Confusing error message when using Self in where clause bounds errornously

[Havvy]

The source trait SelfReferential<T> where T: Self {} gives the error

error[E0411]: expected trait, found self type `Self`
  --> src/main.rs:13:35
   |
13 | trait SelfReferential<T> where T: Self {}
   |                                   ^^^^ `Self` is only available in traits and impls

This is confusing because the Self is in a trait. It just doesn't seem to be valid to use it as the trait bounds of a type in a where clause.

[BGR360]

Potentially related: #91180

@rustbot label +A-traits +D-papercut

[estebank]

The current output is hilariously contradictory:

error[E0411]: expected trait, found self type `Self`
 --> src/lib.rs:1:35
  |
1 | trait SelfReferential<T> where T: Self {}
  |       ---------------             ^^^^ `Self` is only available in impls, traits, and type definitions
  |       |
  |       `Self` not allowed in a trait