next.js/turbopack/crates/turbopack-ecmascript/src/tree_shake/mod.rs

use std::fmt::Write;

use anyhow::{Result, bail};
use rustc_hash::FxHashMap;
use swc_core::{
    common::{DUMMY_SP, GLOBALS, SyntaxContext, comments::Comments, util::take::Take},
    ecma::{
        ast::{
            ExportAll, ExportNamedSpecifier, Expr, ExprStmt, Id, Ident, ImportDecl, Lit, Module,
            ModuleDecl, ModuleExportName, ModuleItem, NamedExport, Program, Stmt,
        },
        codegen::to_code,
    },
};
use turbo_rcstr::RcStr;
use turbo_tasks::{FxIndexSet, ResolvedVc, ValueToString, Vc};
use turbopack_core::{ident::AssetIdent, resolve::ModulePart, source::Source};

use self::graph::{DepGraph, ItemData, ItemId, ItemIdGroupKind, Mode, SplitModuleResult};
pub(crate) use self::graph::{
    PartId, create_turbopack_part_id_assert, find_turbopack_part_id_in_asserts,
};
use crate::{EcmascriptModuleAsset, analyzer::graph::EvalContext, parse::ParseResult};

pub mod asset;
pub mod chunk_item;
mod graph;
pub mod merge;
mod optimizations;
pub mod side_effect_module;
#[cfg(test)]
mod tests;
mod util;

pub(crate) const TURBOPACK_PART_IMPORT_SOURCE: &str = "__TURBOPACK_PART__";

pub struct Analyzer<'a> {
    g: &'a mut DepGraph,
    item_ids: &'a Vec<ItemId>,
    items: &'a mut FxHashMap<ItemId, ItemData>,

    last_side_effects: Vec<ItemId>,

    vars: FxHashMap<Id, VarState>,
}

#[derive(Debug, Default, Clone)]
struct VarState {
    declarator: Option<ItemId>,

    /// The module items that might trigger side effects on that variable.
    /// We also store if this is a `const` write, so no further change will
    /// happen to this var.
    last_writes: Vec<ItemId>,
    /// The module items that might read that variable.
    last_reads: Vec<ItemId>,

    last_op: Option<VarOp>,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum VarOp {
    Read,
    Write,
}

impl Analyzer<'_> {
    pub(super) fn analyze(
        module: &Module,
        comments: &dyn Comments,
        unresolved_ctxt: SyntaxContext,
        top_level_ctxt: SyntaxContext,
    ) -> (DepGraph, FxHashMap<ItemId, ItemData>) {
        let mut g = DepGraph::default();
        let (item_ids, mut items) = g.init(module, comments, unresolved_ctxt, top_level_ctxt);

        let mut analyzer = Analyzer {
            g: &mut g,
            item_ids: &item_ids,
            items: &mut items,
            last_side_effects: Default::default(),
            vars: Default::default(),
        };

        let eventual_ids = analyzer.hoist_vars_and_bindings();

        analyzer.evaluate_immediate(module, &eventual_ids);

        analyzer.evaluate_eventual(module);

        analyzer.handle_exports(module);

        analyzer.handle_explicit_deps();

        (g, items)
    }

    fn handle_explicit_deps(&mut self) {
        for item_id in self.item_ids.iter() {
            if let Some(item) = self.items.get(item_id)
                && !item.explicit_deps.is_empty()
            {
                self.g.add_strong_deps(item_id, item.explicit_deps.iter());
            }
        }
    }

    /// Phase 1: Hoisted Variables and Bindings
    ///
    ///
    /// Returns all (EVENTUAL_READ/WRITE_VARS) in the module.
    fn hoist_vars_and_bindings(&mut self) -> FxIndexSet<Id> {
        let mut eventual_ids = FxIndexSet::default();

        for item_id in self.item_ids.iter() {
            if let Some(item) = self.items.get(item_id) {
                eventual_ids.extend(item.eventual_read_vars.iter().cloned());
                eventual_ids.extend(item.eventual_write_vars.iter().cloned());

                if item.is_hoisted && item.side_effects {
                    self.g
                        .add_strong_deps(item_id, self.last_side_effects.last());

                    self.last_side_effects.push(item_id.clone());
                }

                for id in item.var_decls.iter() {
                    let state = self.vars.entry(id.clone()).or_default();

