Diagnostic for dyn Self could be more specific

[wchargin]

Code (play):

trait S {
    fn f() -> dyn Self;
}

Current diagnostic (1.34.0-nightly 2019-02-15 eac0908):

error[E0411]: expected trait, found self type `Self`
 --> src/lib.rs:2:19
  |
2 |     fn f() -> dyn Self;
  |                   ^^^^ `Self` is only available in impls, traits, and type definitions

@Moongoodboy-K on IRC points out that it might be more helpful for the
diagnostic to say something like,

expected trait, found concrete type Self

(help: Self refers to the concrete type implementing this trait)

(For context: while I’ve since learned that this code is pretty far from
valid, the intent was to find a way to use Self-types while preserving
object-safety. My intuition was that it should suffice to return the
receiver’s vtable from the function, which suggested to me the syntax
dyn Self. Using simply fn f() -> dyn S is object-safe but loses the
self-typing, which might matter if given something like dyn S + Send.)

[theastrallyforged]

TFW you only read the annotation next to the highlight, and entirely miss the error heading. xD

The heading is spot-on; it's just that Self is only available in impls, traits, and type definitions bit that doesn't make sense (what? this is a trait definition!)

[estebank]

Similar case: #50871