rust private

Issue 42545 type inference regression Fix an ICE that results from type inference obligations being dropped on the floor. Specifically, when computing the implied bounds, we would sometimes do normalizations that get stored in the cache, but we would *not* try to solve the resulting obligations. This can sometimes leave type variables uninferred. Occurs only rarely because implied bounds are processed in regionck which happens very late, so usually the cache is populated already from somewhere else. I think that the *proper* fix here is probably lazy normalization. This fix is intentionally very narrow both because this code is on the chopping block and because this needs a beta backport. r? @eddyb cc @arielb1