                    if state.declarator.is_none() {
                        state.declarator = Some(item_id.clone());
                    }

                    if item.is_hoisted {
                        state.last_writes.push(item_id.clone());
                    } else {
                        // TODO(WEB-705): Create a fake module item
                        // state.last_writes.push(item_id.clone());
                    }
                }
            }
        }

        eventual_ids
    }

    /// Phase 2: Immediate evaluation
    fn evaluate_immediate(&mut self, _module: &Module, eventual_ids: &FxIndexSet<Id>) {
        for item_id in self.item_ids.iter() {
            if let Some(item) = self.items.get(item_id) {
                // Ignore HOISTED module items, they have been processed in phase 1 already.
                if item.is_hoisted {
                    continue;
                }

                for id in item.var_decls.iter() {
                    let state = self.vars.entry(id.clone()).or_default();
                    if state.declarator.is_none() {
                        state.declarator = Some(item_id.clone());
                    }
                }

                // For each var in READ_VARS:
                for id in item.read_vars.iter() {
                    // read (last: read) -> ref last_writes, push last_reads
                    // read (last: (read +) write) -> ref last_writes, clear last_reads, push
                    // last_reads

                    // (the writes need to be executed before this read)
                    let state = self.vars.entry(id.clone()).or_default();
                    self.g.add_strong_deps(item_id, state.last_writes.iter());

                    if let Some(declarator) = &state.declarator
                        && declarator != item_id
                    {
                        // A read also depends on the declaration.
                        self.g
                            .add_strong_deps(item_id, [declarator].iter().copied());
                    }

                    if state.last_op == Some(VarOp::Write) && !item.write_vars.contains(id) {
                        state.last_reads.clear();
                    }
                }

                // For each var in WRITE_VARS:
                for id in item.write_vars.iter() {
                    // Create a weak dependency to all module items listed in
                    // LAST_READS for that var.

                    // (the reads need to be executed before this write, when
                    // it’s needed)

                    let state = self.vars.entry(id.clone()).or_default();
                    self.g.add_weak_deps(item_id, state.last_reads.iter());

                    if let Some(declarator) = &state.declarator
                        && declarator != item_id
                    {
                        // A write also depends on the declaration.
                        self.g.add_strong_deps(item_id, [declarator]);
                    }

                    if !item.read_vars.contains(id) {
                        // write (last: read) -> weak_ref last_reads, clear last_writes, push
                        // last_writes

                        if state.last_op == Some(VarOp::Read) {
                            state.last_writes.clear();
                        } else if state.last_op == Some(VarOp::Write) {
                            // write (last: (read +) write) -> weak_ref last_reads, push last_writes
                        }
                    } else {
                        // read+write (last: read) -> weak_ref last_reads, ref last_writes, clear
                        // last_reads, clear last_writes, push last_reads, push last_writes

                        // read+write (last: (read +) write) -> ref last_writes, clear
                        // last_reads, clear last_writes, push
                        // last_reads, push last_writes
                        if state.last_op.is_some() {
                            state.last_reads.clear();
                            state.last_writes.clear();
                        }
                    }
                }

                if item.side_effects {
                    // Create a strong dependency to LAST_SIDE_EFFECT.

                    self.g
                        .add_strong_deps(item_id, self.last_side_effects.last());

                    // Create weak dependencies to all LAST_WRITES and strong
                    // dependencies to LAST_READS.
                    //
                    // We need to create strong dependencies to LAST_READS because
                    // prototype-based methods definitions should be executed before
                    // any usage of those methods, and the usage of those methods are
                    // flagged as a side effect.
                    for id in eventual_ids.iter() {
                        let state = self.vars.entry(id.clone()).or_default();

                        self.g.add_weak_deps(item_id, state.last_writes.iter());
                        self.g.add_strong_deps(item_id, state.last_reads.iter());
                    }
                }

                // For each var in WRITE_VARS:
                for id in item.write_vars.iter() {
                    // Add this module item to LAST_WRITES

                    let state = self.vars.entry(id.clone()).or_default();
                    state.last_writes.push(item_id.clone());

                    // Optimization: Remove each module item to which we
                    // just created a strong dependency from LAST_WRITES

                    state
                        .last_writes
                        .retain(|last_write| !self.g.has_dep(item_id, last_write, true));

