Crate log [] [src]

A lightweight logging facade.

The log crate 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.

If no logging implementation is selected, the facade falls back to a "noop" implementation that ignores all log messages. The overhead in this case is very small - just an integer load, comparison and jump.

A log request consists of a target, a level, and a body. A target is a string which defaults to the module path of the location of the log request, though that default may be overridden. Logger implementations typically use the target to filter requests based on some user configuration.


The basic use of the log crate is through the five logging macros: error!, warn!, info!, debug! and trace! where error! represents the highest-priority log level, and trace! the lowest.

Each of these macros accept format strings similarly to println!.

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.


extern crate log;

pub fn shave_the_yak(yak: &Yak) {
    info!(target: "yak_events", "Commencing yak shaving for {:?}", yak);

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

In executables

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

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


The logging system may only be initialized once.

Available logging implementations

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

Implementing a Logger

Loggers implement the Log trait. Here's a very basic example that simply logs all messages at the Error, Warn or Info levels to stdout:

extern crate log;

use log::{Record, Level, Metadata};

struct SimpleLogger;

impl log::Log for SimpleLogger {
    fn enabled(&self, metadata: &Metadata) -> bool {
        metadata.level() <= Level::Info

    fn log(&self, record: &Record) {
        if self.enabled(record.metadata()) {
            println!("{} - {}", record.level(), record.args());

    fn flush(&self) {}

Loggers are installed by calling the set_logger function. It takes a closure which is provided a MaxLevelFilter token and returns a Log trait object. The MaxLevelFilter token controls the global maximum log level. The logging facade uses this as an optimization to improve performance of log messages at levels that are disabled. In the case of our example logger, we'll want to set the maximum log level to Info, since we ignore any Debug or Trace level log messages. A logging implementation should provide a function that wraps a call to set_logger, handling initialization of the logger:

use log::{SetLoggerError, LevelFilter};

static LOGGER: SimpleLogger = SimpleLogger;

pub fn init() -> Result<(), SetLoggerError> {

Use with std

set_logger requires you to provide a &'static Log, which can be hard if your logger depends on some runtime configuration. The set_boxed_logger function is available with the std Cargo feature. It is identical to set_logger except that it requires you to provide a Box<Log> rather than a &'static Log:

pub fn init() -> Result<(), SetLoggerError> {

Compile time filters

Log levels can be statically disabled at compile time via Cargo features. Log invocations at disabled levels will be skipped and will not even be present in the resulting binary unless the log level is specified dynamically. This level is configured separately for release and debug builds. The features are:

  • max_level_off
  • max_level_error
  • max_level_warn
  • max_level_info
  • max_level_debug
  • max_level_trace
  • release_max_level_off
  • release_max_level_error
  • release_max_level_warn
  • release_max_level_info
  • release_max_level_debug
  • release_max_level_trace

These features control the value of the STATIC_MAX_LEVEL constant. The logging macros check this value before logging a message. By default, no levels are disabled.

For example, a crate can disable trace level logs in debug builds and trace, info, and warn level logs in release builds with the following configuration:

log = { version = "0.4", features = ["max_level_debug", "release_max_level_warn"] }

Version compatibility

The 0.3 and 0.4 versions of the log crate are almost entirely compatible. Log messages made using log 0.3 will forward transparently to a logger implementation using log 0.4. Log messages made using log 0.4 will forward to a logger implementation using log 0.3, but the module path and file name information associated with the message will unfortunately be lost.



Logs a message at the debug level.


Logs a message at the error level.


Logs a message at the info level.


The standard logging macro.


Determines if a message logged at the specified level in that module will be logged.


Logs a message at the trace level.


Logs a message at the warn level.



Metadata about a log message.


Builder for Metadata.


The type returned by from_str when the string doesn't match any of the log levels.


The "payload" of a log message.


Builder for Record.


The type returned by set_logger if set_logger has already been called.



An enum representing the available verbosity levels of the logger.


An enum representing the available verbosity level filters of the logger.



The statically resolved maximum log level.



A trait encapsulating the operations required of a logger.



Returns a reference to the logger.


Returns the current maximum log level.


Sets the global logger to a Box<Log>.


Sets the global logger to a &'static Log.


Sets the global maximum log level.