By RFC1214:
> Before calling a fn, we check that its argument and return types are WF.
The previous code only checked the trait-ref, which was not enough
in several cases.
As this is a soundness fix, it is a [breaking-change]. Some new annotations are needed, which I think are because of #18653 and the imperfection of `projection_must_outlive` (that can probably be worked around by moving the wf obligation later).
Fixes#28609
r? @nikomatsakis
130851e030