pc.children, cc) || !contains(pc.children, tc) || ac == nil {
+		t.Errorf("bad linkage: pc.children = %v, want %v, %v, and an afterFunc",
+			pc.children, cc, tc)
+	}
+	pc.mu.Unlock()
+
+	if p, ok := parentCancelCtx(cc.Context); !ok || p != pc {
+		t.Errorf("bad linkage: parentCancelCtx(cancelChild.Context) = %v, %v want %v, true", p, ok, pc)
+	}
+	if p, ok := parentCancelCtx(tc.Context); !ok || p != pc {
+		t.Errorf("bad linkage: parentCancelCtx(timerChild.Context) = %v, %v want %v, true", p, ok, pc)
+	}
+	if p, ok := parentCancelCtx(ac.Context); !ok || p != pc {
+		t.Errorf("bad linkage: parentCancelCtx(afterChild.Context) = %v, %v want %v, true", p, ok, pc)
+	}
+
+	cancel()
+
+	pc.mu.Lock()
+	if len(pc.children) != 0 {
+		t.Errorf("pc.cancel didn't clear pc.children = %v", pc.children)
+	}
+	pc.mu.Unlock()
+
+	// parent and children should all be finished.
+	check := func(ctx Context, name string) {
+		select {
+		case <-ctx.Done():
+		default:
+			t.Errorf("<-%s.Done() blocked, but shouldn't have", name)
+		}
+		if e := ctx.Err(); e != Canceled {
+			t.Errorf("%s.Err() == %v want %v", name, e, Canceled)
+		}
+	}
+	check(parent, "parent")
+	check(cancelChild, "cancelChild")
+	check(valueChild, "valueChild")
+	check(timerChild, "timerChild")
+
+	// WithCancel should return a canceled context on a canceled parent.
+	precanceledChild := WithValue(parent, "key", "value")
+	select {
+	case <-precanceledChild.Done():
+	default:
+		t.Errorf("<-precanceledChild.Done() blocked, but shouldn't have")
+	}
+	if e := precanceledChild.Err(); e != Canceled {
+		t.Errorf("precanceledChild.Err() == %v want %v", e, Canceled)
+	}
+}
+
+func XTestChildFinishesFirst(t testingT) {
+	cancelable, stop := WithCancel(Background())
+	defer stop()
+	for _, parent := range []Context{Background(), cancelable} {
+		child, cancel := WithCancel(parent)
+
+		select {
+		case x := <-parent.Done():
+			t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
+		case x := <-child.Done():
+			t.Errorf("<-child.Done() == %v want nothing (it should block)", x)
+		default:
+		}
+
+		cc := child.(*cancelCtx)
+		pc, pcok := parent.(*cancelCtx) // pcok == false when parent == Background()
+		if p, ok := parentCancelCtx(cc.Context); ok != pcok || (ok && pc != p) {
+			t.Errorf("bad linkage: parentCancelCtx(cc.Context) = %v, %v want %v, %v", p, ok, pc, pcok)
+		}
+
+		if pcok {
+			pc.mu.Lock()
+			if len(pc.children) != 1 || !contains(pc.children, cc) {
+				t.Errorf("bad linkage: pc.children = %v, cc = %v", pc.children, cc)
+			}
+			pc.mu.Unlock()
+		}
+
+		cancel()
+
+		if pcok {
+			pc.mu.Lock()
+			if len(pc.children) != 0 {
+				t.Errorf("child's cancel didn't remove self from pc.children = %v", pc.children)
+			}
+			pc.mu.Unlock()
+		}
+
+		// child should be finished.
+		select {
+		case <-child.Done():
+		default:
+			t.Errorf("<-child.Done() blocked, but shouldn't have")
+		}
+		if e := child.Err(); e != Canceled {
+			t.Errorf("child.Err() == %v want %v", e, Canceled)
+		}
+
+		// parent should not be finished.
+		select {
+		case x := <-parent.Done():
+			t.Errorf("<-parent.Done() == %v want nothing (it should block)", x)
+		default:
+		}
+		if e := parent.Err(); e != nil {
+			t.Errorf("parent.Err() == %v want nil", e)
+		}
+	}
+}
+
+func XTestCancelRemoves(t testingT) {
+	checkChildren := func(when string, ctx Context, want int) {
+		if got := len(ctx.(*cancelCtx).children); got != want {
+			t.Errorf("%s: context has %d children, want %d", when, got, want)
+		}
+	}
+
+	ctx, _ := WithCancel(Background())
+	checkChildren("after creation", ctx, 0)
+	_, cancel := WithCancel(ctx)
+	checkChildren("with WithCancel child ", ctx, 1)
+	cancel()
+	checkChildren("after canceling WithCancel child", ctx, 0)
+
+	ctx, _ = WithCancel(Background())
+	checkChildren("after creation", ctx, 0)
+	_, cancel = WithTimeout(ctx, 60*time.Minute)
+	checkChildren("with WithTimeout child ", ctx, 1)
+	cancel()
+	checkChildren("after canceling WithTimeout child", ctx, 0)
+
+	ctx, _ = WithCancel(Background())
+	checkChildren("after creation", ctx, 0)
+	stop := AfterFunc(ctx, func() {})
+	checkChildren("with AfterFunc child ", ctx, 1)
+	stop()
+	checkChildren("after stopping AfterFunc child ", ctx, 0)
+}
+
+type myCtx struct {
+	Context
+}
+
+type myDoneCtx struct {
+	Context
+}
+
+func (d *myDoneCtx) Done() <-chan struct{} {
+	c := make(chan struct{})
+	return c
+}
+func XTestCustomContextGoroutines(t testingT) {
+	g := goroutines.Load()
+	checkNoGoroutine := func() {
+		t.Helper()
+		now := goroutines.Load()
+		if now != g {
+			t.Fatalf("%d goroutines created", now-g)
+		}
+	}
+	checkCreatedGoroutine := func() {
+		t.Helper()
+		now := goroutines.Load()
+		if now != g+1 {
+			t.Fatalf("%d goroutines created, want 1", now-g)
+		}
+		g = now
+	}
+
+	_, cancel0 := WithCancel(&myDoneCtx{Background()})
+	cancel0()
+	checkCreatedGoroutine()
+
+	_, cancel0 = WithTimeout(&myDoneCtx{Background()}, veryLongDuration)
+	cancel0()
+	checkCreatedGoroutine()
+
+	checkNoGoroutine()
+	defer checkNoGoroutine()
+
+	ctx1, cancel1 := WithCancel(Background())
+	defer cancel1()
+	checkNoGoroutine()
+
+	ctx2 := &myCtx{ctx1}
+	ctx3, cancel3 := WithCancel(ctx2)
+	defer cancel3()
+	checkNoGoroutine()
+
+	_, cancel3b := WithCancel(&myDoneCtx{ctx2})
+	defer cancel3b()
+	checkCreatedGoroutine() // ctx1 is not providing Done, must not be used
+
+	ctx4, cancel4 := WithTimeout(ctx3, veryLongDuration)
+	defer cancel4()
+	checkNoGoroutine()
+
+	ctx5, cancel5 := WithCancel(ctx4)
+	defer cancel5()
+	checkNoGoroutine()
+
+	cancel5()
+	checkNoGoroutine()
+
+	_, cancel6 := WithTimeout(ctx5, veryLongDuration)
+	defer cancel6()
+	checkNoGoroutine()
+
+	// Check applied to canceled context.
+	cancel6()
+	cancel1()
+	_, cancel7 := WithCancel(ctx5)
+	defer cancel7()
+	checkNoGoroutine()
+}

go/src/context/example_test.go

@@ -0,0 +1,263 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sync"
+	"time"
+)
+
+var neverReady = make(chan struct{}) // never closed
+
+// This example demonstrates the use of a cancelable context to prevent a
+// goroutine leak. By the end of the example function, the goroutine started
+// by gen will return without leaking.
+func ExampleWithCancel() {
+	// gen generates integers in a separate goroutine and
+	// sends them to the returned channel.
+	// The callers of gen need to cancel the context once
+	// they are done consuming generated integers not to leak
+	// the internal goroutine started by gen.
+	gen := func(ctx context.Context) <-chan int {
+		dst := make(chan int)
+		n := 1
+		go func() {
+			for {
+				select {
+				case <-ctx.Done():
+					return // returning not to leak the goroutine
+				case dst <- n:
+					n++
+				}
+			}
+		}()
+		return dst
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel() // cancel when we are finished consuming integers
+
+	for n := range gen(ctx) {
+		fmt.Println(n)
+		if n == 5 {
+			break
+		}
+	}
+	// Output:
+	// 1
+	// 2
+	// 3
+	// 4
+	// 5
+}
+
+// This example passes a context with an arbitrary deadline to tell a blocking
+// function that it should abandon its work as soon as it gets to it.
+func ExampleWithDeadline() {
+	d := time.Now().Add(shortDuration)
+	ctx, cancel := context.WithDeadline(context.Background(), d)
+
+	// Even though ctx will be expired, it is good practice to call its
+	// cancellation function in any case. Failure to do so may keep the
+	// context and its parent alive longer than necessary.
+	defer cancel()
+
+	select {
+	case <-neverReady:
+		fmt.Println("ready")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err())
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example passes a context with a timeout to tell a blocking function that
+// it should abandon its work after the timeout elapses.
+func ExampleWithTimeout() {
+	// Pass a context with a timeout to tell a blocking function that it
+	// should abandon its work after the timeout elapses.
+	ctx, cancel := context.WithTimeout(context.Background(), shortDuration)
+	defer cancel()
+
+	select {
+	case <-neverReady:
+		fmt.Println("ready")
+	case <-ctx.Done():
+		fmt.Println(ctx.Err()) // prints "context deadline exceeded"
+	}
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example demonstrates how a value can be passed to the context
+// and also how to retrieve it if it exists.
+func ExampleWithValue() {
+	type favContextKey string
+
+	f := func(ctx context.Context, k favContextKey) {
+		if v := ctx.Value(k); v != nil {
+			fmt.Println("found value:", v)
+			return
+		}
+		fmt.Println("key not found:", k)
+	}
+
+	k := favContextKey("language")
+	ctx := context.WithValue(context.Background(), k, "Go")
+
+	f(ctx, k)
+	f(ctx, favContextKey("color"))
+
+	// Output:
+	// found value: Go
+	// key not found: color
+}
+
+// This example uses AfterFunc to define a function which waits on a sync.Cond,
+// stopping the wait when a context is canceled.
+func ExampleAfterFunc_cond() {
+	waitOnCond := func(ctx context.Context, cond *sync.Cond, conditionMet func() bool) error {
+		stopf := context.AfterFunc(ctx, func() {
+			// We need to acquire cond.L here to be sure that the Broadcast
+			// below won't occur before the call to Wait, which would result
+			// in a missed signal (and deadlock).
+			cond.L.Lock()
+			defer cond.L.Unlock()
+
+			// If multiple goroutines are waiting on cond simultaneously,
+			// we need to make sure we wake up exactly this one.
+			// That means that we need to Broadcast to all of the goroutines,
+			// which will wake them all up.
+			//
+			// If there are N concurrent calls to waitOnCond, each of the goroutines
+			// will spuriously wake up O(N) other goroutines that aren't ready yet,
+			// so this will cause the overall CPU cost to be O(N²).
+			cond.Broadcast()
+		})
+		defer stopf()
+
+		// Since the wakeups are using Broadcast instead of Signal, this call to
+		// Wait may unblock due to some other goroutine's context being canceled,
+		// so to be sure that ctx is actually canceled we need to check it in a loop.
+		for !conditionMet() {
+			cond.Wait()
+			if ctx.Err() != nil {
+				return ctx.Err()
+			}
+		}
+
+		return nil
+	}
+
+	cond := sync.NewCond(new(sync.Mutex))
+
+	var wg sync.WaitGroup
+	for i := 0; i < 4; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+
+			ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
+			defer cancel()
+
+			cond.L.Lock()
+			defer cond.L.Unlock()
+
+			err := waitOnCond(ctx, cond, func() bool { return false })
+			fmt.Println(err)
+		}()
+	}
+	wg.Wait()
+
+	// Output:
+	// context deadline exceeded
+	// context deadline exceeded
+	// context deadline exceeded
+	// context deadline exceeded
+}
+
+// This example uses AfterFunc to define a function which reads from a net.Conn,
+// stopping the read when a context is canceled.
+func ExampleAfterFunc_connection() {
+	readFromConn := func(ctx context.Context, conn net.Conn, b []byte) (n int, err error) {
+		stopc := make(chan struct{})
+		stop := context.AfterFunc(ctx, func() {
+			conn.SetReadDeadline(time.Now())
+			close(stopc)
+		})
+		n, err = conn.Read(b)
+		if !stop() {
+			// The AfterFunc was started.
+			// Wait for it to complete, and reset the Conn's deadline.
+			<-stopc
+			conn.SetReadDeadline(time.Time{})
+			return n, ctx.Err()
+		}
+		return n, err
+	}
+
+	listener, err := net.Listen("tcp", "localhost:0")
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer listener.Close()
+
+	conn, err := net.Dial(listener.Addr().Network(), listener.Addr().String())
+	if err != nil {
+		fmt.Println(err)
+		return
+	}
+	defer conn.Close()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 1*time.Millisecond)
+	defer cancel()
+
+	b := make([]byte, 1024)
+	_, err = readFromConn(ctx, conn, b)
+	fmt.Println(err)
+
+	// Output:
+	// context deadline exceeded
+}
+
+// This example uses AfterFunc to define a function which combines
+// the cancellation signals of two Contexts.
+func ExampleAfterFunc_merge() {
+	// mergeCancel returns a context that contains the values of ctx,
+	// and which is canceled when either ctx or cancelCtx is canceled.
+	mergeCancel := func(ctx, cancelCtx context.Context) (context.Context, context.CancelFunc) {
+		ctx, cancel := context.WithCancelCause(ctx)
+		stop := context.AfterFunc(cancelCtx, func() {
+			cancel(context.Cause(cancelCtx))
+		})
+		return ctx, func() {
+			stop()
+			cancel(context.Canceled)
+		}
+	}
+
+	ctx1, cancel1 := context.WithCancelCause(context.Background())
+	defer cancel1(errors.New("ctx1 canceled"))
+
+	ctx2, cancel2 := context.WithCancelCause(context.Background())
+
+	mergedCtx, mergedCancel := mergeCancel(ctx1, ctx2)
+	defer mergedCancel()
+
+	cancel2(errors.New("ctx2 canceled"))
+	<-mergedCtx.Done()
+	fmt.Println(context.Cause(mergedCtx))
+
+	// Output:
+	// ctx2 canceled
+}

go/src/context/net_test.go

@@ -0,0 +1,21 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	"context"
+	"net"
+	"testing"
+)
+
+func TestDeadlineExceededIsNetError(t *testing.T) {
+	err, ok := context.DeadlineExceeded.(net.Error)
+	if !ok {
+		t.Fatal("DeadlineExceeded does not implement net.Error")
+	}
+	if !err.Timeout() || !err.Temporary() {
+		t.Fatalf("Timeout() = %v, Temporary() = %v, want true, true", err.Timeout(), err.Temporary())
+	}
+}

