A Rust library providing a lightweight logging facade.

• log documentation
• env_logger documentation

A logging facade provides a single logging API that abstracts over the actual logging implementation. Libraries can use the logging API provided by this crate, and the consumer of those libraries can choose the logging implementation that is most suitable for its use case.

Libraries should link only to the log crate, and use the provided macros to log whatever information will be useful to downstream consumers:

[dependencies]
log = "0.3"

#[macro_use]
extern crate log;

pub fn shave_the_yak(yak: &Yak) {
    trace!("Commencing yak shaving");

    loop {
        match find_a_razor() {
            Ok(razor) => {
                info!("Razor located: {}", razor);
                yak.shave(razor);
                break;
            }
            Err(err) => {
                warn!("Unable to locate a razor: {}, retrying", err);
            }
        }
    }
}

Executables should choose a logger implementation and initialize it early in the runtime of the program. Logger implementations will typically include a function to do this. Any log messages generated before the logger is initialized will be ignored.

The executable itself may use the log crate to log as well.

The env_logger crate provides a logger implementation that mirrors the functionality of the old revision of the log crate.

[dependencies]
log = "0.3"
env_logger = "0.3"

#[macro_use]
extern crate log;
extern crate env_logger;

fn main() {
    env_logger::init().unwrap();

    info!("starting up");

    // ...
}

Then when running the executable, specify a value for the RUST_LOG environment variable that corresponds with the log messages you want to show.

$ RUST_LOG=info ./main
starting up

See the env_logger documentation for the RUST_LOG values that can be used to get log messages with different levels or filtered to different modules.

Tests can use the env_logger crate to see log messages generated during that test:

[dependencies]
log = "0.3"

[dev-dependencies]
env_logger = "0.3"

#[macro_use]
extern crate log;

fn add_one(num: i32) -> i32 {
    info!("add_one called with {}", num);
    num + 1
}

#[cfg(test)]
mod tests {
    use super::*;
    extern crate env_logger;

    #[test]
    fn it_adds_one() {
        let _ = env_logger::init();
        info!("can log from the test too");
        assert_eq!(3, add_one(2));
    }

    #[test]
    fn it_handles_negative_numbers() {
        let _ = env_logger::init();
        info!("logging from another test");
        assert_eq!(-7, add_one(-8));
    }
}

Assuming the module under test is called my_lib, running the tests with the RUST_LOG filtering to info messages from this module looks like:

$ RUST_LOG=my_lib=info cargo test
     Running target/debug/my_lib-...

running 2 tests
INFO:my_lib::tests: logging from another test
INFO:my_lib: add_one called with -8
test tests::it_handles_negative_numbers ... ok
INFO:my_lib::tests: can log from the test too
INFO:my_lib: add_one called with 2
test tests::it_adds_one ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured

Note that env_logger::init() needs to be called in each test in which you want to enable logging. Additionally, the default behavior of tests to run in parallel means that logging output may be interleaved with test output. Either run tests in a single thread by specifying RUST_TEST_THREADS=1 or by running one test by specifying its name as an argument to the test binaries as directed by the cargo test help docs:

$ RUST_LOG=my_lib=info cargo test it_adds_one
     Running target/debug/my_lib-...

running 1 test
INFO:my_lib::tests: can log from the test too
INFO:my_lib: add_one called with 2
test tests::it_adds_one ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

By default, env_logger logs to stderr. If you want to log to stdout instead, you can use the Builder to change the log target:

use std::env;
use env_logger::{Builder, Target};

let mut builder = Builder::new();
builder.target(Target::Stdout);
if env::var("RUST_LOG").is_ok() {
    builder.parse(&env::var("RUST_LOG").unwrap());
}
builder.init().unwrap();