ai_synth/docs/superpowers/specs/2026-03-23-error-location-logging-design.md

Design: Error Location Logging

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

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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:

fmt().with_env_filter(filter).init();

With:

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