go/src/context/x_test.go

@@ -0,0 +1,1198 @@
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package context_test
+
+import (
+	. "context"
+	"errors"
+	"fmt"
+	"internal/asan"
+	"math/rand"
+	"runtime"
+	"strings"
+	"sync"
+	"testing"
+	"time"
+)
+
+// Each XTestFoo in context_test.go must be called from a TestFoo here to run.
+func TestParentFinishesChild(t *testing.T) {
+	XTestParentFinishesChild(t) // uses unexported context types
+}
+func TestChildFinishesFirst(t *testing.T) {
+	XTestChildFinishesFirst(t) // uses unexported context types
+}
+func TestCancelRemoves(t *testing.T) {
+	XTestCancelRemoves(t) // uses unexported context types
+}
+func TestCustomContextGoroutines(t *testing.T) {
+	XTestCustomContextGoroutines(t) // reads the context.goroutines counter
+}
+
+// The following are regular tests in package context_test.
+
+// otherContext is a Context that's not one of the types defined in context.go.
+// This lets us test code paths that differ based on the underlying type of the
+// Context.
+type otherContext struct {
+	Context
+}
+
+const (
+	shortDuration    = 1 * time.Millisecond // a reasonable duration to block in a test
+	veryLongDuration = 1000 * time.Hour     // an arbitrary upper bound on the test's running time
+)
+
+// quiescent returns an arbitrary duration by which the program should have
+// completed any remaining work and reached a steady (idle) state.
+func quiescent(t *testing.T) time.Duration {
+	deadline, ok := t.Deadline()
+	if !ok {
+		return 5 * time.Second
+	}
+
+	const arbitraryCleanupMargin = 1 * time.Second
+	return time.Until(deadline) - arbitraryCleanupMargin
+}
+func TestBackground(t *testing.T) {
+	c := Background()
+	if c == nil {
+		t.Fatalf("Background returned nil")
+	}
+	select {
+	case x := <-c.Done():
+		t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
+	default:
+	}
+	if got, want := fmt.Sprint(c), "context.Background"; got != want {
+		t.Errorf("Background().String() = %q want %q", got, want)
+	}
+}
+
+func TestTODO(t *testing.T) {
+	c := TODO()
+	if c == nil {
+		t.Fatalf("TODO returned nil")
+	}
+	select {
+	case x := <-c.Done():
+		t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
+	default:
+	}
+	if got, want := fmt.Sprint(c), "context.TODO"; got != want {
+		t.Errorf("TODO().String() = %q want %q", got, want)
+	}
+}
+
+func TestWithCancel(t *testing.T) {
+	c1, cancel := WithCancel(Background())
+
+	if got, want := fmt.Sprint(c1), "context.Background.WithCancel"; got != want {
+		t.Errorf("c1.String() = %q want %q", got, want)
+	}
+
+	o := otherContext{c1}
+	c2, _ := WithCancel(o)
+	contexts := []Context{c1, o, c2}
+
+	for i, c := range contexts {
+		if d := c.Done(); d == nil {
+			t.Errorf("c[%d].Done() == %v want non-nil", i, d)
+		}
+		if e := c.Err(); e != nil {
+			t.Errorf("c[%d].Err() == %v want nil", i, e)
+		}
+
+		select {
+		case x := <-c.Done():
+			t.Errorf("<-c.Done() == %v want nothing (it should block)", x)
+		default:
+		}
+	}
+
+	cancel() // Should propagate synchronously.
+	for i, c := range contexts {
+		select {
+		case <-c.Done():
+		default:
+			t.Errorf("<-c[%d].Done() blocked, but shouldn't have", i)
+		}
+		if e := c.Err(); e != Canceled {
+			t.Errorf("c[%d].Err() == %v want %v", i, e, Canceled)
+		}
+	}
+}
+
+func testDeadline(c Context, name string, t *testing.T) {
+	t.Helper()
+	d := quiescent(t)
+	timer := time.NewTimer(d)
+	defer timer.Stop()
+	select {
+	case <-timer.C:
+		t.Fatalf("%s: context not timed out after %v", name, d)
+	case <-c.Done():
+	}
+	if e := c.Err(); e != DeadlineExceeded {
+		t.Errorf("%s: c.Err() == %v; want %v", name, e, DeadlineExceeded)
+	}
+}
+
+func TestDeadline(t *testing.T) {
+	t.Parallel()
+
+	c, _ := WithDeadline(Background(), time.Now().Add(shortDuration))
+	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
+		t.Errorf("c.String() = %q want prefix %q", got, prefix)
+	}
+	testDeadline(c, "WithDeadline", t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(shortDuration))
+	o := otherContext{c}
+	testDeadline(o, "WithDeadline+otherContext", t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(shortDuration))
+	o = otherContext{c}
+	c, _ = WithDeadline(o, time.Now().Add(veryLongDuration))
+	testDeadline(c, "WithDeadline+otherContext+WithDeadline", t)
+
+	c, _ = WithDeadline(Background(), time.Now().Add(-shortDuration))
+	testDeadline(c, "WithDeadline+inthepast", t)
+
+	c, _ = WithDeadline(Background(), time.Now())
+	testDeadline(c, "WithDeadline+now", t)
+}
+
+func TestTimeout(t *testing.T) {
+	t.Parallel()
+
+	c, _ := WithTimeout(Background(), shortDuration)
+	if got, prefix := fmt.Sprint(c), "context.Background.WithDeadline("; !strings.HasPrefix(got, prefix) {
+		t.Errorf("c.String() = %q want prefix %q", got, prefix)
+	}
+	testDeadline(c, "WithTimeout", t)
+
+	c, _ = WithTimeout(Background(), shortDuration)
+	o := otherContext{c}
+	testDeadline(o, "WithTimeout+otherContext", t)
+
+	c, _ = WithTimeout(Background(), shortDuration)
+	o = otherContext{c}
+	c, _ = WithTimeout(o, veryLongDuration)
+	testDeadline(c, "WithTimeout+otherContext+WithTimeout", t)
+}
+
+func TestCanceledTimeout(t *testing.T) {
+	c, _ := WithTimeout(Background(), time.Second)
+	o := otherContext{c}
+	c, cancel := WithTimeout(o, veryLongDuration)
+	cancel() // Should propagate synchronously.
+	select {
+	case <-c.Done():
+	default:
+		t.Errorf("<-c.Done() blocked, but shouldn't have")
+	}
+	if e := c.Err(); e != Canceled {
+		t.Errorf("c.Err() == %v want %v", e, Canceled)
+	}
+}
+
+type key1 int
+type key2 int
+
+func (k key2) String() string { return fmt.Sprintf("%[1]T(%[1]d)", k) }
+
+var k1 = key1(1)
+var k2 = key2(1) // same int as k1, different type
+var k3 = key2(3) // same type as k2, different int
+
+func TestValues(t *testing.T) {
+	check := func(c Context, nm, v1, v2, v3 string) {
+		if v, ok := c.Value(k1).(string); ok == (len(v1) == 0) || v != v1 {
+			t.Errorf(`%s.Value(k1).(string) = %q, %t want %q, %t`, nm, v, ok, v1, len(v1) != 0)
+		}
+		if v, ok := c.Value(k2).(string); ok == (len(v2) == 0) || v != v2 {
+			t.Errorf(`%s.Value(k2).(string) = %q, %t want %q, %t`, nm, v, ok, v2, len(v2) != 0)
+		}
+		if v, ok := c.Value(k3).(string); ok == (len(v3) == 0) || v != v3 {
+			t.Errorf(`%s.Value(k3).(string) = %q, %t want %q, %t`, nm, v, ok, v3, len(v3) != 0)
+		}
+	}
+
+	c0 := Background()
+	check(c0, "c0", "", "", "")
+
+	c1 := WithValue(Background(), k1, "c1k1")
+	check(c1, "c1", "c1k1", "", "")
+
+	if got, want := fmt.Sprint(c1), `context.Background.WithValue(context_test.key1, c1k1)`; got != want {
+		t.Errorf("c.String() = %q want %q", got, want)
+	}
+
+	c2 := WithValue(c1, k2, "c2k2")
+	check(c2, "c2", "c1k1", "c2k2", "")
+
+	if got, want := fmt.Sprint(c2), `context.Background.WithValue(context_test.key1, c1k1).WithValue(context_test.key2(1), c2k2)`; got != want {
+		t.Errorf("c.String() = %q want %q", got, want)
+	}
+
+	c3 := WithValue(c2, k3, "c3k3")
+	check(c3, "c2", "c1k1", "c2k2", "c3k3")
+
+	c4 := WithValue(c3, k1, nil)
+	check(c4, "c4", "", "c2k2", "c3k3")
+
+	if got, want := fmt.Sprint(c4), `context.Background.WithValue(context_test.key1, c1k1).WithValue(context_test.key2(1), c2k2).WithValue(context_test.key2(3), c3k3).WithValue(context_test.key1, <nil>)`; got != want {
+		t.Errorf("c.String() = %q want %q", got, want)
+	}
+
+	o0 := otherContext{Background()}
+	check(o0, "o0", "", "", "")
+
+	o1 := otherContext{WithValue(Background(), k1, "c1k1")}
+	check(o1, "o1", "c1k1", "", "")
+
+	o2 := WithValue(o1, k2, "o2k2")
+	check(o2, "o2", "c1k1", "o2k2", "")
+
+	o3 := otherContext{c4}
+	check(o3, "o3", "", "c2k2", "c3k3")
+
+	o4 := WithValue(o3, k3, nil)
+	check(o4, "o4", "", "c2k2", "")
+}
+
+func TestAllocs(t *testing.T) {
+	if asan.Enabled {
+		t.Skip("test allocates more with -asan")
+	}
+	bg := Background()
+	for _, test := range []struct {
+		desc       string
+		f          func()
+		limit      float64
+		gccgoLimit float64
+	}{
+		{
+			desc:       "Background()",
+			f:          func() { Background() },
+			limit:      0,
+			gccgoLimit: 0,
+		},
+		{
+			desc: fmt.Sprintf("WithValue(bg, %v, nil)", k1),
+			f: func() {
+				c := WithValue(bg, k1, nil)
+				c.Value(k1)
+			},
+			limit:      3,
+			gccgoLimit: 3,
+		},
+		{
+			desc: "WithTimeout(bg, 1*time.Nanosecond)",
+			f: func() {
+				c, _ := WithTimeout(bg, 1*time.Nanosecond)
+				<-c.Done()
+			},
+			limit:      12,
+			gccgoLimit: 15,
+		},
+		{
+			desc: "WithCancel(bg)",
+			f: func() {
+				c, cancel := WithCancel(bg)
+				cancel()
+				<-c.Done()
+			},
+			limit:      5,
+			gccgoLimit: 8,
+		},
+		{
+			desc: "WithTimeout(bg, 5*time.Millisecond)",
+			f: func() {
+				c, cancel := WithTimeout(bg, 5*time.Millisecond)
+				cancel()
+				<-c.Done()
+			},
+			limit:      8,
+			gccgoLimit: 25,
+		},
+	} {
+		limit := test.limit
+		if runtime.Compiler == "gccgo" {
+			// gccgo does not yet do escape analysis.
+			// TODO(iant): Remove this when gccgo does do escape analysis.
+			limit = test.gccgoLimit
+		}
+		numRuns := 100
+		if testing.Short() {
+			numRuns = 10
+		}
+		if n := testing.AllocsPerRun(numRuns, test.f); n > limit {
+			t.Errorf("%s allocs = %f want %d", test.desc, n, int(limit))
+		}
+	}
+}
+
+func TestSimultaneousCancels(t *testing.T) {
+	root, cancel := WithCancel(Background())
+	m := map[Context]CancelFunc{root: cancel}
+	q := []Context{root}
+	// Create a tree of contexts.
+	for len(q) != 0 && len(m) < 100 {
+		parent := q[0]
+		q = q[1:]
+		for i := 0; i < 4; i++ {
+			ctx, cancel := WithCancel(parent)
+			m[ctx] = cancel
+			q = append(q, ctx)
+		}
+	}
+	// Start all the cancels in a random order.
+	var wg sync.WaitGroup
+	wg.Add(len(m))
+	for _, cancel := range m {
+		go func(cancel CancelFunc) {
+			cancel()
+			wg.Done()
+		}(cancel)
+	}
+
+	d := quiescent(t)
+	stuck := make(chan struct{})
+	timer := time.AfterFunc(d, func() { close(stuck) })
+	defer timer.Stop()
+
+	// Wait on all the contexts in a random order.
+	for ctx := range m {
+		select {
+		case <-ctx.Done():
+		case <-stuck:
+			buf := make([]byte, 10<<10)
+			n := runtime.Stack(buf, true)
+			t.Fatalf("timed out after %v waiting for <-ctx.Done(); stacks:\n%s", d, buf[:n])
+		}
+	}
+	// Wait for all the cancel functions to return.
+	done := make(chan struct{})
+	go func() {
+		wg.Wait()
+		close(done)
+	}()
+	select {
+	case <-done:
+	case <-stuck:
+		buf := make([]byte, 10<<10)
+		n := runtime.Stack(buf, true)
+		t.Fatalf("timed out after %v waiting for cancel functions; stacks:\n%s", d, buf[:n])
+	}
+}
+
+func TestInterlockedCancels(t *testing.T) {
+	parent, cancelParent := WithCancel(Background())
+	child, cancelChild := WithCancel(parent)
+	go func() {
+		<-parent.Done()
+		cancelChild()
+	}()
+	cancelParent()
+	d := quiescent(t)
+	timer := time.NewTimer(d)
+	defer timer.Stop()
+	select {
+	case <-child.Done():
+	case <-timer.C:
+		buf := make([]byte, 10<<10)
+		n := runtime.Stack(buf, true)
+		t.Fatalf("timed out after %v waiting for child.Done(); stacks:\n%s", d, buf[:n])
+	}
+}
+
+func TestLayersCancel(t *testing.T) {
+	testLayers(t, time.Now().UnixNano(), false)
+}
+
+func TestLayersTimeout(t *testing.T) {
+	testLayers(t, time.Now().UnixNano(), true)
+}
+
+func testLayers(t *testing.T, seed int64, testTimeout bool) {
+	t.Parallel()
+
+	r := rand.New(rand.NewSource(seed))
+	prefix := fmt.Sprintf("seed=%d", seed)
+	errorf := func(format string, a ...any) {
+		t.Errorf(prefix+format, a...)
+	}
+	const (
+		minLayers = 30
+	)
+	type value int
+	var (
+		vals      []*value
+		cancels   []CancelFunc
+		numTimers int
+		ctx       = Background()
+	)
+	for i := 0; i < minLayers || numTimers == 0 || len(cancels) == 0 || len(vals) == 0; i++ {
+		switch r.Intn(3) {
+		case 0:
+			v := new(value)
+			ctx = WithValue(ctx, v, v)
+			vals = append(vals, v)
+		case 1:
+			var cancel CancelFunc
+			ctx, cancel = WithCancel(ctx)
+			cancels = append(cancels, cancel)
+		case 2:
+			var cancel CancelFunc
+			d := veryLongDuration
+			if testTimeout {
+				d = shortDuration
+			}
+			ctx, cancel = WithTimeout(ctx, d)
+			cancels = append(cancels, cancel)
+			numTimers++
+		}
+	}
+	checkValues := func(when string) {
+		for _, key := range vals {
+			if val := ctx.Value(key).(*value); key != val {
+				errorf("%s: ctx.Value(%p) = %p want %p", when, key, val, key)
+			}
+		}
+	}
+	if !testTimeout {
+		select {
+		case <-ctx.Done():
+			errorf("ctx should not be canceled yet")
+		default:
+		}
+	}
+	if s, prefix := fmt.Sprint(ctx), "context.Background."; !strings.HasPrefix(s, prefix) {
+		t.Errorf("ctx.String() = %q want prefix %q", s, prefix)
+	}
+	t.Log(ctx)
+	checkValues("before cancel")
+	if testTimeout {
+		d := quiescent(t)
+		timer := time.NewTimer(d)
+		defer timer.Stop()
+		select {
+		case <-ctx.Done():
+		case <-timer.C:
+			errorf("ctx should have timed out after %v", d)
+		}
+		checkValues("after timeout")
+	} else {
+		cancel := cancels[r.Intn(len(cancels))]
+		cancel()
+		select {
+		case <-ctx.Done():
+		default:
+			errorf("ctx should be canceled")
+		}
+		checkValues("after cancel")
+	}
+}
+
+func TestWithCancelCanceledParent(t *testing.T) {
+	parent, pcancel := WithCancelCause(Background())
+	cause := fmt.Errorf("Because!")
+	pcancel(cause)
+
+	c, _ := WithCancel(parent)
+	select {
+	case <-c.Done():
+	default:
+		t.Errorf("child not done immediately upon construction")
+	}
+	if got, want := c.Err(), Canceled; got != want {
+		t.Errorf("child not canceled; got = %v, want = %v", got, want)
+	}
+	if got, want := Cause(c), cause; got != want {
+		t.Errorf("child has wrong cause; got = %v, want = %v", got, want)
+	}
+}
+
+func TestWithCancelSimultaneouslyCanceledParent(t *testing.T) {
+	// Cancel the parent goroutine concurrently with creating a child.
+	for i := 0; i < 100; i++ {
+		parent, pcancel := WithCancelCause(Background())
+		cause := fmt.Errorf("Because!")
+		go pcancel(cause)
+
+		c, _ := WithCancel(parent)
+		<-c.Done()
+		if got, want := c.Err(), Canceled; got != want {
+			t.Errorf("child not canceled; got = %v, want = %v", got, want)
+		}
+		if got, want := Cause(c), cause; got != want {
+			t.Errorf("child has wrong cause; got = %v, want = %v", got, want)
+		}
+	}
+}
+
+func TestWithValueChecksKey(t *testing.T) {
+	panicVal := recoveredValue(func() { _ = WithValue(Background(), []byte("foo"), "bar") })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal = recoveredValue(func() { _ = WithValue(Background(), nil, "bar") })
+	if got, want := fmt.Sprint(panicVal), "nil key"; got != want {
+		t.Errorf("panic = %q; want %q", got, want)
+	}
+}
+
+func TestInvalidDerivedFail(t *testing.T) {
+	panicVal := recoveredValue(func() { _, _ = WithCancel(nil) })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal = recoveredValue(func() { _, _ = WithDeadline(nil, time.Now().Add(shortDuration)) })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+	panicVal = recoveredValue(func() { _ = WithValue(nil, "foo", "bar") })
+	if panicVal == nil {
+		t.Error("expected panic")
+	}
+}
+
+func recoveredValue(fn func()) (v any) {
+	defer func() { v = recover() }()
+	fn()
+	return
+}
+
