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(..) => {

src/librustc/infer/combine.rs

@@ -577,7 +577,7 @@ impl TypeRelation<'tcx> for Generalizer<'_, 'tcx> {
 
             ty::RePlaceholder(..)
             | ty::ReVar(..)
-            | ty::ReEmpty
+            | ty::ReEmpty(_)
             | ty::ReStatic
             | ty::ReScope(..)
             | ty::ReEarlyBound(..)

src/librustc/infer/error_reporting/mod.rs

@@ -138,7 +138,10 @@ pub(super) fn note_and_explain_region(
             msg_span_from_free_region(tcx, region)
         }
 
-        ty::ReEmpty => ("the empty lifetime".to_owned(), None),
+        ty::ReEmpty(ty::UniverseIndex::ROOT) => ("the empty lifetime".to_owned(), None),
+
+        // uh oh, hope no user ever sees THIS
+        ty::ReEmpty(ui) => (format!("the empty lifetime in universe {:?}", ui), None),
 
         ty::RePlaceholder(_) => (format!("any other region"), None),
 
@@ -181,7 +184,8 @@ fn msg_span_from_free_region(
             msg_span_from_early_bound_and_free_regions(tcx, region)
         }
         ty::ReStatic => ("the static lifetime".to_owned(), None),
-        ty::ReEmpty => ("an empty lifetime".to_owned(), None),
+        ty::ReEmpty(ty::UniverseIndex::ROOT) => ("an empty lifetime".to_owned(), None),
+        ty::ReEmpty(ui) => (format!("an empty lifetime in universe {:?}", ui), None),
         _ => bug!("{:?}", region),
     }
 }
@@ -375,6 +379,31 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                         }
                     }
 
+                    RegionResolutionError::UpperBoundUniverseConflict(
+                        _,
+                        _,
+                        var_universe,
+                        sup_origin,
+                        sup_r,
+                    ) => {
+                        assert!(sup_r.is_placeholder());
+
+                        // Make a dummy value for the "sub region" --
+                        // this is the initial value of the
+                        // placeholder. In practice, we expect more
+                        // tailored errors that don't really use this
+                        // value.
+                        let sub_r = self.tcx.mk_region(ty::ReEmpty(var_universe));
+
+                        self.report_placeholder_failure(
+                            region_scope_tree,
+                            sup_origin,
+                            sub_r,
+                            sup_r,
+                        )
+                        .emit();
+                    }
+
                     RegionResolutionError::MemberConstraintFailure {
                         opaque_type_def_id,
                         hidden_ty,
@@ -429,6 +458,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             RegionResolutionError::GenericBoundFailure(..) => true,
             RegionResolutionError::ConcreteFailure(..)
             | RegionResolutionError::SubSupConflict(..)
+            | RegionResolutionError::UpperBoundUniverseConflict(..)
             | RegionResolutionError::MemberConstraintFailure { .. } => false,
         };
 
@@ -443,6 +473,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
             RegionResolutionError::ConcreteFailure(ref sro, _, _) => sro.span(),
             RegionResolutionError::GenericBoundFailure(ref sro, _, _) => sro.span(),
             RegionResolutionError::SubSupConflict(_, ref rvo, _, _, _, _) => rvo.span(),
+            RegionResolutionError::UpperBoundUniverseConflict(_, ref rvo, _, _, _) => rvo.span(),
             RegionResolutionError::MemberConstraintFailure { span, .. } => span,
         });
         errors

src/librustc/infer/error_reporting/nice_region_error/different_lifetimes.rs

@@ -45,7 +45,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
     ///
     /// It will later be extended to trait objects.
     pub(super) fn try_report_anon_anon_conflict(&self) -> Option<ErrorReported> {
-        let (span, sub, sup) = self.regions();
+        let (span, sub, sup) = self.regions()?;
 
