tracing

The tracing crate (tokio-rs) is the Rust ecosystem's standard for structured diagnostics — logging, distributed tracing, and instrumentation unified under one API. When a level is disabled, overhead is a single atomic load (~1 ns).

Performance tuning

For hot paths, compile-time level gating eliminates all debug/trace instrumentation from the binary entirely:

[dependencies]
tracing = { version = "0.1", features = ["max_level_info"] }

This means zero-cost debug instrumentation: leave tracing::debug! calls in production code and they compile to nothing.

Ecosystem

• tracing-subscriber — formatting, filtering, and layer composition for output.
• tracing-error — enriches errors with span context via SpanTrace. In async code, SpanTraces show the logical application hierarchy rather than executor frames, making them far more useful than backtraces. Integrates with color-eyre and error-stack.
• tokio-console — real-time async task debugging powered by tracing data.
• fastrace — alternative for library authors who want zero overhead when tracing is completely disabled (tracing still pays the atomic load even when no subscriber is attached; fastrace compiles out entirely).

For library authors

If writing a library crate, instrument with tracing spans and events. Application authors choose their subscriber. This separation of instrumentation from output is tracing's key architectural insight and why it won over log.