+func TestDeadlineExceededSupportsTimeout(t *testing.T) {
+	i, ok := DeadlineExceeded.(interface {
+		Timeout() bool
+	})
+	if !ok {
+		t.Fatal("DeadlineExceeded does not support Timeout interface")
+	}
+	if !i.Timeout() {
+		t.Fatal("wrong value for timeout")
+	}
+}
+func TestCause(t *testing.T) {
+	var (
+		forever       = 1e6 * time.Second
+		parentCause   = fmt.Errorf("parentCause")
+		childCause    = fmt.Errorf("childCause")
+		tooSlow       = fmt.Errorf("tooSlow")
+		finishedEarly = fmt.Errorf("finishedEarly")
+	)
+	for _, test := range []struct {
+		name  string
+		ctx   func() Context
+		err   error
+		cause error
+	}{
+		{
+			name:  "Background",
+			ctx:   Background,
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name:  "TODO",
+			ctx:   TODO,
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name: "WithCancel",
+			ctx: func() Context {
+				ctx, cancel := WithCancel(Background())
+				cancel()
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithCancelCause",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				cancel(parentCause)
+				return ctx
+			},
+			err:   Canceled,
+			cause: parentCause,
+		},
+		{
+			name: "WithCancelCause nil",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				cancel(nil)
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithCancelCause: parent cause before child",
+			ctx: func() Context {
+				ctx, cancelParent := WithCancelCause(Background())
+				ctx, cancelChild := WithCancelCause(ctx)
+				cancelParent(parentCause)
+				cancelChild(childCause)
+				return ctx
+			},
+			err:   Canceled,
+			cause: parentCause,
+		},
+		{
+			name: "WithCancelCause: parent cause after child",
+			ctx: func() Context {
+				ctx, cancelParent := WithCancelCause(Background())
+				ctx, cancelChild := WithCancelCause(ctx)
+				cancelChild(childCause)
+				cancelParent(parentCause)
+				return ctx
+			},
+			err:   Canceled,
+			cause: childCause,
+		},
+		{
+			name: "WithCancelCause: parent cause before nil",
+			ctx: func() Context {
+				ctx, cancelParent := WithCancelCause(Background())
+				ctx, cancelChild := WithCancel(ctx)
+				cancelParent(parentCause)
+				cancelChild()
+				return ctx
+			},
+			err:   Canceled,
+			cause: parentCause,
+		},
+		{
+			name: "WithCancelCause: parent cause after nil",
+			ctx: func() Context {
+				ctx, cancelParent := WithCancelCause(Background())
+				ctx, cancelChild := WithCancel(ctx)
+				cancelChild()
+				cancelParent(parentCause)
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithCancelCause: child cause after nil",
+			ctx: func() Context {
+				ctx, cancelParent := WithCancel(Background())
+				ctx, cancelChild := WithCancelCause(ctx)
+				cancelParent()
+				cancelChild(childCause)
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithCancelCause: child cause before nil",
+			ctx: func() Context {
+				ctx, cancelParent := WithCancel(Background())
+				ctx, cancelChild := WithCancelCause(ctx)
+				cancelChild(childCause)
+				cancelParent()
+				return ctx
+			},
+			err:   Canceled,
+			cause: childCause,
+		},
+		{
+			name: "WithTimeout",
+			ctx: func() Context {
+				ctx, cancel := WithTimeout(Background(), 0)
+				cancel()
+				return ctx
+			},
+			err:   DeadlineExceeded,
+			cause: DeadlineExceeded,
+		},
+		{
+			name: "WithTimeout canceled",
+			ctx: func() Context {
+				ctx, cancel := WithTimeout(Background(), forever)
+				cancel()
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithTimeoutCause",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), 0, tooSlow)
+				cancel()
+				return ctx
+			},
+			err:   DeadlineExceeded,
+			cause: tooSlow,
+		},
+		{
+			name: "WithTimeoutCause canceled",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), forever, tooSlow)
+				cancel()
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "WithTimeoutCause stacked",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx, _ = WithTimeoutCause(ctx, 0, tooSlow)
+				cancel(finishedEarly)
+				return ctx
+			},
+			err:   DeadlineExceeded,
+			cause: tooSlow,
+		},
+		{
+			name: "WithTimeoutCause stacked canceled",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx, _ = WithTimeoutCause(ctx, forever, tooSlow)
+				cancel(finishedEarly)
+				return ctx
+			},
+			err:   Canceled,
+			cause: finishedEarly,
+		},
+		{
+			name: "WithoutCancel",
+			ctx: func() Context {
+				return WithoutCancel(Background())
+			},
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name: "WithoutCancel canceled",
+			ctx: func() Context {
+				ctx, cancel := WithCancelCause(Background())
+				ctx = WithoutCancel(ctx)
+				cancel(finishedEarly)
+				return ctx
+			},
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name: "WithoutCancel timeout",
+			ctx: func() Context {
+				ctx, cancel := WithTimeoutCause(Background(), 0, tooSlow)
+				ctx = WithoutCancel(ctx)
+				cancel()
+				return ctx
+			},
+			err:   nil,
+			cause: nil,
+		},
+		{
+			name: "parent of custom context not canceled",
+			ctx: func() Context {
+				ctx, _ := WithCancelCause(Background())
+				ctx, cancel2 := newCustomContext(ctx)
+				cancel2()
+				return ctx
+			},
+			err:   Canceled,
+			cause: Canceled,
+		},
+		{
+			name: "parent of custom context is canceled before",
+			ctx: func() Context {
+				ctx, cancel1 := WithCancelCause(Background())
+				ctx, cancel2 := newCustomContext(ctx)
+				cancel1(parentCause)
+				cancel2()
+				return ctx
+			},
+			err:   Canceled,
+			cause: parentCause,
+		},
+		{
+			name: "parent of custom context is canceled after",
+			ctx: func() Context {
+				ctx, cancel1 := WithCancelCause(Background())
+				ctx, cancel2 := newCustomContext(ctx)
+				cancel2()
+				cancel1(parentCause)
+				return ctx
+			},
+			err: Canceled,
+			// This isn't really right: the child context was canceled before
+			// the parent context, and shouldn't inherit the parent's cause.
+			// However, since the child is a custom context, Cause has no way
+			// to tell which was canceled first and returns the parent's cause.
+			cause: parentCause,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			ctx := test.ctx()
+			if got, want := ctx.Err(), test.err; want != got {
+				t.Errorf("ctx.Err() = %v want %v", got, want)
+			}
+			if got, want := Cause(ctx), test.cause; want != got {
+				t.Errorf("Cause(ctx) = %v want %v", got, want)
+			}
+		})
+	}
+}
+
+func TestCauseRace(t *testing.T) {
+	cause := errors.New("TestCauseRace")
+	ctx, cancel := WithCancelCause(Background())
+	go func() {
+		cancel(cause)
+	}()
+	for {
+		// Poll Cause, rather than waiting for Done, to test that
+		// access to the underlying cause is synchronized properly.
+		if err := Cause(ctx); err != nil {
+			if err != cause {
+				t.Errorf("Cause returned %v, want %v", err, cause)
+			}
+			break
+		}
+		runtime.Gosched()
+	}
+}
+
+func TestWithoutCancel(t *testing.T) {
+	key, value := "key", "value"
+	ctx := WithValue(Background(), key, value)
+	ctx = WithoutCancel(ctx)
+	if d, ok := ctx.Deadline(); !d.IsZero() || ok != false {
+		t.Errorf("ctx.Deadline() = %v, %v want zero, false", d, ok)
+	}
+	if done := ctx.Done(); done != nil {
+		t.Errorf("ctx.Deadline() = %v want nil", done)
+	}
+	if err := ctx.Err(); err != nil {
+		t.Errorf("ctx.Err() = %v want nil", err)
+	}
+	if v := ctx.Value(key); v != value {
+		t.Errorf("ctx.Value(%q) = %q want %q", key, v, value)
+	}
+}
+
+type customDoneContext struct {
+	Context
+	donec chan struct{}
+}
+
+func (c *customDoneContext) Done() <-chan struct{} {
+	return c.donec
+}
+
+func TestCustomContextPropagation(t *testing.T) {
+	cause := errors.New("TestCustomContextPropagation")
+	donec := make(chan struct{})
+	ctx1, cancel1 := WithCancelCause(Background())
+	ctx2 := &customDoneContext{
+		Context: ctx1,
+		donec:   donec,
+	}
+	ctx3, cancel3 := WithCancel(ctx2)
+	defer cancel3()
+
+	cancel1(cause)
+	close(donec)
+
+	<-ctx3.Done()
+	if got, want := ctx3.Err(), Canceled; got != want {
+		t.Errorf("child not canceled; got = %v, want = %v", got, want)
+	}
+	if got, want := Cause(ctx3), cause; got != want {
+		t.Errorf("child has wrong cause; got = %v, want = %v", got, want)
+	}
+}
+
+// customCauseContext is a custom Context used to test context.Cause.
+type customCauseContext struct {
+	mu   sync.Mutex
+	done chan struct{}
+	err  error
+
+	cancelChild CancelFunc
+}
+
+func (ccc *customCauseContext) Deadline() (deadline time.Time, ok bool) {
+	return
+}
+
+func (ccc *customCauseContext) Done() <-chan struct{} {
+	ccc.mu.Lock()
+	defer ccc.mu.Unlock()
+	return ccc.done
+}
+
+func (ccc *customCauseContext) Err() error {
+	ccc.mu.Lock()
+	defer ccc.mu.Unlock()
+	return ccc.err
+}
+
+func (ccc *customCauseContext) Value(key any) any {
+	return nil
+}
+
+func (ccc *customCauseContext) cancel() {
+	ccc.mu.Lock()
+	ccc.err = Canceled
+	close(ccc.done)
+	cancelChild := ccc.cancelChild
+	ccc.mu.Unlock()
+
+	if cancelChild != nil {
+		cancelChild()
+	}
+}
+
+func (ccc *customCauseContext) setCancelChild(cancelChild CancelFunc) {
+	ccc.cancelChild = cancelChild
+}
+
+func TestCustomContextCause(t *testing.T) {
+	// Test if we cancel a custom context, Err and Cause return Canceled.
+	ccc := &customCauseContext{
+		done: make(chan struct{}),
+	}
+	ccc.cancel()
+	if got := ccc.Err(); got != Canceled {
+		t.Errorf("ccc.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ccc); got != Canceled {
+		t.Errorf("Cause(ccc) = %v, want %v", got, Canceled)
+	}
+
+	// Test that if we pass a custom context to WithCancelCause,
+	// and then cancel that child context with a cause,
+	// that the cause of the child canceled context is correct
+	// but that the parent custom context is not canceled.
+	ccc = &customCauseContext{
+		done: make(chan struct{}),
+	}
+	ctx, causeFunc := WithCancelCause(ccc)
+	cause := errors.New("TestCustomContextCause")
+	causeFunc(cause)
+	if got := ctx.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ctx.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ctx); got != cause {
+		t.Errorf("after CancelCauseFunc Cause(ctx) = %v, want %v", got, cause)
+	}
+	if got := ccc.Err(); got != nil {
+		t.Errorf("after CancelCauseFunc ccc.Err() = %v, want %v", got, nil)
+	}
+	if got := Cause(ccc); got != nil {
+		t.Errorf("after CancelCauseFunc Cause(ccc) = %v, want %v", got, nil)
+	}
+
+	// Test that if we now cancel the parent custom context,
+	// the cause of the child canceled context is still correct,
+	// and the parent custom context is canceled without a cause.
+	ccc.cancel()
+	if got := ctx.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ctx.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ctx); got != cause {
+		t.Errorf("after CancelCauseFunc Cause(ctx) = %v, want %v", got, cause)
+	}
+	if got := ccc.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ccc.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ccc); got != Canceled {
+		t.Errorf("after CancelCauseFunc Cause(ccc) = %v, want %v", got, Canceled)
+	}
+
+	// Test that if we associate a custom context with a child,
+	// then canceling the custom context cancels the child.
+	ccc = &customCauseContext{
+		done: make(chan struct{}),
+	}
+	ctx, cancelFunc := WithCancel(ccc)
+	ccc.setCancelChild(cancelFunc)
+	ccc.cancel()
+	if got := ctx.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ctx.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ctx); got != Canceled {
+		t.Errorf("after CancelCauseFunc Cause(ctx) = %v, want %v", got, Canceled)
+	}
+	if got := ccc.Err(); got != Canceled {
+		t.Errorf("after CancelCauseFunc ccc.Err() = %v, want %v", got, Canceled)
+	}
+	if got := Cause(ccc); got != Canceled {
+		t.Errorf("after CancelCauseFunc Cause(ccc) = %v, want %v", got, Canceled)
+	}
+}
+
+func TestAfterFuncCalledAfterCancel(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	donec := make(chan struct{})
+	stop := AfterFunc(ctx, func() {
+		close(donec)
+	})
+	select {
+	case <-donec:
+		t.Fatalf("AfterFunc called before context is done")
+	case <-time.After(shortDuration):
+	}
+	cancel()
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called after context is canceled")
+	}
+	if stop() {
+		t.Fatalf("stop() = true, want false")
+	}
+}
+
+func TestAfterFuncCalledAfterTimeout(t *testing.T) {
+	ctx, cancel := WithTimeout(Background(), shortDuration)
+	defer cancel()
+	donec := make(chan struct{})
+	AfterFunc(ctx, func() {
+		close(donec)
+	})
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called after context is canceled")
+	}
+}
+
+func TestAfterFuncCalledImmediately(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	cancel()
+	donec := make(chan struct{})
+	AfterFunc(ctx, func() {
+		close(donec)
+	})
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called for already-canceled context")
+	}
+}
+
+func TestAfterFuncNotCalledAfterStop(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	donec := make(chan struct{})
+	stop := AfterFunc(ctx, func() {
+		close(donec)
+	})
+	if !stop() {
+		t.Fatalf("stop() = false, want true")
+	}
+	cancel()
+	select {
+	case <-donec:
+		t.Fatalf("AfterFunc called for already-canceled context")
+	case <-time.After(shortDuration):
+	}
+	if stop() {
+		t.Fatalf("stop() = true, want false")
+	}
+}
+
+// This test verifies that canceling a context does not block waiting for AfterFuncs to finish.
+func TestAfterFuncCalledAsynchronously(t *testing.T) {
+	ctx, cancel := WithCancel(Background())
+	donec := make(chan struct{})
+	stop := AfterFunc(ctx, func() {
+		// The channel send blocks until donec is read from.
+		donec <- struct{}{}
+	})
+	defer stop()
+	cancel()
+	// After cancel returns, read from donec and unblock the AfterFunc.
+	select {
+	case <-donec:
+	case <-time.After(veryLongDuration):
+		t.Fatalf("AfterFunc not called after context is canceled")
+	}
+}
+
+// customContext is a custom Context implementation.
+type customContext struct {
+	parent Context
+
+	doneOnce sync.Once
+	donec    chan struct{}
+	err      error
+}
+
+func newCustomContext(parent Context) (Context, CancelFunc) {
+	c := &customContext{
+		parent: parent,
+		donec:  make(chan struct{}),
+	}
+	AfterFunc(parent, func() {
+		c.doneOnce.Do(func() {
+			c.err = parent.Err()
+			close(c.donec)
+		})
+	})
+	return c, func() {
+		c.doneOnce.Do(func() {
+			c.err = Canceled
+			close(c.donec)
+		})
+	}
+}
+
+func (c *customContext) Deadline() (time.Time, bool) {
+	return c.parent.Deadline()
+}
+
+func (c *customContext) Done() <-chan struct{} {
+	return c.donec
+}
+
+func (c *customContext) Err() error {
+	select {
+	case <-c.donec:
+		return c.err
+	default:
+		return nil
+	}
+}
+
+func (c *customContext) Value(key any) any {
+	return c.parent.Value(key)
+}
+
+// Issue #75533.
+func TestContextErrDoneRace(t *testing.T) {
+	// 4 iterations reliably reproduced #75533.
+	for range 10 {
+		ctx, cancel := WithCancel(Background())
+		donec := ctx.Done()
+		go cancel()
+		for ctx.Err() == nil {
+			if runtime.GOARCH == "wasm" {
+				runtime.Gosched() // need to explicitly yield
+			}
+		}
+		select {
+		case <-donec:
+		default:
+			t.Fatalf("ctx.Err is non-nil, but ctx.Done is not closed")
+		}
+	}
+}

