#logging #testing


An opinionated library for developing and testing rust applications that use logging

17 releases

0.5.3 Dec 9, 2022
0.5.2 Nov 18, 2022
0.5.1 Dec 28, 2021
0.4.0 Dec 24, 2021
0.1.7 Dec 22, 2017

#86 in Debugging

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MIT license

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loggy v0.5.3

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An opinionated library for developing and testing rust applications that use logging.

This was initially inspired by simple-logging implementation, with additional features focusing on development of applications (as opposed to libraries). Structured messages were influenced by slog, but allow for nested structures and provide only a single format targeting human readability.


This library was written to support the development of a non-trivial Rust binary application(s) that uses logging. The functionality provided here was factored out, both to keep it isolated from the application's code itself, and in the hope it might prove useful to others.

In essence, the focus of this library is on using logging to provide information to the human user running the console application, rather than providing logging events for analysis of the behavior of server-like application. For the latter use case, you would want something like slog.

Technically, this library is an implementation for the Rust log facade, with a few additional features thrown in.


The provided features reflect the library's opinionated nature.

Message formatting

Messages are always emitted to the standard error (except for in tests, where they may be captured for use in assertions). The message format is <prefix>[<thread>]: <time> [<level]>] <module or scope>: <message>, where the thread and time may be omitted when you set up the global logger. For example:

extern crate loggy;

fn main() {
    log::set_logger(&loggy::Loggy {
        prefix: "...", // Typically, the name of the program.
        show_time: true, // Or false, if you prefer.
        show_thread: true, // Or false, if you prefer.
    log::set_max_level(log::LevelFilter::Info); // Or whatever level you want.

    // Use loggy facilities in the rest of the code.
    // ...

Logging multi-line messages (that contain \n) will generate multiple log lines, which will always be consecutive (even when logging from multiple threads). The first line will include the log level in upper case (e.g., [ERROR]), all the following will specify it in lower case (e.g., [error]). The time stamp, if included, will be identical for all these lines. This makes log messages easily grep-able, countable, etc.

Logging a message provides a structured way to format relevant additional information. The syntax is an extension of slog, allowing for nested structures. However unlike in slog, the output format is fixed. For example:

extern crate loggy;

fn foo() {
    let value = "bar";
        "some text {}", 1;
        label {
            sub_field => value,

Will generate the message:

program name: [INFO] scope name: some text 1
program name: [info]   value: bar
program name: [info]   label:
program name: [info]     sub_field: bar

Named scopes

By default, log messages are annotated with the name of the module generating them. To better identify specific processing stages and/or tasks, it is common to replace this by an explicit scope name; note this only applies to the current thread. Scopes can be established in three different ways:

extern crate loggy;

#[loggy::scope("scope name")]
fn foo() {
    // Log messages generated here will be prefixed by the scope name instead of the module name.
    // ...

fn bar() {
    // Log messages generated here will be prefixed by the function name `bar` instead of the module name.
    // ...

fn baz() {
    loggy::with_scope("scope name", || {
        // Log messages generated here will be prefixed by the scope name instead of the module name.
        // ...

    if some_condition {
        let _scope = loggy::Scope::new("scope name");
        // Log messages generated here will be prefixed by the scope name instead of the module name.
        // ...
    } else {

Logging levels

Log levels are given stronger semantics:

  • A loggy::panic! is logged using the Error level (that is, it is formatted as a log message), but is always converted to a std::panic! (that is, terminates the current thread).

  • A loggy::error! serves a different purpose. It also indicates a non-recoverable error, but allows the code to continue, possibly reporting additional errors, and automatically calls panic! at the end of the current named scope, indicating this scope has failed and reporting the total number of errors. Calling loggy::error! outside a named scope is not allowed; it is converted to a generic "error! must only be used inside a scope" std::panic! message. Errors, like panics, are always reported, regardless of the logging level.

  • A loggy::warn! is only reported if the logging level is at least Warn, and is otherwise silently ignored. Warnings designate abnormal situations where the code has a reasonable way to recover and continue normal execution. This may be used outside any named scope.

