# T-011: `tracing` Log Infrastructure — Subscriber Init, RUST_LOG Filter, Rerun Sink Feature Flag

**Type:** Task  
**Phase:** 0 — Foundation  
**Autonomy:** `agent:autonomous` — Mechanical wiring, no decisions required.  
**Stack:** `stack:rust`  
**Version:** v0.1  
**Iteration:** iter-1  
**Effort:** XS (1 hour)

---

> ⚠️ **Agent Scope:** T-004 already calls `tracing::info!`, `tracing::warn!`, `tracing::error!` throughout `audio/engine.rs`. Right now those calls are silent — no subscriber is registered. T-011 initialises the subscriber so those calls produce output. Add the `rerun-telemetry` Cargo feature as a stub (prints "Rerun sink active" but does no real Rerun work — T-121 owns the real implementation). Nothing else.

---

## Context

`tracing` is already a workspace dependency (AMENDMENT-001) and is called throughout `kansas/`. Without a subscriber, every `tracing::info!` call is a no-op and developers get no log output at all — including audio device errors, transport state transitions, and model inference latencies. This task is the one-line initialisation that activates all of it.

The `rerun-telemetry` feature flag gates the Rerun SDK dependency. Production builds ship without it (`cargo tauri build` — no Rerun). Development builds opt in with `cargo tauri dev --features rerun-telemetry`. T-121 fills the stub with real Rerun log calls.

---

## Prerequisites

- [ ] T-001 merged — workspace `Cargo.toml` with `tracing` and `tracing-subscriber` already listed

Verify both are present in `[workspace.dependencies]`. If missing, add:

```toml
tracing            = { version = "*" }
tracing-subscriber = { version = "*", features = ["env-filter", "fmt"] }
```

---

## Acceptance Criteria

- [ ] `cargo tauri dev` prints structured log lines to stdout — at minimum the `[audio]` lines from T-004
- [ ] `RUST_LOG=debug cargo tauri dev` prints debug-level output
- [ ] `RUST_LOG=warn cargo tauri dev` shows only warnings and errors
- [ ] Without `RUST_LOG` set, default filter is `kansas=info,warn` (kansas at info, everything else at warn)
- [ ] `cargo tauri dev --features rerun-telemetry` prints `[logging] Rerun telemetry sink active (stub — see T-121)` and does not crash
- [ ] `cargo tauri build` (without `rerun-telemetry`) compiles without error — Rerun SDK is not linked
- [ ] `cargo check --workspace` — zero errors
- [ ] `cargo clippy --workspace -- -D warnings` — zero warnings

---

## New Cargo Feature

Add to `kansas/Cargo.toml`:

```toml
[features]
default = []
rerun-telemetry = []   # T-121 adds: dep:rerun when this is filled in

[dependencies]
tracing            = { workspace = true }
tracing-subscriber = { workspace = true }
# rerun dep added by T-121 under rerun-telemetry feature
```

---

## New File: `kansas/src/logging.rs`

```rust
//! Logging infrastructure for the Synesthesia Tauri backend.
//!
//! Initialise once at startup via [`setup`].
//! Log level controlled by the `RUST_LOG` environment variable.
//!
//! # Feature: `rerun-telemetry`
//! When compiled with `--features rerun-telemetry`, a stub Rerun sink is
//! registered alongside the stdout subscriber. T-121 replaces the stub with
//! real Rerun log calls.

use tracing_subscriber::{
    fmt,
    layer::SubscriberExt,
    util::SubscriberInitExt,
    EnvFilter,
};

/// Default log filter when `RUST_LOG` is not set.
/// - `kansas` crate: INFO and above (our own logs)
/// - Everything else: WARN and above (suppress noisy deps)
const DEFAULT_FILTER: &str = "kansas=info,warn";

/// Initialise the global tracing subscriber.
/// Must be called exactly once, before any `tracing::` calls.
/// Panics if called a second time (tracing subscriber is a global singleton).
pub fn setup() {
    let filter = EnvFilter::try_from_default_env()
        .unwrap_or_else(|_| EnvFilter::new(DEFAULT_FILTER));

    let fmt_layer = fmt::layer()
        .with_target(true)          // show module path
        .with_thread_names(false)   // too noisy for audio threads
        .with_file(false)           // file:line in DEBUG only
        .compact();

    let registry = tracing_subscriber::registry()
        .with(filter)
        .with(fmt_layer);

    #[cfg(feature = "rerun-telemetry")]
    let registry = registry.with(RerunStubLayer);

    registry.init();

    #[cfg(feature = "rerun-telemetry")]
    tracing::info!("[logging] Rerun telemetry sink active (stub — see T-121)");

    tracing::info!("[logging] Tracing initialised — RUST_LOG={}", 
        std::env::var("RUST_LOG").unwrap_or_else(|_| DEFAULT_FILTER.into()));
}

// ─── Rerun stub layer ─────────────────────────────────────────────────────────

#[cfg(feature = "rerun-telemetry")]
struct RerunStubLayer;

#[cfg(feature = "rerun-telemetry")]
impl<S: tracing::Subscriber> tracing_subscriber::Layer<S> for RerunStubLayer {
    // T-121 overrides on_event() to forward structured log data to the Rerun SDK.
    // Stub: no-op layer that satisfies the trait bound.
}
```

