#[must_use] on a trait method impl does nothing.

[theemathas]

What it does

The lint should warn if #[must_use] is applied to a function in an impl Trait for Type { .... } block, and suggest applying it to the trait definition instead.

Advantage

As per the reference, #[must_use] in the impl Trait for Type block does absolutely nothing, and is therefore almost surely a mistake. On the other hand, #[must_use] in the trait definition applies to all implementations of the trait.

Drawbacks

No response

Example

trait Foo {
    fn bar(self) -> i32;
}
impl Foo for () {
    #[must_use]
    fn bar(self) -> i32 {
        1
    }
}

Could be written as:

trait Foo {
    #[must_use]
    fn bar(self) -> i32;
}
impl Foo for () {
    fn bar(self) -> i32 {
        1
    }
}

[y21]

Maybe this should be an extension to rustc's unused_attributes lint instead? That one is already used as a general lint for any kind of attribute used in some places where it doesn't do anything.

[samueltardieu]

I agree with @y21. I've opened rust-lang/rust#136923 to extend the rustc lint. Closing this one.