go/src/crypto/crypto.go

@@ -0,0 +1,273 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package crypto collects common cryptographic constants.
+package crypto
+
+import (
+	"hash"
+	"io"
+	"strconv"
+)
+
+// Hash identifies a cryptographic hash function that is implemented in another
+// package.
+type Hash uint
+
+// HashFunc simply returns the value of h so that [Hash] implements [SignerOpts].
+func (h Hash) HashFunc() Hash {
+	return h
+}
+
+func (h Hash) String() string {
+	switch h {
+	case MD4:
+		return "MD4"
+	case MD5:
+		return "MD5"
+	case SHA1:
+		return "SHA-1"
+	case SHA224:
+		return "SHA-224"
+	case SHA256:
+		return "SHA-256"
+	case SHA384:
+		return "SHA-384"
+	case SHA512:
+		return "SHA-512"
+	case MD5SHA1:
+		return "MD5+SHA1"
+	case RIPEMD160:
+		return "RIPEMD-160"
+	case SHA3_224:
+		return "SHA3-224"
+	case SHA3_256:
+		return "SHA3-256"
+	case SHA3_384:
+		return "SHA3-384"
+	case SHA3_512:
+		return "SHA3-512"
+	case SHA512_224:
+		return "SHA-512/224"
+	case SHA512_256:
+		return "SHA-512/256"
+	case BLAKE2s_256:
+		return "BLAKE2s-256"
+	case BLAKE2b_256:
+		return "BLAKE2b-256"
+	case BLAKE2b_384:
+		return "BLAKE2b-384"
+	case BLAKE2b_512:
+		return "BLAKE2b-512"
+	default:
+		return "unknown hash value " + strconv.Itoa(int(h))
+	}
+}
+
+const (
+	MD4         Hash = 1 + iota // import golang.org/x/crypto/md4
+	MD5                         // import crypto/md5
+	SHA1                        // import crypto/sha1
+	SHA224                      // import crypto/sha256
+	SHA256                      // import crypto/sha256
+	SHA384                      // import crypto/sha512
+	SHA512                      // import crypto/sha512
+	MD5SHA1                     // no implementation; MD5+SHA1 used for TLS RSA
+	RIPEMD160                   // import golang.org/x/crypto/ripemd160
+	SHA3_224                    // import crypto/sha3
+	SHA3_256                    // import crypto/sha3
+	SHA3_384                    // import crypto/sha3
+	SHA3_512                    // import crypto/sha3
+	SHA512_224                  // import crypto/sha512
+	SHA512_256                  // import crypto/sha512
+	BLAKE2s_256                 // import golang.org/x/crypto/blake2s
+	BLAKE2b_256                 // import golang.org/x/crypto/blake2b
+	BLAKE2b_384                 // import golang.org/x/crypto/blake2b
+	BLAKE2b_512                 // import golang.org/x/crypto/blake2b
+	maxHash
+)
+
+var digestSizes = []uint8{
+	MD4:         16,
+	MD5:         16,
+	SHA1:        20,
+	SHA224:      28,
+	SHA256:      32,
+	SHA384:      48,
+	SHA512:      64,
+	SHA512_224:  28,
+	SHA512_256:  32,
+	SHA3_224:    28,
+	SHA3_256:    32,
+	SHA3_384:    48,
+	SHA3_512:    64,
+	MD5SHA1:     36,
+	RIPEMD160:   20,
+	BLAKE2s_256: 32,
+	BLAKE2b_256: 32,
+	BLAKE2b_384: 48,
+	BLAKE2b_512: 64,
+}
+
+// Size returns the length, in bytes, of a digest resulting from the given hash
+// function. It doesn't require that the hash function in question be linked
+// into the program.
+func (h Hash) Size() int {
+	if h > 0 && h < maxHash {
+		return int(digestSizes[h])
+	}
+	panic("crypto: Size of unknown hash function")
+}
+
+var hashes = make([]func() hash.Hash, maxHash)
+
+// New returns a new hash.Hash calculating the given hash function. New panics
+// if the hash function is not linked into the binary.
+func (h Hash) New() hash.Hash {
+	if h > 0 && h < maxHash {
+		f := hashes[h]
+		if f != nil {
+			return f()
+		}
+	}
+	panic("crypto: requested hash function #" + strconv.Itoa(int(h)) + " is unavailable")
+}
+
+// Available reports whether the given hash function is linked into the binary.
+func (h Hash) Available() bool {
+	return h < maxHash && hashes[h] != nil
+}
+
+// RegisterHash registers a function that returns a new instance of the given
+// hash function. This is intended to be called from the init function in
+// packages that implement hash functions.
+func RegisterHash(h Hash, f func() hash.Hash) {
+	if h >= maxHash {
+		panic("crypto: RegisterHash of unknown hash function")
+	}
+	hashes[h] = f
+}
+
+// PublicKey represents a public key using an unspecified algorithm.
+//
+// Although this type is an empty interface for backwards compatibility reasons,
+// all public key types in the standard library implement the following interface
+//
+//	interface{
+//	    Equal(x crypto.PublicKey) bool
+//	}
+//
+// which can be used for increased type safety within applications.
+type PublicKey any
+
+// PrivateKey represents a private key using an unspecified algorithm.
+//
+// Although this type is an empty interface for backwards compatibility reasons,
+// all private key types in the standard library implement the following interface
+//
+//	interface{
+//	    Public() crypto.PublicKey
+//	    Equal(x crypto.PrivateKey) bool
+//	}
+//
+// as well as purpose-specific interfaces such as [Signer] and [Decrypter], which
+// can be used for increased type safety within applications.
+type PrivateKey any
+
+// Signer is an interface for an opaque private key that can be used for
+// signing operations. For example, an RSA key kept in a hardware module.
+type Signer interface {
+	// Public returns the public key corresponding to the opaque,
+	// private key.
+	Public() PublicKey
+
+	// Sign signs digest with the private key, possibly using entropy from
+	// rand. For an RSA key, the resulting signature should be either a
+	// PKCS #1 v1.5 or PSS signature (as indicated by opts). For an (EC)DSA
+	// key, it should be a DER-serialised, ASN.1 signature structure.
+	//
+	// Hash implements the SignerOpts interface and, in most cases, one can
+	// simply pass in the hash function used as opts. Sign may also attempt
+	// to type assert opts to other types in order to obtain algorithm
+	// specific values. See the documentation in each package for details.
+	//
+	// Note that when a signature of a hash of a larger message is needed,
+	// the caller is responsible for hashing the larger message and passing
+	// the hash (as digest) and the hash function (as opts) to Sign.
+	Sign(rand io.Reader, digest []byte, opts SignerOpts) (signature []byte, err error)
+}
+
+// MessageSigner is an interface for an opaque private key that can be used for
+// signing operations where the message is not pre-hashed by the caller.
+// It is a superset of the Signer interface so that it can be passed to APIs
+// which accept Signer, which may try to do an interface upgrade.
+//
+// MessageSigner.SignMessage and MessageSigner.Sign should produce the same
+// result given the same opts. In particular, MessageSigner.SignMessage should
+// only accept a zero opts.HashFunc if the Signer would also accept messages
+// which are not pre-hashed.
+//
+// Implementations which do not provide the pre-hashed Sign API should implement
+// Signer.Sign by always returning an error.
+type MessageSigner interface {
+	Signer
+	SignMessage(rand io.Reader, msg []byte, opts SignerOpts) (signature []byte, err error)
+}
+
+// SignerOpts contains options for signing with a [Signer].
+type SignerOpts interface {
+	// HashFunc returns an identifier for the hash function used to produce
+	// the message passed to Signer.Sign, or else zero to indicate that no
+	// hashing was done.
+	HashFunc() Hash
+}
+
+// Decrypter is an interface for an opaque private key that can be used for
+// asymmetric decryption operations. An example would be an RSA key
+// kept in a hardware module.
+type Decrypter interface {
+	// Public returns the public key corresponding to the opaque,
+	// private key.
+	Public() PublicKey
+
+	// Decrypt decrypts msg. The opts argument should be appropriate for
+	// the primitive used. See the documentation in each implementation for
+	// details.
+	Decrypt(rand io.Reader, msg []byte, opts DecrypterOpts) (plaintext []byte, err error)
+}
+
+type DecrypterOpts any
+
+// SignMessage signs msg with signer. If signer implements [MessageSigner],
+// [MessageSigner.SignMessage] is called directly. Otherwise, msg is hashed
+// with opts.HashFunc() and signed with [Signer.Sign].
+func SignMessage(signer Signer, rand io.Reader, msg []byte, opts SignerOpts) (signature []byte, err error) {
+	if ms, ok := signer.(MessageSigner); ok {
+		return ms.SignMessage(rand, msg, opts)
+	}
+	if opts.HashFunc() != 0 {
+		h := opts.HashFunc().New()
+		h.Write(msg)
+		msg = h.Sum(nil)
+	}
+	return signer.Sign(rand, msg, opts)
+}
+
+// Decapsulator is an interface for an opaque private KEM key that can be used for
+// decapsulation operations. For example, an ML-KEM key kept in a hardware module.
+//
+// It is implemented, for example, by [crypto/mlkem.DecapsulationKey768].
+type Decapsulator interface {
+	Encapsulator() Encapsulator
+	Decapsulate(ciphertext []byte) (sharedKey []byte, err error)
+}
+
+// Encapsulator is an interface for a public KEM key that can be used for
+// encapsulation operations.
+//
+// It is implemented, for example, by [crypto/mlkem.EncapsulationKey768].
+type Encapsulator interface {
+	Bytes() []byte
+	Encapsulate() (sharedKey, ciphertext []byte)
+}

go/src/crypto/crypto_test.go

@@ -0,0 +1,143 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package crypto_test
+
+import (
+	"bytes"
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
+	"errors"
+	"internal/testenv"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strings"
+	"testing"
+)
+
+type messageSignerOnly struct {
+	k *rsa.PrivateKey
+}
+
+func (s *messageSignerOnly) Public() crypto.PublicKey {
+	return s.k.Public()
+}
+
+func (s *messageSignerOnly) Sign(_ io.Reader, _ []byte, _ crypto.SignerOpts) ([]byte, error) {
+	return nil, errors.New("unimplemented")
+}
+
+func (s *messageSignerOnly) SignMessage(rand io.Reader, msg []byte, opts crypto.SignerOpts) ([]byte, error) {
+	h := opts.HashFunc().New()
+	h.Write(msg)
+	digest := h.Sum(nil)
+	return s.k.Sign(rand, digest, opts)
+}
+
+func TestSignMessage(t *testing.T) {
+	block, _ := pem.Decode([]byte(strings.ReplaceAll(
+		`-----BEGIN RSA TESTING KEY-----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+-----END RSA TESTING KEY-----`, "TESTING KEY", "PRIVATE KEY")))
+	k, _ := x509.ParsePKCS1PrivateKey(block.Bytes)
+
+	msg := []byte("hello :)")
+
+	h := crypto.SHA256.New()
+	h.Write(msg)
+	digest := h.Sum(nil)
+
+	sig, err := crypto.SignMessage(k, rand.Reader, msg, &rsa.PSSOptions{Hash: crypto.SHA256})
+	if err != nil {
+		t.Fatalf("SignMessage failed with Signer: %s", err)
+	}
+	if err := rsa.VerifyPSS(&k.PublicKey, crypto.SHA256, digest, sig, nil); err != nil {
+		t.Errorf("VerifyPSS failed for Signer signature: %s", err)
+	}
+
+	sig, err = crypto.SignMessage(&messageSignerOnly{k}, rand.Reader, msg, &rsa.PSSOptions{Hash: crypto.SHA256})
+	if err != nil {
+		t.Fatalf("SignMessage failed with MessageSigner: %s", err)
+	}
+	if err := rsa.VerifyPSS(&k.PublicKey, crypto.SHA256, digest, sig, nil); err != nil {
+		t.Errorf("VerifyPSS failed for MessageSigner signature: %s", err)
+	}
+}
+
+func TestDisallowedAssemblyInstructions(t *testing.T) {
+	// This test enforces the cryptography assembly policy rule that we do not
+	// use BYTE or WORD instructions, since these instructions can obscure what
+	// the assembly is actually doing. If we do not support specific
+	// instructions in the assembler, we should not be using them until we do.
+	//
+	// Instead of using the output of the 'go tool asm' tool, we take the simple
+	// approach and just search the text of .s files for usage of BYTE and WORD.
+	// We do this because the assembler itself will sometimes insert WORD
+	// instructions for things like function preambles etc.
+
+	boringSigPath := filepath.Join("internal", "boring", "sig")
+
+	matcher, err := regexp.Compile(`(^|;)\s(BYTE|WORD)\s`)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := filepath.WalkDir(filepath.Join(testenv.GOROOT(t), "src/crypto"), func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			t.Fatal(err)
+		}
+		if d.IsDir() || !strings.HasSuffix(path, ".s") {
+			return nil
+		}
+		if strings.Contains(path, boringSigPath) {
+			return nil
+		}
+
+		f, err := os.ReadFile(path)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		i := 1
+		for line := range bytes.Lines(f) {
+			if matcher.Match(line) {
+				t.Errorf("%s:%d assembly contains BYTE or WORD instruction (%q)", path, i, string(line))
+			}
+			i++
+		}
+
+		return nil
+	}); err != nil {
+		t.Fatal(err)
+	}
+}