---

## Update `kansas/src/main.rs`

Add one line at the top of `main()`, before anything else:

```rust
mod logging;
// ... other mods

fn main() {
    // MUST be first — initialises global tracing subscriber before any
    // tracing::* calls in AudioEngine::new(), TransportHandle::new(), etc.
    logging::setup();

    let audio_engine = AudioEngine::new()
        .expect("Failed to initialise audio engine");

    tauri::Builder::default()
        // ...
```

---

## Add `logging` Module to `kansas/src/lib.rs`

```rust
pub mod logging;
```

---

## Implementation Notes

### Why `compact()` not `pretty()`

`pretty()` emits multi-line ANSI output. In a terminal it looks great; in a Windows file redirect or CI log it produces garbled escape codes and hard-to-grep output. `compact()` gives single-line structured output readable in both.

### `tracing::instrument` for hot paths

Do **not** add `#[tracing::instrument]` to the audio callback or the tick loop — these fire 200+ times per second and the span overhead accumulates. Reserve `#[instrument]` for IPC command handlers and one-shot initialisation paths. T-011 does not add any `#[instrument]` attributes; that is left to individual task authors.

### The `RerunStubLayer` no-op trait impl

`tracing_subscriber::Layer<S>` has default implementations for all methods. The empty `impl` is valid Rust — it compiles to a zero-cost abstraction that adds no overhead when `rerun-telemetry` is disabled (the feature gates the entire struct out of the binary). When T-121 arrives it fills in `fn on_event(...)` to forward log fields to the Rerun SDK.

### Log output format

After T-011, `cargo tauri dev` produces lines like:

```
2026-03-19T18:45:01.234Z  INFO kansas::logging: [logging] Tracing initialised — RUST_LOG=kansas=info,warn
2026-03-19T18:45:01.235Z  INFO kansas::audio::engine: [audio] Output: Speakers (Realtek) | 48000Hz | 2ch
2026-03-19T18:45:01.236Z  INFO kansas::audio::engine: [audio] Input: Microphone (USB) | 48000Hz | 2ch
```

---

## Testing

```bash
# Default filter — should see INFO from kansas, WARN from deps
cargo tauri dev

# Debug filter — verbose output
RUST_LOG=debug cargo tauri dev

# Warn only — mostly silent
RUST_LOG=warn cargo tauri dev

# Rerun stub
cargo tauri dev --features rerun-telemetry
# Expected line: [logging] Rerun telemetry sink active (stub — see T-121)

# Release build must NOT include rerun
cargo tauri build
# Verify: ldd/dumpbin on the .exe shows no rerun .dll
```

---

## GitHub CLI

```bash
gh issue create \
  --title "T-011: tracing log infrastructure — subscriber init, RUST_LOG filter, Rerun sink stub" \
  --label "type:task,stack:rust,agent:autonomous,priority:high,status:ready,day:1" \
  --body-file T-011.md
```

---

**Parent:** GENESIS  
**Blocks:** T-121 (fills `RerunStubLayer::on_event` with real Rerun calls)  
**Blocked By:** T-001  
**Version:** v0.1 · **Iteration:** iter-1 · **Effort:** XS (1 hour)