Error messages for where `impl Trait` is allowed do not account for return-position-impl-trait-in-trait (RPITIT)

[shepmaster]

Code

fn repro(a: impl Fn() -> impl Drop) {}

Current output

error[E0562]: `impl Trait` only allowed in function and inherent method argument and return types, not in `Fn` trait return types
 --> src/lib.rs:1:26
  |
1 | fn repro(a: impl Fn() -> impl Drop) {}
  |                          ^^^^^^^^^

Desired output

I believe this message should mention the newly-stabilized possibilities of RPITIT. For example, this is not a function or an inherent method:

trait Moo {
    fn moo() -> impl Drop;
}

Rationale and extra context

No response

Other cases

My gut tells me that there are other errors with the same now-incorrect subset of locations.

Anything else?

Seen in nightly (2024-01-04 f688dd6)

[compiler-errors]

We probably should not make this diagnostic have an exhaustive list of allowed places anymore