`async fn` in trait should require `async fn` in impl

[compiler-errors]

For now, we've decided that an async fn() -> T in a trait should not be satisfied by a fn() -> Type where Type: Future<Output = T> until we at least hammer out how #[refine] is gonna work, etc.

So let's just do a HIR check that the signatures are compatible.

[compiler-errors]

Assigning to @ComputerDruid who expressed interest in working on this.

[compiler-errors]

@rustbot assign @ComputerDruid

idk if that works, the github UI doesn't let me assign to you 😝

[compiler-errors]

ugh lol

[ComputerDruid]

@rustbot claim

[zirconium-n]

Should it be satisfied with fn() -> impl Future<Output = T> ?

[compiler-errors]

I think not for now, and certainly not without the user enabling the return_position_impl_trait_in_trait feature gate