                    // Drop all writes which are not reachable from this item.
                    //
                    // For
                    //
                    // var x = 0
                    // x = 1
                    // x = 2
                    // x += 3
                    //
                    // this will drop `x = 1` as not reachable from `x += 3`.

                    state
                        .last_writes
                        .retain(|last_write| self.g.has_path_connecting(item_id, last_write));
                }

                // For each var in READ_VARS:
                for id in item.read_vars.iter() {
                    // Add this module item to LAST_READS

                    let state = self.vars.entry(id.clone()).or_default();
                    state.last_reads.push(item_id.clone());

                    // Optimization: Remove each module item to which we
                    // have a strong dependency

                    state
                        .last_reads
                        .retain(|last_read| !self.g.has_dep(item_id, last_read, true));

                    state.last_op = Some(VarOp::Read);
                }

                for id in item.write_vars.iter() {
                    let state = self.vars.entry(id.clone()).or_default();
                    state.last_op = Some(VarOp::Write);
                }

                if item.side_effects {
                    self.last_side_effects.push(item_id.clone());
                }
            }
        }
    }

    /// Phase 3: Eventual evaluation
    fn evaluate_eventual(&mut self, _module: &Module) {
        for item_id in self.item_ids.iter() {
            if let Some(item) = self.items.get(item_id) {
                // For each var in EVENTUAL_READ_VARS:

                for id in item.eventual_read_vars.iter() {
                    // Create a strong dependency to all module items listed in
                    // LAST_WRITES for that var.

                    let state = self.vars.entry(id.clone()).or_default();
                    self.g.add_strong_deps(item_id, state.last_writes.iter());

                    if let Some(declarator) = &state.declarator
                        && declarator != item_id
                    {
                        // A read also depends on the declaration.
                        self.g.add_strong_deps(item_id, [declarator]);
                    }
                }

                // For each var in EVENTUAL_WRITE_VARS:
                for id in item.eventual_write_vars.iter() {
                    // Create a weak dependency to all module items listed in
                    // LAST_READS for that var.

                    let state = self.vars.entry(id.clone()).or_default();

                    self.g.add_weak_deps(item_id, state.last_reads.iter());

                    if let Some(declarator) = &state.declarator
                        && declarator != item_id
                    {
                        // A write also depends on the declaration.
                        self.g.add_strong_deps(item_id, [declarator]);
                    }
                }

                // (no state update happens, since this is only triggered by
                // side effects, which we already handled)
            }
        }
    }

    /// Phase 4: Exports
    fn handle_exports(&mut self, _module: &Module) {
        // We use the last side effect as a module evaluation
        if let Some(last) = self.last_side_effects.last()
            && let Some(item) = self.items.get_mut(last)
        {
            item.is_module_evaluation = true;
        }

        for item_id in self.item_ids.iter() {
            if let ItemId::Group(ItemIdGroupKind::Export(local, _)) = item_id {
                // Create a strong dependency to LAST_WRITES for this var

                let state = self.vars.entry(local.clone()).or_default();

                self.g.add_strong_deps(item_id, state.last_writes.iter());
            }
        }
    }
}

#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub(crate) enum Key {
    ModuleEvaluation,
    Export(RcStr),
    Exports,
    StarExports,
}

/// Converts [ModulePart] to the index.
async fn get_part_id(result: &SplitResult, part: &ModulePart) -> Result<u32> {
    // TODO implement ModulePart::Facade
    let key = match part {
        ModulePart::Evaluation => Key::ModuleEvaluation,
        ModulePart::Export(export) => Key::Export(export.clone()),
        ModulePart::Exports => Key::Exports,
        ModulePart::Internal(part_id) => return Ok(*part_id),
        ModulePart::Locals
        | ModulePart::Facade
        | ModulePart::RenamedExport { .. }
        | ModulePart::RenamedNamespace { .. } => {
            bail!("invalid module part")
        }
    };

    let SplitResult::Ok {
        entrypoints,
        modules,
        ..
    } = &result
    else {
        bail!("split failed")
    };

    if let Some(id) = entrypoints.get(&key) {
        return Ok(*id);
    }

    // This is required to handle `export * from 'foo'`
    if let ModulePart::Export(..) = part
        && let Some(&v) = entrypoints
            .get(&Key::StarExports)
            .or_else(|| entrypoints.get(&Key::Exports))
    {
        return Ok(v);
    }

    let mut dump = String::new();

    for (idx, m) in modules.iter().enumerate() {
        let ParseResult::Ok { program, .. } = &*m.await? else {
            bail!("failed to get module")
        };

