Rollup merge of #139828 - compiler-errors:rigid-trait, r=lcnr

Don't require rigid alias's trait to hold

See test for write-up. TL;DR is that we don't need the trait bound to hold, since we enforce it during WF.

I think this is preferable to introducing (if we even could do so) a more specific hack around coroutine interiors, higher ranked types, etc, since this is just a manifestation of more pervasive issues w/ lifetime erasure in coroutines. This just doesn't manifest in the old solver b/c it doesn't try to prove `T: Trait` holds when rigidly projecting `<T as Trait>::Assoc`.

It's pretty clear that this affects quite a few traits (rust-lang/rust#139763), so I think this needs fixing.

r? lcnr

Fixes rust-lang/trait-system-refactor-initiative#177

Author: matthiaskrgr