RS.CLIPPY.MISSING_ASSERT_MESSAGE

Checks assertions without a custom panic message

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: missing_assert_message. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks assertions without a custom panic message.

Why restrict this?

Without a good custom message, it'd be hard to understand what went wrong when the assertion fails. A good custom message should be more about why the failure of the assertion is problematic and not what is failed because the assertion already conveys that.

Although the same reasoning applies to testing functions, this lint ignores them as they would be too noisy. Also, in most cases understanding the test failure would be easier compared to understanding a complex invariant distributed around the codebase.

Known problems

This lint cannot check the quality of the custom panic messages. Hence, you can suppress this lint simply by adding placeholder messages like "assertion failed". However, we recommend coming up with good messages that provide useful information instead of placeholder messages that don't provide any extra information.

Example

fn call(service: Service) {
    assert!(service.ready);
}

Use instead:

fn call(service: Service) {
    assert!(service.ready, "`service.poll_ready()` must be called first to ensure that service is ready to receive requests");
}