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Auto merge of #65232 - nikomatsakis:lazy-norm-anon-const-push-2, r=ma…
…tthewjasper 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
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