replace the leak check with universes, take 2
This PR is an attempt to revive the "universe-based region check", which is an important step towards lazy normalization. Unlike before, we also modify the definition of `'empty` so that it is indexed by a universe. This sidesteps some of the surprising effects we saw before -- at the core, we no longer think that `exists<'a> { forall<'b> { 'b: 'a } }` is solveable. The new region lattice looks like this:
```
static ----------+-----...------+ (greatest)
| | |
early-bound and | |
free regions | |
| | |
scope regions | |
| | |
empty(root) placeholder(U1) |
| / |
| / placeholder(Un)
empty(U1) -- /
| /
... /
| /
empty(Un) -------- (smallest)
```
This PR has three effects:
* It changes a fair number of error messages, I think for the better.
* It fixes a number of bugs. The old algorithm was too conservative and caused us to reject legal subtypings.
* It also causes two regressions (things that used to compile, but now do not).
* `coherence-subtyping.rs` gets an additional error. This is expected.
* `issue-57639.rs` regresses as before, for the reasons covered in #57639.
Both of the regressions stem from the same underlying property: without the leak check, the instantaneous "subtype" check is not able to tell whether higher-ranked subtyping will succeed or not. In both cases, we might be able to fix the problem by doing a 'leak-check like change' at some later point (e.g., as part of coherence).
This is a draft PR because:
* I didn't finish ripping out the leak-check completely.
* We might want to consider a crater run before landing this.
* We might want some kind of design meeting to cover the overall strategy.
* I just remembered I never finished 100% integrating this into the canonicalization code.
* I should also review what happens in NLL region checking -- it probably still has a notion of bottom (empty set).
r? @matthewjasper
8498c5f5b0