go/src/crypto/issue21104_test.go

@@ -0,0 +1,61 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package crypto_test
+
+import (
+	"crypto/aes"
+	"crypto/cipher"
+	"crypto/rc4"
+	"testing"
+)
+
+func TestRC4OutOfBoundsWrite(t *testing.T) {
+	// This cipherText is encrypted "0123456789"
+	cipherText := []byte{238, 41, 187, 114, 151, 2, 107, 13, 178, 63}
+	cipher, err := rc4.NewCipher([]byte{0})
+	if err != nil {
+		panic(err)
+	}
+	test(t, "RC4", cipherText, cipher.XORKeyStream)
+}
+func TestCTROutOfBoundsWrite(t *testing.T) {
+	testBlock(t, "CTR", cipher.NewCTR)
+}
+func TestOFBOutOfBoundsWrite(t *testing.T) {
+	testBlock(t, "OFB", cipher.NewOFB)
+}
+func TestCFBEncryptOutOfBoundsWrite(t *testing.T) {
+	testBlock(t, "CFB Encrypt", cipher.NewCFBEncrypter)
+}
+func TestCFBDecryptOutOfBoundsWrite(t *testing.T) {
+	testBlock(t, "CFB Decrypt", cipher.NewCFBDecrypter)
+}
+func testBlock(t *testing.T, name string, newCipher func(cipher.Block, []byte) cipher.Stream) {
+	// This cipherText is encrypted "0123456789"
+	cipherText := []byte{86, 216, 121, 231, 219, 191, 26, 12, 176, 117}
+	var iv, key [16]byte
+	block, err := aes.NewCipher(key[:])
+	if err != nil {
+		panic(err)
+	}
+	stream := newCipher(block, iv[:])
+	test(t, name, cipherText, stream.XORKeyStream)
+}
+func test(t *testing.T, name string, cipherText []byte, xor func([]byte, []byte)) {
+	want := "abcdefghij"
+	plainText := []byte(want)
+	shorterLen := len(cipherText) / 2
+	defer func() {
+		err := recover()
+		if err == nil {
+			t.Errorf("%v XORKeyStream expected to panic on len(dst) < len(src), but didn't", name)
+		}
+		const plain = "0123456789"
+		if plainText[shorterLen] == plain[shorterLen] {
+			t.Errorf("%v XORKeyStream did out of bounds write, want %v, got %v", name, want, string(plainText))
+		}
+	}()
+	xor(plainText[:shorterLen], cipherText)
+}

go/src/crypto/purego_test.go

@@ -0,0 +1,67 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package crypto_test
+
+import (
+	"go/build"
+	"internal/testenv"
+	"log"
+	"os"
+	"strings"
+	"testing"
+)
+
+// TestPureGoTag checks that when built with the purego build tag, crypto
+// packages don't require any assembly. This is used by alternative compilers
+// such as TinyGo. See also the "crypto/...:purego" test in cmd/dist, which
+// ensures the packages build correctly.
+func TestPureGoTag(t *testing.T) {
+	cmd := testenv.Command(t, testenv.GoToolPath(t), "list", "-e", "crypto/...", "math/big")
+	cmd = testenv.CleanCmdEnv(cmd)
+	cmd.Env = append(cmd.Environ(), "GOOS=linux", "GOFIPS140=off")
+	cmd.Stderr = os.Stderr
+	out, err := cmd.Output()
+	if err != nil {
+		log.Fatalf("loading package list: %v\n%s", err, out)
+	}
+	pkgs := strings.Split(strings.TrimSpace(string(out)), "\n")
+
+	cmd = testenv.Command(t, testenv.GoToolPath(t), "tool", "dist", "list")
+	cmd.Stderr = os.Stderr
+	out, err = testenv.CleanCmdEnv(cmd).Output()
+	if err != nil {
+		log.Fatalf("loading architecture list: %v\n%s", err, out)
+	}
+	allGOARCH := make(map[string]bool)
+	for _, pair := range strings.Split(strings.TrimSpace(string(out)), "\n") {
+		GOARCH := strings.Split(pair, "/")[1]
+		allGOARCH[GOARCH] = true
+	}
+
+	for _, pkgName := range pkgs {
+		if strings.Contains(pkgName, "/boring") {
+			continue
+		}
+
+		for GOARCH := range allGOARCH {
+			context := build.Context{
+				GOOS:      "linux", // darwin has custom assembly
+				GOARCH:    GOARCH,
+				GOROOT:    testenv.GOROOT(t),
+				Compiler:  build.Default.Compiler,
+				BuildTags: []string{"purego", "math_big_pure_go"},
+			}
+
+			pkg, err := context.Import(pkgName, "", 0)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if len(pkg.SFiles) == 0 {
+				continue
+			}
+			t.Errorf("package %s has purego assembly files on %s: %v", pkgName, GOARCH, pkg.SFiles)
+		}
+	}
+}

go/src/embed/embed.go

@@ -0,0 +1,436 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package embed provides access to files embedded in the running Go program.
+//
+// Go source files that import "embed" can use the //go:embed directive
+// to initialize a variable of type string, []byte, or [FS] with the contents of
+// files read from the package directory or subdirectories at compile time.
+//
+// For example, here are three ways to embed a file named hello.txt
+// and then print its contents at run time.
+//
+// Embedding one file into a string:
+//
+//	import _ "embed"
+//
+//	//go:embed hello.txt
+//	var s string
+//	print(s)
+//
+// Embedding one file into a slice of bytes:
+//
+//	import _ "embed"
+//
+//	//go:embed hello.txt
+//	var b []byte
+//	print(string(b))
+//
+// Embedded one or more files into a file system:
+//
+//	import "embed"
+//
+//	//go:embed hello.txt
+//	var f embed.FS
+//	data, _ := f.ReadFile("hello.txt")
+//	print(string(data))
+//
+// # Directives
+//
+// A //go:embed directive above a variable declaration specifies which files to embed,
+// using one or more path.Match patterns.
+//
+// The directive must immediately precede a line containing the declaration of a single variable.
+// Only blank lines and ‘//’ line comments are permitted between the directive and the declaration.
+//
+// The type of the variable must be a string type, or a slice of a byte type,
+// or [FS] (or an alias of [FS]).
+//
+// For example:
+//
+//	package server
+//
+//	import "embed"
+//
+//	// content holds our static web server content.
+//	//go:embed image/* template/*
+//	//go:embed html/index.html
+//	var content embed.FS
+//
+// The Go build system will recognize the directives and arrange for the declared variable
+// (in the example above, content) to be populated with the matching files from the file system.
+//
+// The //go:embed directive accepts multiple space-separated patterns for
+// brevity, but it can also be repeated, to avoid very long lines when there are
+// many patterns. The patterns are interpreted relative to the package directory
+// containing the source file. The path separator is a forward slash, even on
+// Windows systems. Patterns may not contain ‘.’ or ‘..’ or empty path elements,
+// nor may they begin or end with a slash. To match everything in the current
+// directory, use ‘*’ instead of ‘.’. To allow for naming files with spaces in
+// their names, patterns can be written as Go double-quoted or back-quoted
+// string literals.
+//
+// If a pattern names a directory, all files in the subtree rooted at that directory are
+// embedded (recursively), except that files with names beginning with ‘.’ or ‘_’
+// are excluded. So the variable in the above example is almost equivalent to:
+//
+//	// content is our static web server content.
+//	//go:embed image template html/index.html
+//	var content embed.FS
+//
+// The difference is that ‘image/*’ embeds ‘image/.tempfile’ while ‘image’ does not.
+// Neither embeds ‘image/dir/.tempfile’.
+//
+// If a pattern begins with the prefix ‘all:’, then the rule for walking directories is changed
+// to include those files beginning with ‘.’ or ‘_’. For example, ‘all:image’ embeds
+// both ‘image/.tempfile’ and ‘image/dir/.tempfile’.
+//
+// The //go:embed directive can be used with both exported and unexported variables,
+// depending on whether the package wants to make the data available to other packages.
+// It can only be used with variables at package scope, not with local variables.
+//
+// Patterns must not match files outside the package's module, such as ‘.git/*’, symbolic links,
+// 'vendor/', or any directories containing go.mod (these are separate modules).
+// Patterns must not match files whose names include the special punctuation characters  " * < > ? ` ' | / \ and :.
+// Matches for empty directories are ignored. After that, each pattern in a //go:embed line
+// must match at least one file or non-empty directory.
+//
+// If any patterns are invalid or have invalid matches, the build will fail.
+//
+// # Strings and Bytes
+//
+// The //go:embed line for a variable of type string or []byte can have only a single pattern,
+// and that pattern can match only a single file. The string or []byte is initialized with
+// the contents of that file.
+//
+// The //go:embed directive requires importing "embed", even when using a string or []byte.
+// In source files that don't refer to [embed.FS], use a blank import (import _ "embed").
+//
+// # File Systems
+//
+// For embedding a single file, a variable of type string or []byte is often best.
+// The [FS] type enables embedding a tree of files, such as a directory of static
+// web server content, as in the example above.
+//
+// FS implements the [io/fs] package's [FS] interface, so it can be used with any package that
+// understands file systems, including [net/http], [text/template], and [html/template].
+//
+// For example, given the content variable in the example above, we can write:
+//
+//	http.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.FS(content))))
+//
+//	template.ParseFS(content, "*.tmpl")
+//
+// # Tools
+//
+// To support tools that analyze Go packages, the patterns found in //go:embed lines
+// are available in “go list” output. See the EmbedPatterns, TestEmbedPatterns,
+// and XTestEmbedPatterns fields in the “go help list” output.
+package embed
+
+import (
+	"errors"
+	"internal/bytealg"
+	"internal/stringslite"
+	"io"
+	"io/fs"
+	"time"
+)
+
+// An FS is a read-only collection of files, usually initialized with a //go:embed directive.
+// When declared without a //go:embed directive, an FS is an empty file system.
+//
+// An FS is a read-only value, so it is safe to use from multiple goroutines
+// simultaneously and also safe to assign values of type FS to each other.
+//
+// FS implements fs.FS, so it can be used with any package that understands
+// file system interfaces, including net/http, text/template, and html/template.
+//
+// See the package documentation for more details about initializing an FS.
+type FS struct {
+	// The compiler knows the layout of this struct.
+	// See cmd/compile/internal/staticdata's WriteEmbed.
+	//
+	// The files list is sorted by name but not by simple string comparison.
+	// Instead, each file's name takes the form "dir/elem" or "dir/elem/".
+	// The optional trailing slash indicates that the file is itself a directory.
+	// The files list is sorted first by dir (if dir is missing, it is taken to be ".")
+	// and then by base, so this list of files:
+	//
+	//	p
+	//	q/
+	//	q/r
+	//	q/s/
+	//	q/s/t
+	//	q/s/u
+	//	q/v
+	//	w
+	//
+	// is actually sorted as:
+	//
+	//	p       # dir=.    elem=p
+	//	q/      # dir=.    elem=q
+	//	w       # dir=.    elem=w
+	//	q/r     # dir=q    elem=r
+	//	q/s/    # dir=q    elem=s
+	//	q/v     # dir=q    elem=v
+	//	q/s/t   # dir=q/s  elem=t
+	//	q/s/u   # dir=q/s  elem=u
+	//
+	// This order brings directory contents together in contiguous sections
+	// of the list, allowing a directory read to use binary search to find
+	// the relevant sequence of entries.
+	files *[]file
+}
+
+// split splits the name into dir and elem as described in the
+// comment in the FS struct above. isDir reports whether the
+// final trailing slash was present, indicating that name is a directory.
+func split(name string) (dir, elem string, isDir bool) {
+	name, isDir = stringslite.CutSuffix(name, "/")
+	i := bytealg.LastIndexByteString(name, '/')
+	if i < 0 {
+		return ".", name, isDir
+	}
+	return name[:i], name[i+1:], isDir
+}
+
+var (
+	_ fs.ReadDirFS  = FS{}
+	_ fs.ReadFileFS = FS{}
+)
+
+// A file is a single file in the FS.
+// It implements fs.FileInfo and fs.DirEntry.
+type file struct {
+	// The compiler knows the layout of this struct.
+	// See cmd/compile/internal/staticdata's WriteEmbed.
+	name string
+	data string
+	hash [16]byte // truncated SHA256 hash
+}
+
+var (
+	_ fs.FileInfo = (*file)(nil)
+	_ fs.DirEntry = (*file)(nil)
+)
+
+func (f *file) Name() string               { _, elem, _ := split(f.name); return elem }
+func (f *file) Size() int64                { return int64(len(f.data)) }
+func (f *file) ModTime() time.Time         { return time.Time{} }
+func (f *file) IsDir() bool                { _, _, isDir := split(f.name); return isDir }
+func (f *file) Sys() any                   { return nil }
+func (f *file) Type() fs.FileMode          { return f.Mode().Type() }
+func (f *file) Info() (fs.FileInfo, error) { return f, nil }
+
+func (f *file) Mode() fs.FileMode {
+	if f.IsDir() {
+		return fs.ModeDir | 0555
+	}
+	return 0444
+}
+
+func (f *file) String() string {
+	return fs.FormatFileInfo(f)
+}
+
+// dotFile is a file for the root directory,
+// which is omitted from the files list in a FS.
+var dotFile = &file{name: "./"}
+
+// lookup returns the named file, or nil if it is not present.
+func (f FS) lookup(name string) *file {
+	if !fs.ValidPath(name) {
+		// The compiler should never emit a file with an invalid name,
+		// so this check is not strictly necessary (if name is invalid,
+		// we shouldn't find a match below), but it's a good backstop anyway.
+		return nil
+	}
+	if name == "." {
+		return dotFile
+	}
+	if f.files == nil {
+		return nil
+	}
+
+	// Binary search to find where name would be in the list,
+	// and then check if name is at that position.
+	dir, elem, _ := split(name)
+	files := *f.files
+	i := sortSearch(len(files), func(i int) bool {
+		idir, ielem, _ := split(files[i].name)
+		return idir > dir || idir == dir && ielem >= elem
+	})
+	if i < len(files) && stringslite.TrimSuffix(files[i].name, "/") == name {
+		return &files[i]
+	}
+	return nil
+}
+
+// readDir returns the list of files corresponding to the directory dir.
+func (f FS) readDir(dir string) []file {
+	if f.files == nil {
+		return nil
+	}
+	// Binary search to find where dir starts and ends in the list
+	// and then return that slice of the list.
+	files := *f.files
+	i := sortSearch(len(files), func(i int) bool {
+		idir, _, _ := split(files[i].name)
+		return idir >= dir
+	})
+	j := sortSearch(len(files), func(j int) bool {
+		jdir, _, _ := split(files[j].name)
+		return jdir > dir
+	})
+	return files[i:j]
+}
+
+// Open opens the named file for reading and returns it as an [fs.File].
+//
+// The returned file implements [io.Seeker] and [io.ReaderAt] when the file is not a directory.
+func (f FS) Open(name string) (fs.File, error) {
+	file := f.lookup(name)
+	if file == nil {
+		return nil, &fs.PathError{Op: "open", Path: name, Err: fs.ErrNotExist}
+	}
+	if file.IsDir() {
+		return &openDir{file, f.readDir(name), 0}, nil
+	}
+	return &openFile{file, 0}, nil
+}
+
+// ReadDir reads and returns the entire named directory.
+func (f FS) ReadDir(name string) ([]fs.DirEntry, error) {
+	file, err := f.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	dir, ok := file.(*openDir)
+	if !ok {
+		return nil, &fs.PathError{Op: "read", Path: name, Err: errors.New("not a directory")}
+	}
+	list := make([]fs.DirEntry, len(dir.files))
+	for i := range list {
+		list[i] = &dir.files[i]
+	}
+	return list, nil
+}
+
+// ReadFile reads and returns the content of the named file.
+func (f FS) ReadFile(name string) ([]byte, error) {
+	file, err := f.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	ofile, ok := file.(*openFile)
+	if !ok {
+		return nil, &fs.PathError{Op: "read", Path: name, Err: errors.New("is a directory")}
+	}
+	return []byte(ofile.f.data), nil
+}
+
+// An openFile is a regular file open for reading.
+type openFile struct {
+	f      *file // the file itself
+	offset int64 // current read offset
+}
+
+var (
+	_ io.Seeker   = (*openFile)(nil)
+	_ io.ReaderAt = (*openFile)(nil)
+)
+
+func (f *openFile) Close() error               { return nil }
+func (f *openFile) Stat() (fs.FileInfo, error) { return f.f, nil }
+
+func (f *openFile) Read(b []byte) (int, error) {
+	if f.offset >= int64(len(f.f.data)) {
+		return 0, io.EOF
+	}
+	if f.offset < 0 {
+		return 0, &fs.PathError{Op: "read", Path: f.f.name, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.data[f.offset:])
+	f.offset += int64(n)
+	return n, nil
+}
+
+func (f *openFile) Seek(offset int64, whence int) (int64, error) {
+	switch whence {
+	case 0:
+		// offset += 0
+	case 1:
+		offset += f.offset
+	case 2:
+		offset += int64(len(f.f.data))
+	}
+	if offset < 0 || offset > int64(len(f.f.data)) {
+		return 0, &fs.PathError{Op: "seek", Path: f.f.name, Err: fs.ErrInvalid}
+	}
+	f.offset = offset
+	return offset, nil
+}
+
+func (f *openFile) ReadAt(b []byte, offset int64) (int, error) {
+	if offset < 0 || offset > int64(len(f.f.data)) {
+		return 0, &fs.PathError{Op: "read", Path: f.f.name, Err: fs.ErrInvalid}
+	}
+	n := copy(b, f.f.data[offset:])
+	if n < len(b) {
+		return n, io.EOF
+	}
+	return n, nil
+}
+
+// An openDir is a directory open for reading.
+type openDir struct {
+	f      *file  // the directory file itself
+	files  []file // the directory contents
+	offset int    // the read offset, an index into the files slice
+}
+
+func (d *openDir) Close() error               { return nil }
+func (d *openDir) Stat() (fs.FileInfo, error) { return d.f, nil }
+
+func (d *openDir) Read([]byte) (int, error) {
+	return 0, &fs.PathError{Op: "read", Path: d.f.name, Err: errors.New("is a directory")}
+}
+
+func (d *openDir) ReadDir(count int) ([]fs.DirEntry, error) {
+	n := len(d.files) - d.offset
+	if n == 0 {
+		if count <= 0 {
+			return nil, nil
+		}
+		return nil, io.EOF
+	}
+	if count > 0 && n > count {
+		n = count
+	}
+	list := make([]fs.DirEntry, n)
+	for i := range list {
+		list[i] = &d.files[d.offset+i]
+	}
+	d.offset += n
+	return list, nil
+}
+
+// sortSearch is like sort.Search, avoiding an import.
+func sortSearch(n int, f func(int) bool) int {
+	// Define f(-1) == false and f(n) == true.
+	// Invariant: f(i-1) == false, f(j) == true.
+	i, j := 0, n
+	for i < j {
+		h := int(uint(i+j) >> 1) // avoid overflow when computing h
+		// i ≤ h < j
+		if !f(h) {
+			i = h + 1 // preserves f(i-1) == false
+		} else {
+			j = h // preserves f(j) == true
+		}
+	}
+	// i == j, f(i-1) == false, and f(j) (= f(i)) == true  =>  answer is i.
+	return i
+}