  • A loggy::note! allows the code to implement the "treat warnings as errors" functionality. It takes an additional boolean flag specifying whether this should be treated as a error! or a warn!. Since this may be treated as an error! it must only be used inside a scope. It is often useful to have program-wide flags determining whether a certain class of warnings should become errors, for example using a command-line flag. This can be done as follows:

extern crate loggy;

mod some_condition {
    loggy::is_an_error!(false); // By default, not an error.

fn main() {
    // ...
    // ...
    loggy::with_scope("scope name", || { // Errors must be inside some scope.
        // ...
        if test_for_some_condition {
            note!(some_condition::is_an_error(), "some condition"); // Will be an error or a warning depending on the command line flag.
        // ...
    // ...
  • A loggy::info! is only reported if the logging level is at least Info, and is otherwise silently ignored. Information messages should be few and far between, to indicate overall program progress.

  • A loggy::debug! is meant specifically for debugging the program, and targets the code developers rather than the program's users. Debug messages are always emitted in debug builds; in release builds they are only emitted if the logging level is at least Debug. The format of debug messages includes an additional <file>:<line>: prefix to identify their exact source code location. Finally, debug messages are always emitted to the standard error, and are never captured in tests (see below), which makes it possible to debug tests that examine the expected log.

  • A loggy::todox! is identical to loggy::debug!. It allows using the cargo todox extension to ensure no leftover debug messages are left in the code when debugging is over.

  • A loggy::trace! is only reported if the logging level is at least Trace, and is otherwise silently ignored. Trace messages describe program progress in high detail so may generate a very large log.

You can also use loggy::log!(level, ...) to specify the level of a message. Note that if this level is Error, the message can only be generated inside a named scope. There is no way to force a panic! this way (use note! instead).


Testing logging faces the following inconvenient truths:

  • The rust log facade mandates using a single global logger shared by all threads. You can't even replace it once you have set it up once.

  • By default, cargo test uses multiple threads to run tests in parallel.

  • A test capturing logged messages should capture everything generated from all sub-threads spawned by the test.

Therefore, the following following assertions take a global lock to ensure messages from different tests do not interfere with each other. This has several implications:

  • The test assertions have to setup a logger that captures the messages, so do not combine logging tests with any code that sets up the global logger.

  • The test assertions will run serially, one at a time, regardless of the number of threads spawned by cargo test. This still allows non-logging tests (that do not use the following assertions) to run in parallel.

  • Nesting the logging assertions will cause a deadlock. It doesn't make sense to do this in the 1st place, so just don't.

All that said, testing the actual log messages generated by some code is a convenient and surprisingly powerful way of ensuring it behaves as expected. It also ensures that the log messages contain the expected data, something that is otherwise difficult to verify. The following assertions are available to support this:

  • assert_logs(expected_log, || { ... }) executes some code and asserts that the actual log is identical to the (unindented) expected_log. Crucially, this can be nested, so you can examine the log in parts. The collected log for an outer assert_logs (or assert_logs_panics) does not include the log captured by an internal assert_logs.

  • assert_panics(expected_panic, || { ... }) executes some code and asserts that it panics with the (unindented) expected_panic, ignoring the log.

  • assert_logs_panics(expected_log, expected_panic, || { ... }) executes some code and asserts that both the actual log is as expected, and that the code also panics with the expected message.

  • assert_writes(expected_text, |writer| { ... }) is provided for convenience, asserting that the code writes the (unindented) expected_text to the writer: &mut dyn IoWrite. This really should be in a more generic crate.

These are intentionally not attribute macros attached to the test (like the standard #[should_panic]. This allows the expected texts to be dynamically formatted.

Setting the LOGGY_MIRROR_TO_STDERR environment variable to any non-empty value will cause all messages to be emitted to the standard error stream, together with any debug messages, even in tests. This places the debug messages in the context of the other messages, helping in debugging of tests.

Ideally, the standard error content is only reported for failing tests (this includes any debug messages). In practice, the rust mechanism for capturing the standard error does not work properly when the test spawns new threads, so any debug messages emitted from worker threads will be visible even for passing tests. This isn't a show stopper given such messages and the LOGGY_MIRROR_TO_STDERR variable are only used when actively debugging an issue.


loggy is licensed under the MIT License.


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