# Design: Error Location Logging

**Date**: 2026-03-23
**Scope**: Add source file and line number to WARN and ERROR log lines

---

## Context

During the first Docker deployment, several runtime errors (non-existent table, VARCHAR constraint violation) produced log lines like `ERROR ai_synth_backend::errors: Database error: PgDatabaseError { ... }` with no indication of which source file or function triggered the error. Diagnosing required grep-ing the codebase for the SQL string.

## Goal

WARN and ERROR log lines include the source file and line number. INFO and DEBUG remain unchanged.

## Approach

Create a custom `tracing_subscriber` event formatter that conditionally appends `[file:line]` when the event level is WARN or ERROR.

### New file: `backend/src/logging.rs`

~30 lines. Implements `FormatEvent` trait:
- For WARN/ERROR: appends `[src/file.rs:123]` after the target module
- For INFO/DEBUG/TRACE: standard format, no file:line

### Changes to `backend/src/main.rs`

Replace:
```rust
fmt().with_env_filter(filter).init();
```

With:
```rust
fmt().with_env_filter(filter).event_format(logging::ErrorLocationFormat).init();
```

Add `mod logging;` to `main.rs` (alongside the existing `mod cli;` — both are binary-only concerns).

### Output examples

**INFO (unchanged):**
```
2026-03-23T10:00:00Z  INFO ai_synth_backend::services::auth: Magic link token created email="test@example.com"
```

**ERROR (with file:line):**
```
2026-03-23T10:00:00Z ERROR ai_synth_backend::errors [src/errors.rs:91]: Database error: PgDatabaseError { ... }
```

**WARN (with file:line):**
```
2026-03-23T10:00:00Z  WARN ai_synth_backend::handlers::auth [src/handlers/auth.rs:75]: Rate limit exceeded for email
```

## What does NOT change

- No changes to any `tracing::error!` or `tracing::warn!` call sites
- No changes to error types or error handling logic
- No changes to the JSON error response sent to clients
- INFO/DEBUG/TRACE output format unchanged

## Testing

- `cargo test --lib` — existing tests pass (formatter is only used in main.rs, not in tests)
- Manual: run the app, trigger an error, verify file:line appears in log output