go/src/embed/example_test.go

@@ -0,0 +1,23 @@
+// Copyright 2021 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package embed_test
+
+import (
+	"embed"
+	"log"
+	"net/http"
+)
+
+//go:embed internal/embedtest/testdata/*.txt
+var content embed.FS
+
+func Example() {
+	mux := http.NewServeMux()
+	mux.Handle("/", http.FileServer(http.FS(content)))
+	err := http.ListenAndServe(":8080", mux)
+	if err != nil {
+		log.Fatal(err)
+	}
+}

go/src/encoding/encoding.go

@@ -0,0 +1,78 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package encoding defines interfaces shared by other packages that
+// convert data to and from byte-level and textual representations.
+// Packages that check for these interfaces include encoding/gob,
+// encoding/json, and encoding/xml. As a result, implementing an
+// interface once can make a type useful in multiple encodings.
+// Standard types that implement these interfaces include time.Time and net.IP.
+// The interfaces come in pairs that produce and consume encoded data.
+//
+// Adding encoding/decoding methods to existing types may constitute a breaking change,
+// as they can be used for serialization in communicating with programs
+// written with different library versions.
+// The policy for packages maintained by the Go project is to only allow
+// the addition of marshaling functions if no existing, reasonable marshaling exists.
+package encoding
+
+// BinaryMarshaler is the interface implemented by an object that can
+// marshal itself into a binary form.
+//
+// MarshalBinary encodes the receiver into a binary form and returns the result.
+type BinaryMarshaler interface {
+	MarshalBinary() (data []byte, err error)
+}
+
+// BinaryUnmarshaler is the interface implemented by an object that can
+// unmarshal a binary representation of itself.
+//
+// UnmarshalBinary must be able to decode the form generated by MarshalBinary.
+// UnmarshalBinary must copy the data if it wishes to retain the data
+// after returning.
+type BinaryUnmarshaler interface {
+	UnmarshalBinary(data []byte) error
+}
+
+// BinaryAppender is the interface implemented by an object
+// that can append the binary representation of itself.
+// If a type implements both [BinaryAppender] and [BinaryMarshaler],
+// then v.MarshalBinary() must be semantically identical to v.AppendBinary(nil).
+type BinaryAppender interface {
+	// AppendBinary appends the binary representation of itself to the end of b
+	// (allocating a larger slice if necessary) and returns the updated slice.
+	//
+	// Implementations must not retain b, nor mutate any bytes within b[:len(b)].
+	AppendBinary(b []byte) ([]byte, error)
+}
+
+// TextMarshaler is the interface implemented by an object that can
+// marshal itself into a textual form.
+//
+// MarshalText encodes the receiver into UTF-8-encoded text and returns the result.
+type TextMarshaler interface {
+	MarshalText() (text []byte, err error)
+}
+
+// TextUnmarshaler is the interface implemented by an object that can
+// unmarshal a textual representation of itself.
+//
+// UnmarshalText must be able to decode the form generated by MarshalText.
+// UnmarshalText must copy the text if it wishes to retain the text
+// after returning.
+type TextUnmarshaler interface {
+	UnmarshalText(text []byte) error
+}
+
+// TextAppender is the interface implemented by an object
+// that can append the textual representation of itself.
+// If a type implements both [TextAppender] and [TextMarshaler],
+// then v.MarshalText() must be semantically identical to v.AppendText(nil).
+type TextAppender interface {
+	// AppendText appends the textual representation of itself to the end of b
+	// (allocating a larger slice if necessary) and returns the updated slice.
+	//
+	// Implementations must not retain b, nor mutate any bytes within b[:len(b)].
+	AppendText(b []byte) ([]byte, error)
+}

go/src/errors/errors.go

@@ -0,0 +1,90 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package errors implements functions to manipulate errors.
+//
+// The [New] function creates errors whose only content is a text message.
+//
+// An error e wraps another error if e's type has one of the methods
+//
+//	Unwrap() error
+//	Unwrap() []error
+//
+// If e.Unwrap() returns a non-nil error w or a slice containing w,
+// then we say that e wraps w. A nil error returned from e.Unwrap()
+// indicates that e does not wrap any error. It is invalid for an
+// Unwrap method to return an []error containing a nil error value.
+//
+// An easy way to create wrapped errors is to call [fmt.Errorf] and apply
+// the %w verb to the error argument:
+//
+//	wrapsErr := fmt.Errorf("... %w ...", ..., err, ...)
+//
+// Successive unwrapping of an error creates a tree. The [Is] and [As]
+// functions inspect an error's tree by examining first the error
+// itself followed by the tree of each of its children in turn
+// (pre-order, depth-first traversal).
+//
+// See https://go.dev/blog/go1.13-errors for a deeper discussion of the
+// philosophy of wrapping and when to wrap.
+//
+// [Is] examines the tree of its first argument looking for an error that
+// matches the second. It reports whether it finds a match. It should be
+// used in preference to simple equality checks:
+//
+//	if errors.Is(err, fs.ErrExist)
+//
+// is preferable to
+//
+//	if err == fs.ErrExist
+//
+// because the former will succeed if err wraps [io/fs.ErrExist].
+//
+// [AsType] examines the tree of its argument looking for an error whose
+// type matches its type argument. If it succeeds, it returns the
+// corresponding value of that type and true. Otherwise, it returns the
+// zero value of that type and false. The form
+//
+//	if perr, ok := errors.AsType[*fs.PathError](err); ok {
+//		fmt.Println(perr.Path)
+//	}
+//
+// is preferable to
+//
+//	if perr, ok := err.(*fs.PathError); ok {
+//		fmt.Println(perr.Path)
+//	}
+//
+// because the former will succeed if err wraps an [*io/fs.PathError].
+package errors
+
+// New returns an error that formats as the given text.
+// Each call to New returns a distinct error value even if the text is identical.
+func New(text string) error {
+	return &errorString{text}
+}
+
+// errorString is a trivial implementation of error.
+type errorString struct {
+	s string
+}
+
+func (e *errorString) Error() string {
+	return e.s
+}
+
+// ErrUnsupported indicates that a requested operation cannot be performed,
+// because it is unsupported. For example, a call to [os.Link] when using a
+// file system that does not support hard links.
+//
+// Functions and methods should not return this error but should instead
+// return an error including appropriate context that satisfies
+//
+//	errors.Is(err, errors.ErrUnsupported)
+//
+// either by directly wrapping ErrUnsupported or by implementing an [Is] method.
+//
+// Functions and methods should document the cases in which an error
+// wrapping this will be returned.
+var ErrUnsupported = New("unsupported operation")

go/src/errors/errors_test.go

@@ -0,0 +1,33 @@
+// Copyright 2011 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestNewEqual(t *testing.T) {
+	// Different allocations should not be equal.
+	if errors.New("abc") == errors.New("abc") {
+		t.Errorf(`New("abc") == New("abc")`)
+	}
+	if errors.New("abc") == errors.New("xyz") {
+		t.Errorf(`New("abc") == New("xyz")`)
+	}
+
+	// Same allocation should be equal to itself (not crash).
+	err := errors.New("jkl")
+	if err != err {
+		t.Errorf(`err != err`)
+	}
+}
+
+func TestErrorMethod(t *testing.T) {
+	err := errors.New("abc")
+	if err.Error() != "abc" {
+		t.Errorf(`New("abc").Error() = %q, want %q`, err.Error(), "abc")
+	}
+}

go/src/errors/example_test.go

@@ -0,0 +1,238 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"time"
+)
+
+// MyError is an error implementation that includes a time and message.
+type MyError struct {
+	When time.Time
+	What string
+}
+
+func (e MyError) Error() string {
+	return fmt.Sprintf("%v: %v", e.When, e.What)
+}
+
+func oops() error {
+	return MyError{
+		time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
+		"the file system has gone away",
+	}
+}
+
+func Example() {
+	if err := oops(); err != nil {
+		fmt.Println(err)
+	}
+	// Output: 1989-03-15 22:30:00 +0000 UTC: the file system has gone away
+}
+
+func ExampleNew() {
+	err := errors.New("emit macho dwarf: elf header corrupted")
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: emit macho dwarf: elf header corrupted
+}
+
+func OopsNew() error {
+	return errors.New("an error")
+}
+
+var ErrSentinel = errors.New("an error")
+
+func OopsSentinel() error {
+	return ErrSentinel
+}
+
+// Each call to [errors.New] returns an unique instance of the error,
+// even if the arguments are the same. To match against errors
+// created by [errors.New], declare a sentinel error and reuse it.
+func ExampleNew_unique() {
+	err1 := OopsNew()
+	err2 := OopsNew()
+	fmt.Println("Errors using distinct errors.New calls:")
+	fmt.Printf("Is(%q, %q) = %v\n", err1, err2, errors.Is(err1, err2))
+
+	err3 := OopsSentinel()
+	err4 := OopsSentinel()
+	fmt.Println()
+	fmt.Println("Errors using a sentinel error:")
+	fmt.Printf("Is(%q, %q) = %v\n", err3, err4, errors.Is(err3, err4))
+
+	// Output:
+	// Errors using distinct errors.New calls:
+	// Is("an error", "an error") = false
+	//
+	// Errors using a sentinel error:
+	// Is("an error", "an error") = true
+}
+
+// The fmt package's Errorf function lets us use the package's formatting
+// features to create descriptive error messages.
+func ExampleNew_errorf() {
+	const name, id = "bimmler", 17
+	err := fmt.Errorf("user %q (id %d) not found", name, id)
+	if err != nil {
+		fmt.Print(err)
+	}
+	// Output: user "bimmler" (id 17) not found
+}
+
+func ExampleJoin() {
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+	err := errors.Join(err1, err2)
+	fmt.Println(err)
+	if errors.Is(err, err1) {
+		fmt.Println("err is err1")
+	}
+	if errors.Is(err, err2) {
+		fmt.Println("err is err2")
+	}
+	fmt.Println(err.(interface{ Unwrap() []error }).Unwrap())
+	// Output:
+	// err1
+	// err2
+	// err is err1
+	// err is err2
+	// [err1 err2]
+}
+
+func ExampleIs() {
+	if _, err := os.Open("non-existing"); err != nil {
+		if errors.Is(err, fs.ErrNotExist) {
+			fmt.Println("file does not exist")
+		} else {
+			fmt.Println(err)
+		}
+	}
+
+	// Output:
+	// file does not exist
+}
+
+type MyIsError struct {
+	err string
+}
+
+func (e MyIsError) Error() string {
+	return e.err
+}
+func (e MyIsError) Is(err error) bool {
+	return err == fs.ErrPermission
+}
+
+// Custom errors can implement a method "Is(error) bool" to match other error values,
+// overriding the default matching of [errors.Is].
+func ExampleIs_custom_match() {
+	var err error = MyIsError{"an error"}
+	fmt.Println("Error equals fs.ErrPermission:", err == fs.ErrPermission)
+	fmt.Println("Error is fs.ErrPermission:", errors.Is(err, fs.ErrPermission))
+
+	// Output:
+	// Error equals fs.ErrPermission: false
+	// Error is fs.ErrPermission: true
+}
+
+func ExampleAs() {
+	if _, err := os.Open("non-existing"); err != nil {
+		var pathError *fs.PathError
+		if errors.As(err, &pathError) {
+			fmt.Println("Failed at path:", pathError.Path)
+		} else {
+			fmt.Println(err)
+		}
+	}
+
+	// Output:
+	// Failed at path: non-existing
+}
+
+func ExampleAsType() {
+	if _, err := os.Open("non-existing"); err != nil {
+		if pathError, ok := errors.AsType[*fs.PathError](err); ok {
+			fmt.Println("Failed at path:", pathError.Path)
+		} else {
+			fmt.Println(err)
+		}
+	}
+	// Output:
+	// Failed at path: non-existing
+}
+
+type MyAsError struct {
+	err string
+}
+
+func (e MyAsError) Error() string {
+	return e.err
+}
+func (e MyAsError) As(target any) bool {
+	pe, ok := target.(**fs.PathError)
+	if !ok {
+		return false
+	}
+	*pe = &fs.PathError{
+		Op:   "custom",
+		Path: "/",
+		Err:  errors.New(e.err),
+	}
+	return true
+}
+
+// Custom errors can implement a method "As(any) bool" to match against other error types,
+// overriding the default matching of [errors.As].
+func ExampleAs_custom_match() {
+	var err error = MyAsError{"an error"}
+	fmt.Println("Error:", err)
+	fmt.Printf("TypeOf err: %T\n", err)
+
+	var pathError *fs.PathError
+	ok := errors.As(err, &pathError)
+	fmt.Println("Error as fs.PathError:", ok)
+	fmt.Println("fs.PathError:", pathError)
+
+	// Output:
+	// Error: an error
+	// TypeOf err: errors_test.MyAsError
+	// Error as fs.PathError: true
+	// fs.PathError: custom /: an error
+}
+
+// Custom errors can implement a method "As(any) bool" to match against other error types,
+// overriding the default matching of [errors.AsType].
+func ExampleAsType_custom_match() {
+	var err error = MyAsError{"an error"}
+	fmt.Println("Error:", err)
+	fmt.Printf("TypeOf err: %T\n", err)
+
+	pathError, ok := errors.AsType[*fs.PathError](err)
+	fmt.Println("Error as fs.PathError:", ok)
+	fmt.Println("fs.PathError:", pathError)
+
+	// Output:
+	// Error: an error
+	// TypeOf err: errors_test.MyAsError
+	// Error as fs.PathError: true
+	// fs.PathError: custom /: an error
+}
+
+func ExampleUnwrap() {
+	err1 := errors.New("error1")
+	err2 := fmt.Errorf("error2: [%w]", err1)
+	fmt.Println(err2)
+	fmt.Println(errors.Unwrap(err2))
+	// Output:
+	// error2: [error1]
+	// error1
+}

