Crate tracing_test

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Helper functions and macros that allow for easier testing of crates that use tracing.

The focus is on testing the logging, not on debugging the tests. That’s why the library ensures that the logs do not depend on external state. For example, the RUST_LOG env variable is not used for log filtering.

Similar crates:


This crate should mainly be used through the #[traced_test] macro.

First, add a dependency on tracing-test in Cargo.toml:

tokio = { version = "1", features = ["rt-multi-thread", "macros"] }
tracing = "0.1"
tracing-test = "0.1"

Then, annotate your test function with the #[traced_test] macro.

use tracing::{info, warn};
use tracing_test::traced_test;

async fn test_logs_are_captured() {
    // Local log
    info!("This is being logged on the info level");

    // Log from a spawned task (which runs in a separate thread)
    tokio::spawn(async {
        warn!("This is being logged on the warn level from a spawned task");

    // Ensure that certain strings are or aren't logged
    assert!(logs_contain("logged on the info level"));
    assert!(logs_contain("logged on the warn level"));
    assert!(!logs_contain("logged on the error level"));

    // Ensure that the string `logged` is logged exactly twice
    logs_assert(|lines: &[&str]| {
        match lines.iter().filter(|line| line.contains("logged")).count() {
            2 => Ok(()),
            n => Err(format!("Expected two matching logs, but found {}", n)),

Done! You can write assertions using one of two injected functions:

  • logs_contain(&str) -> bool: Use this within an assert! call to ensure that a certain string is (or isn’t) logged anywhere in the logs.
  • logs_assert(f: impl Fn(&[&str]) -> Result<(), String>): Run a function against the log lines. If the function returns an Err, panic. This can be used to run arbitrary assertion logic against the logs.

Logs are written to stdout, so they are captured by the cargo test runner by default, but printed if the test fails.

Of course, you can also annotate regular non-async tests:

use tracing::info;
use tracing_test::traced_test;

fn plain_old_test() {
    assert!(!logs_contain("Logging from a non-async test"));
    info!("Logging from a non-async test");
    assert!(logs_contain("Logging from a non-async test"));
    assert!(!logs_contain("This was never logged"));

Rationale / Why You Need This

Tracing allows you to set a default subscriber within a scope:

let response = tracing::dispatcher::with_default(&subscriber, || get_response(req));

This works fine, as long as no threads are involved. As soon as you use a multi-threaded test runtime (e.g. the #[tokio::test] with the rt-multi-thread feature) and spawn tasks, the tracing logs in those tasks will not be captured by the subscriber.

The macro provided in this crate registers a global default subscriber instead. This subscriber contains a writer which logs into a global static in-memory buffer.

At the beginning of every test, the macro injects span opening code. The span uses the name of the test function (unless it’s already taken, then a counter is appended). This means that the logs from a test are prefixed with the test name, which helps when debugging.

Finally, a function called logs_contain(value: &str) is injected into every annotated test. It filters the logs in the buffer to include only lines containing {span_name}: and then searches the value in the matching log lines. This can be used to assert that a message was logged during a test.

Per-crate Filtering

By default, tracing-test sets an env filter that filters out all logs except the ones from your crate (equivalent to RUST_LOG=<your_crate>=trace). If you need to capture logs from other crates as well, you can turn off this log filtering globally by enabling the no-env-filter Cargo feature:

tracing-test = { version = "0.1", features = ["no-env-filter"] }

Note that this will result in all logs from all your dependencies being captured! This means that the logs_contain function may become less useful, and you might need to use logs_assert instead, with your own custom filtering logic.

Note: Rust “integration tests” (in the tests/ directory) are each built into a separate crate from the crate they test. As a result, integration tests must use no-env-filter to capture and observe logs.


Internal functionality used by the #[traced_test] macro.

Attribute Macros

A procedural macro that ensures that a global logger is registered for the annotated test.