RS.CLIPPY.RETURN_SELF_NOT_MUST_USE

Missing `#[must_use]` annotation on a method returning `Self`

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

What it does

This lint warns when a method returning Self doesn't have the #[must_use] attribute.

Why is this bad?

Methods returning Self often create new values, having the #[must_use] attribute prevents users from "forgetting" to use the newly created value.

The #[must_use] attribute can be added to the type itself to ensure that instances are never forgotten. Functions returning a type marked with #[must_use] will not be linted, as the usage is already enforced by the type attribute.

Limitations

This lint is only applied on methods taking a self argument. It would be mostly noise if it was added on constructors for example.

Example

pub struct Bar;
impl Bar {
    // Missing attribute
    pub fn bar(&self) -> Self {
        Self
    }
}

Use instead:

// It\'s better to have the `#[must_use]` attribute on the method like this:
pub struct Bar;
impl Bar {
    #[must_use]
    pub fn bar(&self) -> Self {
        Self
    }
}

// Or on the type definition like this:
#[must_use]
pub struct Bar;
impl Bar {
    pub fn bar(&self) -> Self {
        Self
    }
}