Rust Observability

A log line is a structured event with named fields, and a library never installs a subscriber. This skill governs how Rust code reports what it is doing; what an error type contains is /rust-errors, and span behaviour across cancellation is /async-rust.

A log line is an event, not a sentence

The core move: fields stay fields. error!(user_id, %path, "config load failed") is queryable — user_id and path are named columns. The interpolated form, error!("config load failed for user {user_id} at {path}"), is a string someone will later write a regex against. The difference shows up the first time somebody needs "all failures for one user" — the moment logging either pays for itself or does not.

The three layers, and which to use