Check projection predicates satisfy bounds

[matthewjasper]

This restores the behavior from before #73905.

closes #78893

r? @nikomatsakis

[matthewjasper]

So, failure is due to problems like this:

trait T {
    type Item;

    fn f() where Self: T<Item = str> {}
}

This is allowed on stable because the trait has a <Self as T>::Item: Sized predicate, which normalizes to str: Sized bound, making this code compile. On nightly/beta this predicate no longer exists.

[nikomatsakis]

this looks good, although I do have a question

[nikomatsakis]

So -- where I got stuck here was that I assumed we would need to normalize after substitution. Does that happen elsewhere?

(I still feel like (not in this PR...) I'd like to explore having the "bounds" predicate return a Binder<Predicate>, with the binder being the "self" type.)

[spastorino]

discussed in T-compiler meeting, declined to backport.

@rustbot modify labels: -beta-nominated

[nikomatsakis]

So @matthewjasper -- we had discussed the possibility of including associated type bounds in elaboration. However, I'm not sure if that's really a good idea. I guess we ought to carve out some space to have a longer conversation about the implications of accepting this change.

[nikomatsakis]

Or, by that I mean: the implications of doing nothing about the change in semantics, because that is a tempting option.

[pnkfelix]

(Note for future prioritization visits: to be clear, we cannot land the PR as it stands now, because it literally breaks bootstrapping of rustc.)

[matthewjasper]

closing, I think that we're going to accept the change in behaviour