Auto merge of #109788 - compiler-errors:trait-item-from-non-trait, r=petrochenkov

More descriptive error when qself path doesnt have a trait on the RHS of `as`

`<Ty as Enum>::Assoc` should report that `Enum` is a trait. Main question is whether to eagerly report the error, or raise it with `return Err(..)` -- i'll note that in an inline comment though.

cc `@GuillaumeGomez` who said this came up at a Paris Rust meetup.

r? `@petrochenkov`

Author: bors