Auto merge of #33816 - nikomatsakis:projection-cache-2, r=arielb1
Projection cache and better warnings for #32330
This PR does three things:
- it lays the groundwork for the more precise subtyping rules discussed in #32330, but does not enable them;
- it issues warnings when the result of a leak-check or subtyping check relies on a late-bound region which will late become early-bound when #32330 is fixed;
- it introduces a cache for projection in the inference context.
I'm not 100% happy with the approach taken by the cache here, but it seems like a step in the right direction. It results in big wins on some test cases, but not as big as previous versions -- I think because it is caching the `Vec<Obligation>` (whereas before I just returned the normalized type with an empty vector). However, that change was needed to fix an ICE in @alexcrichton's future-rs module (I haven't fully tracked the cause of that ICE yet). Also, because trans/the collector use a fresh inference context for every call to `fulfill_obligation`, they don't profit nearly as much from this cache as they ought to.
Still, here are the results from the future-rs `retry.rs`:
~~Another example is the example from #31849. For that, I get 34s to run item-bodies without any cache. The version of the cache included here takes 2s to run item-bodies type-checking. An alternative version which doesn't track nested obligations takes 0.2s, but that version ICEs on @alexcrichton's future-rs (and may well be incorrect, I've not fully convinced myself of that). So, a definite win, but I think there's definitely room for further progress.~~
Pushed a modified version which improves performance of the case from #31849:
```
lunch-box. time rustc --stage0 ~/tmp/issue-31849.rs -Z no-trans
real 0m33.539s
user 0m32.932s
sys 0m0.570s
lunch-box. time rustc --stage2 ~/tmp/issue-31849.rs -Z no-trans
real 0m0.195s
user 0m0.154s
sys 0m0.042s
```
Some sort of cache is also needed for unblocking further work on lazy normalization, since that will lean even more heavily on the cache, and will also require cycle detection.