go/src/errors/join.go

@@ -0,0 +1,63 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors
+
+import (
+	"unsafe"
+)
+
+// Join returns an error that wraps the given errors.
+// Any nil error values are discarded.
+// Join returns nil if every value in errs is nil.
+// The error formats as the concatenation of the strings obtained
+// by calling the Error method of each element of errs, with a newline
+// between each string.
+//
+// A non-nil error returned by Join implements the Unwrap() []error method.
+// The errors may be inspected with [Is] and [As].
+func Join(errs ...error) error {
+	n := 0
+	for _, err := range errs {
+		if err != nil {
+			n++
+		}
+	}
+	if n == 0 {
+		return nil
+	}
+	e := &joinError{
+		errs: make([]error, 0, n),
+	}
+	for _, err := range errs {
+		if err != nil {
+			e.errs = append(e.errs, err)
+		}
+	}
+	return e
+}
+
+type joinError struct {
+	errs []error
+}
+
+func (e *joinError) Error() string {
+	// Since Join returns nil if every value in errs is nil,
+	// e.errs cannot be empty.
+	if len(e.errs) == 1 {
+		return e.errs[0].Error()
+	}
+
+	b := []byte(e.errs[0].Error())
+	for _, err := range e.errs[1:] {
+		b = append(b, '\n')
+		b = append(b, err.Error()...)
+	}
+	// At this point, b has at least one byte '\n'.
+	return unsafe.String(&b[0], len(b))
+}
+
+func (e *joinError) Unwrap() []error {
+	return e.errs
+}

go/src/errors/join_test.go

@@ -0,0 +1,76 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"reflect"
+	"testing"
+)
+
+func TestJoinReturnsNil(t *testing.T) {
+	if err := errors.Join(); err != nil {
+		t.Errorf("errors.Join() = %v, want nil", err)
+	}
+	if err := errors.Join(nil); err != nil {
+		t.Errorf("errors.Join(nil) = %v, want nil", err)
+	}
+	if err := errors.Join(nil, nil); err != nil {
+		t.Errorf("errors.Join(nil, nil) = %v, want nil", err)
+	}
+}
+
+func TestJoin(t *testing.T) {
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+	merr := multiErr{errors.New("err3")}
+	for _, test := range []struct {
+		errs []error
+		want []error
+	}{{
+		errs: []error{err1},
+		want: []error{err1},
+	}, {
+		errs: []error{err1, err2},
+		want: []error{err1, err2},
+	}, {
+		errs: []error{err1, nil, err2},
+		want: []error{err1, err2},
+	}, {
+		errs: []error{merr},
+		want: []error{merr},
+	}} {
+		got := errors.Join(test.errs...).(interface{ Unwrap() []error }).Unwrap()
+		if !reflect.DeepEqual(got, test.want) {
+			t.Errorf("Join(%v) = %v; want %v", test.errs, got, test.want)
+		}
+		if len(got) != cap(got) {
+			t.Errorf("Join(%v) returns errors with len=%v, cap=%v; want len==cap", test.errs, len(got), cap(got))
+		}
+	}
+}
+
+func TestJoinErrorMethod(t *testing.T) {
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+	for _, test := range []struct {
+		errs []error
+		want string
+	}{{
+		errs: []error{err1},
+		want: "err1",
+	}, {
+		errs: []error{err1, err2},
+		want: "err1\nerr2",
+	}, {
+		errs: []error{err1, nil, err2},
+		want: "err1\nerr2",
+	}} {
+		got := errors.Join(test.errs...).Error()
+		if got != test.want {
+			t.Errorf("Join(%v).Error() = %q; want %q", test.errs, got, test.want)
+		}
+	}
+}

go/src/errors/wrap.go

@@ -0,0 +1,209 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors
+
+import (
+	"internal/reflectlite"
+)
+
+// Unwrap returns the result of calling the Unwrap method on err, if err's
+// type contains an Unwrap method returning error.
+// Otherwise, Unwrap returns nil.
+//
+// Unwrap only calls a method of the form "Unwrap() error".
+// In particular Unwrap does not unwrap errors returned by [Join].
+func Unwrap(err error) error {
+	u, ok := err.(interface {
+		Unwrap() error
+	})
+	if !ok {
+		return nil
+	}
+	return u.Unwrap()
+}
+
+// Is reports whether any error in err's tree matches target.
+// The target must be comparable.
+//
+// The tree consists of err itself, followed by the errors obtained by repeatedly
+// calling its Unwrap() error or Unwrap() []error method. When err wraps multiple
+// errors, Is examines err followed by a depth-first traversal of its children.
+//
+// An error is considered to match a target if it is equal to that target or if
+// it implements a method Is(error) bool such that Is(target) returns true.
+//
+// An error type might provide an Is method so it can be treated as equivalent
+// to an existing error. For example, if MyError defines
+//
+//	func (m MyError) Is(target error) bool { return target == fs.ErrExist }
+//
+// then Is(MyError{}, fs.ErrExist) returns true. See [syscall.Errno.Is] for
+// an example in the standard library. An Is method should only shallowly
+// compare err and the target and not call [Unwrap] on either.
+func Is(err, target error) bool {
+	if err == nil || target == nil {
+		return err == target
+	}
+
+	isComparable := reflectlite.TypeOf(target).Comparable()
+	return is(err, target, isComparable)
+}
+
+func is(err, target error, targetComparable bool) bool {
+	for {
+		if targetComparable && err == target {
+			return true
+		}
+		if x, ok := err.(interface{ Is(error) bool }); ok && x.Is(target) {
+			return true
+		}
+		switch x := err.(type) {
+		case interface{ Unwrap() error }:
+			err = x.Unwrap()
+			if err == nil {
+				return false
+			}
+		case interface{ Unwrap() []error }:
+			for _, err := range x.Unwrap() {
+				if is(err, target, targetComparable) {
+					return true
+				}
+			}
+			return false
+		default:
+			return false
+		}
+	}
+}
+
+// As finds the first error in err's tree that matches target, and if one is found, sets
+// target to that error value and returns true. Otherwise, it returns false.
+//
+// For most uses, prefer [AsType]. As is equivalent to [AsType] but sets its target
+// argument rather than returning the matching error and doesn't require its target
+// argument to implement error.
+//
+// The tree consists of err itself, followed by the errors obtained by repeatedly
+// calling its Unwrap() error or Unwrap() []error method. When err wraps multiple
+// errors, As examines err followed by a depth-first traversal of its children.
+//
+// An error matches target if the error's concrete value is assignable to the value
+// pointed to by target, or if the error has a method As(any) bool such that
+// As(target) returns true. In the latter case, the As method is responsible for
+// setting target.
+//
+// An error type might provide an As method so it can be treated as if it were a
+// different error type.
+//
+// As panics if target is not a non-nil pointer to either a type that implements
+// error, or to any interface type.
+func As(err error, target any) bool {
+	if err == nil {
+		return false
+	}
+	if target == nil {
+		panic("errors: target cannot be nil")
+	}
+	val := reflectlite.ValueOf(target)
+	typ := val.Type()
+	if typ.Kind() != reflectlite.Ptr || val.IsNil() {
+		panic("errors: target must be a non-nil pointer")
+	}
+	targetType := typ.Elem()
+	if targetType.Kind() != reflectlite.Interface && !targetType.Implements(errorType) {
+		panic("errors: *target must be interface or implement error")
+	}
+	return as(err, target, val, targetType)
+}
+
+func as(err error, target any, targetVal reflectlite.Value, targetType reflectlite.Type) bool {
+	for {
+		if reflectlite.TypeOf(err).AssignableTo(targetType) {
+			targetVal.Elem().Set(reflectlite.ValueOf(err))
+			return true
+		}
+		if x, ok := err.(interface{ As(any) bool }); ok && x.As(target) {
+			return true
+		}
+		switch x := err.(type) {
+		case interface{ Unwrap() error }:
+			err = x.Unwrap()
+			if err == nil {
+				return false
+			}
+		case interface{ Unwrap() []error }:
+			for _, err := range x.Unwrap() {
+				if err == nil {
+					continue
+				}
+				if as(err, target, targetVal, targetType) {
+					return true
+				}
+			}
+			return false
+		default:
+			return false
+		}
+	}
+}
+
+var errorType = reflectlite.TypeOf((*error)(nil)).Elem()
+
+// AsType finds the first error in err's tree that matches the type E, and
+// if one is found, returns that error value and true. Otherwise, it
+// returns the zero value of E and false.
+//
+// The tree consists of err itself, followed by the errors obtained by
+// repeatedly calling its Unwrap() error or Unwrap() []error method. When
+// err wraps multiple errors, AsType examines err followed by a
+// depth-first traversal of its children.
+//
+// An error err matches the type E if the type assertion err.(E) holds,
+// or if the error has a method As(any) bool such that err.As(target)
+// returns true when target is a non-nil *E. In the latter case, the As
+// method is responsible for setting target.
+func AsType[E error](err error) (E, bool) {
+	if err == nil {
+		var zero E
+		return zero, false
+	}
+	var pe *E // lazily initialized
+	return asType(err, &pe)
+}
+
+func asType[E error](err error, ppe **E) (_ E, _ bool) {
+	for {
+		if e, ok := err.(E); ok {
+			return e, true
+		}
+		if x, ok := err.(interface{ As(any) bool }); ok {
+			if *ppe == nil {
+				*ppe = new(E)
+			}
+			if x.As(*ppe) {
+				return **ppe, true
+			}
+		}
+		switch x := err.(type) {
+		case interface{ Unwrap() error }:
+			err = x.Unwrap()
+			if err == nil {
+				return
+			}
+		case interface{ Unwrap() []error }:
+			for _, err := range x.Unwrap() {
+				if err == nil {
+					continue
+				}
+				if x, ok := asType(err, ppe); ok {
+					return x, true
+				}
+			}
+			return
+		default:
+			return
+		}
+	}
+}

go/src/errors/wrap_test.go

@@ -0,0 +1,438 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package errors_test
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"reflect"
+	"testing"
+)
+
+func TestIs(t *testing.T) {
+	err1 := errors.New("1")
+	erra := wrapped{"wrap 2", err1}
+	errb := wrapped{"wrap 3", erra}
+
+	err3 := errors.New("3")
+
+	poser := &poser{"either 1 or 3", func(err error) bool {
+		return err == err1 || err == err3
+	}}
+
+	testCases := []struct {
+		err    error
+		target error
+		match  bool
+	}{
+		{nil, nil, true},
+		{nil, err1, false},
+		{err1, nil, false},
+		{err1, err1, true},
+		{erra, err1, true},
+		{errb, err1, true},
+		{err1, err3, false},
+		{erra, err3, false},
+		{errb, err3, false},
+		{poser, err1, true},
+		{poser, err3, true},
+		{poser, erra, false},
+		{poser, errb, false},
+		{errorUncomparable{}, errorUncomparable{}, true},
+		{errorUncomparable{}, &errorUncomparable{}, false},
+		{&errorUncomparable{}, errorUncomparable{}, true},
+		{&errorUncomparable{}, &errorUncomparable{}, false},
+		{errorUncomparable{}, err1, false},
+		{&errorUncomparable{}, err1, false},
+		{multiErr{}, err1, false},
+		{multiErr{err1, err3}, err1, true},
+		{multiErr{err3, err1}, err1, true},
+		{multiErr{err1, err3}, errors.New("x"), false},
+		{multiErr{err3, errb}, errb, true},
+		{multiErr{err3, errb}, erra, true},
+		{multiErr{err3, errb}, err1, true},
+		{multiErr{errb, err3}, err1, true},
+		{multiErr{poser}, err1, true},
+		{multiErr{poser}, err3, true},
+		{multiErr{nil}, nil, false},
+	}
+	for _, tc := range testCases {
+		t.Run("", func(t *testing.T) {
+			if got := errors.Is(tc.err, tc.target); got != tc.match {
+				t.Errorf("Is(%v, %v) = %v, want %v", tc.err, tc.target, got, tc.match)
+			}
+		})
+	}
+}
+
+type poser struct {
+	msg string
+	f   func(error) bool
+}
+
+var poserPathErr = &fs.PathError{Op: "poser"}
+
+func (p *poser) Error() string     { return p.msg }
+func (p *poser) Is(err error) bool { return p.f(err) }
+func (p *poser) As(err any) bool {
+	switch x := err.(type) {
+	case **poser:
+		*x = p
+	case *errorT:
+		*x = errorT{"poser"}
+	case **fs.PathError:
+		*x = poserPathErr
+	default:
+		return false
+	}
+	return true
+}
+
+func TestAs(t *testing.T) {
+	var errT errorT
+	var errP *fs.PathError
+	var timeout interface{ Timeout() bool }
+	var p *poser
+	_, errF := os.Open("non-existing")
+	poserErr := &poser{"oh no", nil}
+
+	testCases := []struct {
+		err    error
+		target any
+		match  bool
+		want   any // value of target on match
+	}{{
+		nil,
+		&errP,
+		false,
+		nil,
+	}, {
+		wrapped{"pitied the fool", errorT{"T"}},
+		&errT,
+		true,
+		errorT{"T"},
+	}, {
+		errF,
+		&errP,
+		true,
+		errF,
+	}, {
+		errorT{},
+		&errP,
+		false,
+		nil,
+	}, {
+		wrapped{"wrapped", nil},
+		&errT,
+		false,
+		nil,
+	}, {
+		&poser{"error", nil},
+		&errT,
+		true,
+		errorT{"poser"},
+	}, {
+		&poser{"path", nil},
+		&errP,
+		true,
+		poserPathErr,
+	}, {
+		poserErr,
+		&p,
+		true,
+		poserErr,
+	}, {
+		errors.New("err"),
+		&timeout,
+		false,
+		nil,
+	}, {
+		errF,
+		&timeout,
+		true,
+		errF,
+	}, {
+		wrapped{"path error", errF},
+		&timeout,
+		true,
+		errF,
+	}, {
+		multiErr{},
+		&errT,
+		false,
+		nil,
+	}, {
+		multiErr{errors.New("a"), errorT{"T"}},
+		&errT,
+		true,
+		errorT{"T"},
+	}, {
+		multiErr{errorT{"T"}, errors.New("a")},
+		&errT,
+		true,
+		errorT{"T"},
+	}, {
+		multiErr{errorT{"a"}, errorT{"b"}},
+		&errT,
+		true,
+		errorT{"a"},
+	}, {
+		multiErr{multiErr{errors.New("a"), errorT{"a"}}, errorT{"b"}},
+		&errT,
+		true,
+		errorT{"a"},
+	}, {
+		multiErr{wrapped{"path error", errF}},
+		&timeout,
+		true,
+		errF,
+	}, {
+		multiErr{nil},
+		&errT,
+		false,
+		nil,
+	}}
+	for i, tc := range testCases {
+		name := fmt.Sprintf("%d:As(Errorf(..., %v), %v)", i, tc.err, tc.target)
+		// Clear the target pointer, in case it was set in a previous test.
+		rtarget := reflect.ValueOf(tc.target)
+		rtarget.Elem().Set(reflect.Zero(reflect.TypeOf(tc.target).Elem()))
+		t.Run(name, func(t *testing.T) {
+			match := errors.As(tc.err, tc.target)
+			if match != tc.match {
+				t.Fatalf("match: got %v; want %v", match, tc.match)
+			}
+			if !match {
+				return
+			}
+			if got := rtarget.Elem().Interface(); got != tc.want {
+				t.Fatalf("got %#v, want %#v", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestAsValidation(t *testing.T) {
+	var s string
+	testCases := []any{
+		nil,
+		(*int)(nil),
+		"error",
+		&s,
+	}
+	err := errors.New("error")
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%T(%v)", tc, tc), func(t *testing.T) {
+			defer func() {
+				recover()
+			}()
+			if errors.As(err, tc) {
+				t.Errorf("As(err, %T(%v)) = true, want false", tc, tc)
+				return
+			}
+			t.Errorf("As(err, %T(%v)) did not panic", tc, tc)
+		})
+	}
+}
+
+func TestAsType(t *testing.T) {
+	var errT errorT
+	var errP *fs.PathError
+	type timeout interface {
+		Timeout() bool
+		error
+	}
+	_, errF := os.Open("non-existing")
+	poserErr := &poser{"oh no", nil}
+
+	testAsType(t,
+		nil,
+		errP,
+		false,
+	)
+	testAsType(t,
+		wrapped{"pitied the fool", errorT{"T"}},
+		errorT{"T"},
+		true,
+	)
+	testAsType(t,
+		errF,
+		errF,
+		true,
+	)
+	testAsType(t,
+		errT,
+		errP,
+		false,
+	)
+	testAsType(t,
+		wrapped{"wrapped", nil},
+		errT,
+		false,
+	)
+	testAsType(t,
+		&poser{"error", nil},
+		errorT{"poser"},
+		true,
+	)
+	testAsType(t,
+		&poser{"path", nil},
+		poserPathErr,
+		true,
+	)
+	testAsType(t,
+		poserErr,
+		poserErr,
+		true,
+	)
+	testAsType(t,
+		errors.New("err"),
+		timeout(nil),
+		false,
+	)
+	testAsType(t,
+		errF,
+		errF.(timeout),
+		true)
+	testAsType(t,
+		wrapped{"path error", errF},
+		errF.(timeout),
+		true,
+	)
+	testAsType(t,
+		multiErr{},
+		errT,
+		false,
+	)
+	testAsType(t,
+		multiErr{errors.New("a"), errorT{"T"}},
+		errorT{"T"},
+		true,
+	)
+	testAsType(t,
+		multiErr{errorT{"T"}, errors.New("a")},
+		errorT{"T"},
+		true,
+	)
+	testAsType(t,
+		multiErr{errorT{"a"}, errorT{"b"}},
+		errorT{"a"},
+		true,
+	)
+	testAsType(t,
+		multiErr{multiErr{errors.New("a"), errorT{"a"}}, errorT{"b"}},
+		errorT{"a"},
+		true,
+	)
+	testAsType(t,
+		multiErr{wrapped{"path error", errF}},
+		errF.(timeout),
+		true,
+	)
+	testAsType(t,
+		multiErr{nil},
+		errT,
+		false,
+	)
+}
+
+type compError interface {
+	comparable
+	error
+}
+
+func testAsType[E compError](t *testing.T, err error, want E, wantOK bool) {
+	t.Helper()
+	name := fmt.Sprintf("AsType[%T](Errorf(..., %v))", want, err)
+	t.Run(name, func(t *testing.T) {
+		got, gotOK := errors.AsType[E](err)
+		if gotOK != wantOK || got != want {
+			t.Fatalf("got %v, %t; want %v, %t", got, gotOK, want, wantOK)
+		}
+	})
+}
+
+func BenchmarkIs(b *testing.B) {
+	err1 := errors.New("1")
+	err2 := multiErr{multiErr{multiErr{err1, errorT{"a"}}, errorT{"b"}}}
+
+	for i := 0; i < b.N; i++ {
+		if !errors.Is(err2, err1) {
+			b.Fatal("Is failed")
+		}
+	}
+}
+
+func BenchmarkAs(b *testing.B) {
+	err := multiErr{multiErr{multiErr{errors.New("a"), errorT{"a"}}, errorT{"b"}}}
+	for i := 0; i < b.N; i++ {
+		var target errorT
+		if !errors.As(err, &target) {
+			b.Fatal("As failed")
+		}
+	}
+}
+
+func BenchmarkAsType(b *testing.B) {
+	err := multiErr{multiErr{multiErr{errors.New("a"), errorT{"a"}}, errorT{"b"}}}
+	for range b.N {
+		if _, ok := errors.AsType[errorT](err); !ok {
+			b.Fatal("AsType failed")
+		}
+	}
+}
+
+func TestUnwrap(t *testing.T) {
+	err1 := errors.New("1")
+	erra := wrapped{"wrap 2", err1}
+
+	testCases := []struct {
+		err  error
+		want error
+	}{
+		{nil, nil},
+		{wrapped{"wrapped", nil}, nil},
+		{err1, nil},
+		{erra, err1},
+		{wrapped{"wrap 3", erra}, erra},
+	}
+	for _, tc := range testCases {
+		if got := errors.Unwrap(tc.err); got != tc.want {
+			t.Errorf("Unwrap(%v) = %v, want %v", tc.err, got, tc.want)
+		}
+	}
+}
+
+type errorT struct{ s string }
+
+func (e errorT) Error() string { return fmt.Sprintf("errorT(%s)", e.s) }
+
+type wrapped struct {
+	msg string
+	err error
+}
+
+func (e wrapped) Error() string { return e.msg }
+func (e wrapped) Unwrap() error { return e.err }
+
+type multiErr []error
+
+func (m multiErr) Error() string   { return "multiError" }
+func (m multiErr) Unwrap() []error { return []error(m) }
+
+type errorUncomparable struct {
+	f []string
+}
+
+func (errorUncomparable) Error() string {
+	return "uncomparable error"
+}
+
+func (errorUncomparable) Is(target error) bool {
+	_, ok := target.(errorUncomparable)
+	return ok
+}

