Rust Logging (tracing)

Level	Usage
error	At handling boundary only — not at every ?
warn	Degraded but recoverable
info	Program flow
debug	Decision paths, intermediate values
trace	Heavy diagnostic, scripts only

Log at the handling boundary — one log per error, at the point where propagation stops. Never log at every ?.

Never discard errors silently with let _ = ... — log before discarding.

Never log secret values — log the key name only.

Structured Fields

info!(email = %email, "Creating user");  // % = Display, ? = Debug, no sigil = typed value

Instrumentation

#[instrument(skip_all, fields(user_id = %cmd.user_id))]
async fn create_user(cmd: CreateUser, repository: &dyn UserRepository) -> Result<()> { }

#[instrument(skip_all, ret)]
async fn find_by_id(&self, id: UserId) -> Result<Option<User>> { }

• skip_all + explicit fields(...) — never auto-log params that may contain secrets
• Instrument entry points only — not hot-path inner functions
• Any crate performing I/O must declare tracing as a dependency