author Andreas Tolfsen <>
Thu, 14 Jun 2018 12:49:09 -0700
changeset 422748 e018d6a7da4527e75ff0f7434a9b19e6d5d6afe3
parent 403419 9cca7938b734df92088d35729dbfe69243570ee6
child 436372 362b43ebdc0ac81b3804e18ea90aceee6754aaa1
permissions -rw-r--r--
Bug 1441204 - Upgrade log crate from 0.4.1 to 0.4.2. r=maja_zf MozReview-Commit-ID: FMIeUf55uzU


A Rust library providing a lightweight logging *facade*.

[![Build Status](](
[![Build status](](

* [`log` 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.

## Usage

## In libraries

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

log = "0.4"

extern crate log;

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

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

## In executables

In order to produce log output executables have to use a logger implementation compatible with the facade.
There are many available implementations to chose from, here are some of the most popular ones:

* Simple minimal loggers:
    * [`env_logger`](*/env_logger/)
    * [`simple_logger`](
    * [`simplelog`](
    * [`pretty_env_logger`](*/pretty_env_logger/)
    * [`stderrlog`](*/stderrlog/)
    * [`flexi_logger`](*/flexi_logger/)
* Complex configurable frameworks:
    * [`log4rs`](*/log4rs/)
    * [`fern`](*/fern/)
* Adaptors for other facilities:
    * [`syslog`](*/syslog/)
    * [`slog-stdlog`](*/slog_stdlog/)
    * [`android_log`](*/android_log/)

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.