What is the log crate?

log (2014) is the standard logging facade for Rust — not a full logger. It defines macros (info!, warn!, error!, …) and the Log trait. A separate crate must be installed as the logger implementation at runtime.

Libraries use log so applications can choose any backend without forcing a dependency on tracing or slog.

Simple example

use log::{info, warn};

fn main() {
    env_logger::init(); // backend: reads RUST_LOG env var

    info!("server starting");
    warn!("cache miss for key={}", "user:42");
}

Run with filter:

RUST_LOG=info cargo run

Common backends

See also: tracing for structured logging and spans in async services.