#[must_use] on traits, e.g. on Future

[emilk]

Putting #[must_use] on a trait is currently a no-op (it doesn't seem to do anything, but it is allowed). See example on playground.

One important use-case for this would be to have #[must_use] on std::Future. When returning impl Future<…> or Box<dyn Future<…>> from a function, it should lead to a warning if that future is not used, as that is almost certainly a bug.

[Centril]

cc @varkor

[zackmdavis]

(Also see previous discussion on #51560.)

[varkor]

This is now possible. It should be easy to make a pull request adding #[must_use] to Future (with a test).

We probably want to also consider what other traits would benefit from being #[must_use]. Iterator is a fairly obvious choice, for one.

[F001]

Fn FnMut FnOnce are good candidates as well.

[Centril]

@F001 Can you elaborate? If Output = () then I probably don't want the warning...?

[F001]

@Centril Well, I think this warning means this:

let c : impl Fn(i32) = ... ;
c(); // `c` must be used. If `c` is discarded, it probably indicates something wrong.

let r = c(); // I think this attribute does not care `r` is used or not

[Centril]

@F001 Yeah that makes sense.