        {
            let code = to_code(&program);

            writeln!(dump, "# Module #{idx}:\n{code}\n\n\n")?;
        }
    }

    bail!(
        "could not find part id for module part {:?} in {:?}\n\nModule dump:\n{dump}",
        key,
        entrypoints
    )
}

#[turbo_tasks::value(shared, serialization = "none", eq = "manual")]
pub(crate) enum SplitResult {
    Ok {
        asset_ident: ResolvedVc<AssetIdent>,

        /// `u32` is a index to `modules`.
        #[turbo_tasks(trace_ignore)]
        entrypoints: FxHashMap<Key, u32>,

        #[turbo_tasks(debug_ignore, trace_ignore)]
        modules: Vec<ResolvedVc<ParseResult>>,

        #[turbo_tasks(trace_ignore)]
        deps: FxHashMap<u32, Vec<PartId>>,

        #[turbo_tasks(debug_ignore, trace_ignore)]
        star_reexports: Vec<ExportAll>,
    },
    Failed {
        parse_result: ResolvedVc<ParseResult>,
    },
}

impl PartialEq for SplitResult {
    fn eq(&self, other: &Self) -> bool {
        match (self, other) {
            (Self::Ok { .. }, Self::Ok { .. }) => false,
            _ => core::mem::discriminant(self) == core::mem::discriminant(other),
        }
    }
}

#[turbo_tasks::function]
pub(super) fn split_module(asset: Vc<EcmascriptModuleAsset>) -> Result<Vc<SplitResult>> {
    Ok(split(asset.source().ident(), asset.source(), asset.parse()))
}

#[turbo_tasks::function]
pub(super) async fn split(
    ident: ResolvedVc<AssetIdent>,
    source: ResolvedVc<Box<dyn Source>>,
    parsed: ResolvedVc<ParseResult>,
) -> Result<Vc<SplitResult>> {
    // Do not split already split module
    if !ident.await?.parts.is_empty() {
        return Ok(SplitResult::Failed {
            parse_result: parsed,
        }
        .cell());
    }

    // Turbopack has a bug related to parsing of CJS files where the package.json has
    // a `"type": "module"` and the file is a CJS file.
    let name = ident.to_string().await?;
    if name.ends_with(".cjs") {
        return Ok(SplitResult::Failed {
            parse_result: parsed,
        }
        .cell());
    }

    let parse_result = parsed.await?;

    match &*parse_result {
        ParseResult::Ok {
            program,
            comments,
            eval_context,
            source_map,
            globals,
            ..
        } => {
            // If the script file is a common js file, we cannot split the module
            if util::should_skip_tree_shaking(program) {
                return Ok(SplitResult::Failed {
                    parse_result: parsed,
                }
                .cell());
            }

            let module = match program {
                Program::Module(module) => module,
                Program::Script(..) => unreachable!("CJS is already handled"),
            };

            // We copy directives like `use client` or `use server` to each module
            let directives = module
                .body
                .iter()
                .take_while(|item| {
                    matches!(
                        item,
                        ModuleItem::Stmt(Stmt::Expr(ExprStmt {
                            expr: box Expr::Lit(Lit::Str(..)),
                            ..
                        }))
                    )
                })
                .cloned()
                .collect::<Vec<_>>();

            let (mut dep_graph, items) = GLOBALS.set(globals, || {
                Analyzer::analyze(
                    module,
                    comments,
                    SyntaxContext::empty().apply_mark(eval_context.unresolved_mark),
                    SyntaxContext::empty().apply_mark(eval_context.top_level_mark),
                )
            });

            dep_graph.handle_weak(Mode::Production);

            let SplitModuleResult {
                entrypoints,
                part_deps,
                modules,
                star_reexports,
            } = dep_graph.split_module(&directives, &items);

            assert_ne!(modules.len(), 0, "modules.len() == 0;\nModule: {module:?}",);

            for &v in entrypoints.values() {
                debug_assert!(
                    v < modules.len() as u32,
                    "Invalid entrypoint '{}' while there are only '{}' modules",
                    v,
                    modules.len()
                );
            }
            let modules = modules
                .into_iter()
                .map(|module| {
                    let program = Program::Module(module);
                    let eval_context = EvalContext::new(
                        &program,
                        eval_context.unresolved_mark,
                        eval_context.top_level_mark,
                        eval_context.force_free_values.clone(),
                        None,
                        Some(source),
                    );