go/src/expvar/expvar.go

@@ -0,0 +1,417 @@
+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package expvar provides a standardized interface to public variables, such
+// as operation counters in servers. It exposes these variables via HTTP at
+// /debug/vars in JSON format. As of Go 1.22, the /debug/vars request must
+// use GET.
+//
+// Operations to set or modify these public variables are atomic.
+//
+// In addition to adding the HTTP handler, this package registers the
+// following variables:
+//
+//	cmdline   os.Args
+//	memstats  runtime.Memstats
+//
+// The package is sometimes only imported for the side effect of
+// registering its HTTP handler and the above variables. To use it
+// this way, link this package into your program:
+//
+//	import _ "expvar"
+package expvar
+
+import (
+	"encoding/json"
+	"internal/godebug"
+	"log"
+	"math"
+	"net/http"
+	"os"
+	"runtime"
+	"slices"
+	"strconv"
+	"sync"
+	"sync/atomic"
+	"unicode/utf8"
+)
+
+// Var is an abstract type for all exported variables.
+type Var interface {
+	// String returns a valid JSON value for the variable.
+	// Types with String methods that do not return valid JSON
+	// (such as time.Time) must not be used as a Var.
+	String() string
+}
+
+type jsonVar interface {
+	// appendJSON appends the JSON representation of the receiver to b.
+	appendJSON(b []byte) []byte
+}
+
+// Int is a 64-bit integer variable that satisfies the [Var] interface.
+type Int struct {
+	i atomic.Int64
+}
+
+func (v *Int) Value() int64 {
+	return v.i.Load()
+}
+
+func (v *Int) String() string {
+	return string(v.appendJSON(nil))
+}
+
+func (v *Int) appendJSON(b []byte) []byte {
+	return strconv.AppendInt(b, v.i.Load(), 10)
+}
+
+func (v *Int) Add(delta int64) {
+	v.i.Add(delta)
+}
+
+func (v *Int) Set(value int64) {
+	v.i.Store(value)
+}
+
+// Float is a 64-bit float variable that satisfies the [Var] interface.
+type Float struct {
+	f atomic.Uint64
+}
+
+func (v *Float) Value() float64 {
+	return math.Float64frombits(v.f.Load())
+}
+
+func (v *Float) String() string {
+	return string(v.appendJSON(nil))
+}
+
+func (v *Float) appendJSON(b []byte) []byte {
+	return strconv.AppendFloat(b, math.Float64frombits(v.f.Load()), 'g', -1, 64)
+}
+
+// Add adds delta to v.
+func (v *Float) Add(delta float64) {
+	for {
+		cur := v.f.Load()
+		curVal := math.Float64frombits(cur)
+		nxtVal := curVal + delta
+		nxt := math.Float64bits(nxtVal)
+		if v.f.CompareAndSwap(cur, nxt) {
+			return
+		}
+	}
+}
+
+// Set sets v to value.
+func (v *Float) Set(value float64) {
+	v.f.Store(math.Float64bits(value))
+}
+
+// Map is a string-to-Var map variable that satisfies the [Var] interface.
+type Map struct {
+	m      sync.Map // map[string]Var
+	keysMu sync.RWMutex
+	keys   []string // sorted
+}
+
+// KeyValue represents a single entry in a [Map].
+type KeyValue struct {
+	Key   string
+	Value Var
+}
+
+func (v *Map) String() string {
+	return string(v.appendJSON(nil))
+}
+
+func (v *Map) appendJSON(b []byte) []byte {
+	return v.appendJSONMayExpand(b, false)
+}
+
+func (v *Map) appendJSONMayExpand(b []byte, expand bool) []byte {
+	afterCommaDelim := byte(' ')
+	mayAppendNewline := func(b []byte) []byte { return b }
+	if expand {
+		afterCommaDelim = '\n'
+		mayAppendNewline = func(b []byte) []byte { return append(b, '\n') }
+	}
+
+	b = append(b, '{')
+	b = mayAppendNewline(b)
+	first := true
+	v.Do(func(kv KeyValue) {
+		if !first {
+			b = append(b, ',', afterCommaDelim)
+		}
+		first = false
+		b = appendJSONQuote(b, kv.Key)
+		b = append(b, ':', ' ')
+		switch v := kv.Value.(type) {
+		case nil:
+			b = append(b, "null"...)
+		case jsonVar:
+			b = v.appendJSON(b)
+		default:
+			b = append(b, v.String()...)
+		}
+	})
+	b = mayAppendNewline(b)
+	b = append(b, '}')
+	b = mayAppendNewline(b)
+	return b
+}
+
+// Init removes all keys from the map.
+func (v *Map) Init() *Map {
+	v.keysMu.Lock()
+	defer v.keysMu.Unlock()
+	v.keys = v.keys[:0]
+	v.m.Clear()
+	return v
+}
+
+// addKey updates the sorted list of keys in v.keys.
+func (v *Map) addKey(key string) {
+	v.keysMu.Lock()
+	defer v.keysMu.Unlock()
+	// Using insertion sort to place key into the already-sorted v.keys.
+	i, found := slices.BinarySearch(v.keys, key)
+	if found {
+		return
+	}
+	v.keys = slices.Insert(v.keys, i, key)
+}
+
+func (v *Map) Get(key string) Var {
+	i, _ := v.m.Load(key)
+	av, _ := i.(Var)
+	return av
+}
+
+func (v *Map) Set(key string, av Var) {
+	// Before we store the value, check to see whether the key is new. Try a Load
+	// before LoadOrStore: LoadOrStore causes the key interface to escape even on
+	// the Load path.
+	if _, ok := v.m.Load(key); !ok {
+		if _, dup := v.m.LoadOrStore(key, av); !dup {
+			v.addKey(key)
+			return
+		}
+	}
+
+	v.m.Store(key, av)
+}
+
+// Add adds delta to the *[Int] value stored under the given map key.
+func (v *Map) Add(key string, delta int64) {
+	i, ok := v.m.Load(key)
+	if !ok {
+		var dup bool
+		i, dup = v.m.LoadOrStore(key, new(Int))
+		if !dup {
+			v.addKey(key)
+		}
+	}
+
+	// Add to Int; ignore otherwise.
+	if iv, ok := i.(*Int); ok {
+		iv.Add(delta)
+	}
+}
+
+// AddFloat adds delta to the *[Float] value stored under the given map key.
+func (v *Map) AddFloat(key string, delta float64) {
+	i, ok := v.m.Load(key)
+	if !ok {
+		var dup bool
+		i, dup = v.m.LoadOrStore(key, new(Float))
+		if !dup {
+			v.addKey(key)
+		}
+	}
+
+	// Add to Float; ignore otherwise.
+	if iv, ok := i.(*Float); ok {
+		iv.Add(delta)
+	}
+}
+
+// Delete deletes the given key from the map.
+func (v *Map) Delete(key string) {
+	v.keysMu.Lock()
+	defer v.keysMu.Unlock()
+	i, found := slices.BinarySearch(v.keys, key)
+	if found {
+		v.keys = slices.Delete(v.keys, i, i+1)
+		v.m.Delete(key)
+	}
+}
+
+// Do calls f for each entry in the map.
+// The map is locked during the iteration,
+// but existing entries may be concurrently updated.
+func (v *Map) Do(f func(KeyValue)) {
+	v.keysMu.RLock()
+	defer v.keysMu.RUnlock()
+	for _, k := range v.keys {
+		i, _ := v.m.Load(k)
+		val, _ := i.(Var)
+		f(KeyValue{k, val})
+	}
+}
+
+// String is a string variable, and satisfies the [Var] interface.
+type String struct {
+	s atomic.Value // string
+}
+
+func (v *String) Value() string {
+	p, _ := v.s.Load().(string)
+	return p
+}
+
+// String implements the [Var] interface. To get the unquoted string
+// use [String.Value].
+func (v *String) String() string {
+	return string(v.appendJSON(nil))
+}
+
+func (v *String) appendJSON(b []byte) []byte {
+	return appendJSONQuote(b, v.Value())
+}
+
+func (v *String) Set(value string) {
+	v.s.Store(value)
+}
+
+// Func implements [Var] by calling the function
+// and formatting the returned value using JSON.
+type Func func() any
+
+func (f Func) Value() any {
+	return f()
+}
+
+func (f Func) String() string {
+	v, _ := json.Marshal(f())
+	return string(v)
+}
+
+// All published variables.
+var vars Map
+
+// Publish declares a named exported variable. This should be called from a
+// package's init function when it creates its Vars. If the name is already
+// registered then this will log.Panic.
+func Publish(name string, v Var) {
+	if _, dup := vars.m.LoadOrStore(name, v); dup {
+		log.Panicln("Reuse of exported var name:", name)
+	}
+	vars.keysMu.Lock()
+	defer vars.keysMu.Unlock()
+	vars.keys = append(vars.keys, name)
+	slices.Sort(vars.keys)
+}
+
+// Get retrieves a named exported variable. It returns nil if the name has
+// not been registered.
+func Get(name string) Var {
+	return vars.Get(name)
+}
+
+// Convenience functions for creating new exported variables.
+
+func NewInt(name string) *Int {
+	v := new(Int)
+	Publish(name, v)
+	return v
+}
+
+func NewFloat(name string) *Float {
+	v := new(Float)
+	Publish(name, v)
+	return v
+}
+
+func NewMap(name string) *Map {
+	v := new(Map).Init()
+	Publish(name, v)
+	return v
+}
+
+func NewString(name string) *String {
+	v := new(String)
+	Publish(name, v)
+	return v
+}
+
+// Do calls f for each exported variable.
+// The global variable map is locked during the iteration,
+// but existing entries may be concurrently updated.
+func Do(f func(KeyValue)) {
+	vars.Do(f)
+}
+
+func expvarHandler(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	w.Write(vars.appendJSONMayExpand(nil, true))
+}
+
+// Handler returns the expvar HTTP Handler.
+//
+// This is only needed to install the handler in a non-standard location.
+func Handler() http.Handler {
+	return http.HandlerFunc(expvarHandler)
+}
+
+func cmdline() any {
+	return os.Args
+}
+
+func memstats() any {
+	stats := new(runtime.MemStats)
+	runtime.ReadMemStats(stats)
+	return *stats
+}
+
+func init() {
+	if godebug.New("httpmuxgo121").Value() == "1" {
+		http.HandleFunc("/debug/vars", expvarHandler)
+	} else {
+		http.HandleFunc("GET /debug/vars", expvarHandler)
+	}
+	Publish("cmdline", Func(cmdline))
+	Publish("memstats", Func(memstats))
+}
+
+// TODO: Use json.appendString instead.
+func appendJSONQuote(b []byte, s string) []byte {
+	const hex = "0123456789abcdef"
+	b = append(b, '"')
+	for _, r := range s {
+		switch {
+		case r < ' ' || r == '\\' || r == '"' || r == '<' || r == '>' || r == '&' || r == '\u2028' || r == '\u2029':
+			switch r {
+			case '\\', '"':
+				b = append(b, '\\', byte(r))
+			case '\n':
+				b = append(b, '\\', 'n')
+			case '\r':
+				b = append(b, '\\', 'r')
+			case '\t':
+				b = append(b, '\\', 't')
+			default:
+				b = append(b, '\\', 'u', hex[(r>>12)&0xf], hex[(r>>8)&0xf], hex[(r>>4)&0xf], hex[(r>>0)&0xf])
+			}
+		case r < utf8.RuneSelf:
+			b = append(b, byte(r))
+		default:
+			b = utf8.AppendRune(b, r)
+		}
+	}
+	b = append(b, '"')
+	return b
+}