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

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

Author: bors

src/librustc/ich/impls_ty.rs

@@ -63,9 +63,12 @@ impl<'a> HashStable<StableHashingContext<'a>> for ty::RegionKind {
     fn hash_stable(&self, hcx: &mut StableHashingContext<'a>, hasher: &mut StableHasher) {
         mem::discriminant(self).hash_stable(hcx, hasher);
         match *self {
-            ty::ReErased | ty::ReStatic | ty::ReEmpty => {
+            ty::ReErased | ty::ReStatic => {
                 // No variant fields to hash for these ...
             }
+            ty::ReEmpty(universe) => {
+                universe.hash_stable(hcx, hasher);
+            }
             ty::ReLateBound(db, ty::BrAnon(i)) => {
                 db.hash_stable(hcx, hasher);
                 i.hash_stable(hcx, hasher);

src/librustc/infer/canonical/canonicalizer.rs

@@ -167,18 +167,29 @@ impl CanonicalizeRegionMode for CanonicalizeQueryResponse {
         r: ty::Region<'tcx>,
     ) -> ty::Region<'tcx> {
         match r {
-            ty::ReFree(_) | ty::ReEmpty | ty::ReErased | ty::ReStatic | ty::ReEarlyBound(..) => r,
+            ty::ReFree(_)
+            | ty::ReErased
+            | ty::ReStatic
+            | ty::ReEmpty(ty::UniverseIndex::ROOT)
+            | ty::ReEarlyBound(..) => r,
+
             ty::RePlaceholder(placeholder) => canonicalizer.canonical_var_for_region(
                 CanonicalVarInfo { kind: CanonicalVarKind::PlaceholderRegion(*placeholder) },
                 r,
             ),
+
             ty::ReVar(vid) => {
                 let universe = canonicalizer.region_var_universe(*vid);
                 canonicalizer.canonical_var_for_region(
                     CanonicalVarInfo { kind: CanonicalVarKind::Region(universe) },
                     r,
                 )
             }
+
+            ty::ReEmpty(ui) => {
+                bug!("canonicalizing 'empty in universe {:?}", ui) // FIXME
+            }
+
             _ => {
                 // Other than `'static` or `'empty`, the query
                 // response should be executing in a fully
@@ -213,7 +224,7 @@ impl CanonicalizeRegionMode for CanonicalizeUserTypeAnnotation {
         r: ty::Region<'tcx>,
     ) -> ty::Region<'tcx> {
         match r {
-            ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReErased | ty::ReEmpty | ty::ReStatic => r,
+            ty::ReEarlyBound(_) | ty::ReFree(_) | ty::ReErased | ty::ReStatic => r,
             ty::ReVar(_) => canonicalizer.canonical_var_for_region_in_root_universe(r),
             _ => {
                 // We only expect region names that the user can type.
@@ -320,8 +331,8 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for Canonicalizer<'cx, 'tcx> {
             | ty::ReEarlyBound(..)
             | ty::ReFree(_)
             | ty::ReScope(_)
+            | ty::ReEmpty(_)
             | ty::RePlaceholder(..)
-            | ty::ReEmpty
             | ty::ReErased => self.canonicalize_region_mode.canonicalize_free_region(self, r),
 
             ty::ReClosureBound(..) => {