                    ParseResult::resolved_cell(ParseResult::Ok {
                        program,
                        globals: globals.clone(),
                        comments: comments.clone(),
                        source_map: source_map.clone(),
                        eval_context,
                    })
                })
                .collect();

            Ok(SplitResult::Ok {
                asset_ident: ident,
                entrypoints,
                deps: part_deps,
                modules,
                star_reexports,
            }
            .cell())
        }

        _ => Ok(SplitResult::Failed {
            parse_result: parsed,
        }
        .cell()),
    }
}

#[turbo_tasks::function]
pub(crate) async fn part_of_module(
    split_data: Vc<SplitResult>,
    part: ModulePart,
) -> Result<Vc<ParseResult>> {
    let split_data = split_data.await?;

    match &*split_data {
        SplitResult::Ok {
            asset_ident,
            modules,
            entrypoints,
            deps,
            star_reexports,
            ..
        } => {
            debug_assert_ne!(modules.len(), 0, "modules.len() == 0");

            if part == ModulePart::Facade {
                if let ParseResult::Ok {
                    comments,
                    eval_context,
                    globals,
                    source_map,
                    ..
                } = &*modules[0].await?
                {
                    let mut module = Module::dummy();

                    let mut export_names = entrypoints
                        .keys()
                        .filter_map(|key| {
                            if let Key::Export(v) = key {
                                Some(v.clone())
                            } else {
                                None
                            }
                        })
                        .collect::<Vec<_>>();
                    export_names.sort();

                    module
                        .body
                        .push(ModuleItem::ModuleDecl(ModuleDecl::Import(ImportDecl {
                            span: DUMMY_SP,
                            specifiers: vec![],
                            src: Box::new(TURBOPACK_PART_IMPORT_SOURCE.into()),
                            type_only: false,
                            with: Some(Box::new(create_turbopack_part_id_assert(
                                PartId::ModuleEvaluation,
                            ))),
                            phase: Default::default(),
                        })));

                    let specifiers = export_names
                        .into_iter()
                        .map(|export_name| {
                            swc_core::ecma::ast::ExportSpecifier::Named(ExportNamedSpecifier {
                                span: DUMMY_SP,
                                orig: ModuleExportName::Ident(Ident::new(
                                    export_name.as_str().into(),
                                    DUMMY_SP,
                                    Default::default(),
                                )),
                                exported: None,
                                is_type_only: false,
                            })
                        })
                        .collect::<Vec<_>>();

                    module
                        .body
                        .push(ModuleItem::ModuleDecl(ModuleDecl::ExportNamed(
                            NamedExport {
                                span: DUMMY_SP,
                                specifiers,
                                src: Some(Box::new(TURBOPACK_PART_IMPORT_SOURCE.into())),
                                type_only: false,
                                with: Some(Box::new(create_turbopack_part_id_assert(
                                    PartId::Exports,
                                ))),
                            },
                        )));

                    module.body.extend(star_reexports.iter().map(|export_all| {
                        ModuleItem::ModuleDecl(ModuleDecl::ExportAll(export_all.clone()))
                    }));

                    let program = Program::Module(module);
                    let eval_context = EvalContext::new(
                        &program,
                        eval_context.unresolved_mark,
                        eval_context.top_level_mark,
                        eval_context.force_free_values.clone(),
                        None,
                        None,
                    );

                    return Ok(ParseResult::Ok {
                        program,
                        comments: comments.clone(),
                        eval_context,
                        globals: globals.clone(),
                        source_map: source_map.clone(),
                    }
                    .cell());
                } else {
                    unreachable!()
                }
            }

            let part_id = get_part_id(&split_data, &part).await?;

            if part_id as usize >= modules.len() {
                bail!(
                    "part_id is out of range: {part_id} >= {}; asset = {}; entrypoints = \
                     {entrypoints:?}: part_deps = {deps:?}",
                    asset_ident.to_string().await?,
                    modules.len(),
                );
            }

            Ok(*modules[part_id as usize])
        }
        SplitResult::Failed { parse_result } => Ok(**parse_result),
    }
}