RS.CLIPPY.MUST_USE_CANDIDATE

Function or method that could take a `#[must_use]` attribute

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

What it does

Checks for public functions that have no #[must_use] attribute, but return something not already marked must-use, have no mutable arg and mutate no statics.

Why is this bad?

Not bad at all, this lint just shows places where you could add the attribute.

Known problems

The lint only checks the arguments for mutable types without looking if they are actually changed. On the other hand, it also ignores a broad range of potentially interesting side effects, because we cannot decide whether the programmer intends the function to be called for the side effect or the result. Expect many false positives. At least we don't lint if the result type is unit or already #[must_use].

Examples

// this could be annotated with `#[must_use]`.
pub fn id<T>(t: T) -> T { t }