         // Determine whether the sub and sup consist of both anonymous (elided) regions.
         let anon_reg_sup = self.tcx().is_suitable_region(sup)?;

src/librustc/infer/error_reporting/nice_region_error/mod.rs

@@ -17,11 +17,6 @@ mod util;
 
 impl<'cx, 'tcx> InferCtxt<'cx, 'tcx> {
     pub fn try_report_nice_region_error(&self, error: &RegionResolutionError<'tcx>) -> bool {
-        match *error {
-            ConcreteFailure(..) | SubSupConflict(..) => {}
-            _ => return false, // inapplicable
-        }
-
         if let Some(tables) = self.in_progress_tables {
             let tables = tables.borrow();
             NiceRegionError::new(self, error.clone(), Some(&tables)).try_report().is_some()
@@ -79,13 +74,14 @@ impl<'cx, 'tcx> NiceRegionError<'cx, 'tcx> {
             .or_else(|| self.try_report_impl_not_conforming_to_trait())
     }
 
-    pub fn regions(&self) -> (Span, ty::Region<'tcx>, ty::Region<'tcx>) {
+    pub fn regions(&self) -> Option<(Span, ty::Region<'tcx>, ty::Region<'tcx>)> {
         match (&self.error, self.regions) {
-            (Some(ConcreteFailure(origin, sub, sup)), None) => (origin.span(), sub, sup),
-            (Some(SubSupConflict(_, _, origin, sub, _, sup)), None) => (origin.span(), sub, sup),
-            (None, Some((span, sub, sup))) => (span, sub, sup),
-            (Some(_), Some(_)) => panic!("incorrectly built NiceRegionError"),
-            _ => panic!("trying to report on an incorrect lifetime failure"),
+            (Some(ConcreteFailure(origin, sub, sup)), None) => Some((origin.span(), sub, sup)),
+            (Some(SubSupConflict(_, _, origin, sub, _, sup)), None) => {
+                Some((origin.span(), sub, sup))
+            }
+            (None, Some((span, sub, sup))) => Some((span, sub, sup)),
+            _ => None,
         }
     }
 }

src/librustc/infer/error_reporting/nice_region_error/named_anon_conflict.rs

@@ -9,7 +9,7 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
     /// When given a `ConcreteFailure` for a function with parameters containing a named region and
     /// an anonymous region, emit an descriptive diagnostic error.
     pub(super) fn try_report_named_anon_conflict(&self) -> Option<DiagnosticBuilder<'a>> {
-        let (span, sub, sup) = self.regions();
+        let (span, sub, sup) = self.regions()?;
 
         debug!(
             "try_report_named_anon_conflict(sub={:?}, sup={:?}, error={:?})",

src/librustc/infer/error_reporting/nice_region_error/placeholder_error.rs

@@ -107,6 +107,25 @@ impl NiceRegionError<'me, 'tcx> {
                 found.substs,
             )),
 
+            Some(RegionResolutionError::UpperBoundUniverseConflict(
+                vid,
+                _,
+                _,
+                SubregionOrigin::Subtype(box TypeTrace {
+                    cause,
+                    values: ValuePairs::TraitRefs(ExpectedFound { expected, found }),
+                }),
+                sup_placeholder @ ty::RePlaceholder(_),
+            )) if expected.def_id == found.def_id => Some(self.try_report_placeholders_trait(
+                Some(self.tcx().mk_region(ty::ReVar(*vid))),
+                cause,
+                None,
+                Some(*sup_placeholder),
+                expected.def_id,
+                expected.substs,
+                found.substs,
+            )),
+
             Some(RegionResolutionError::ConcreteFailure(
                 SubregionOrigin::Subtype(box TypeTrace {
                     cause,

src/librustc/infer/freshen.rs

@@ -130,7 +130,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for TypeFreshener<'a, 'tcx> {
             | ty::ReScope(_)
             | ty::ReVar(_)
             | ty::RePlaceholder(..)
-            | ty::ReEmpty
+            | ty::ReEmpty(_)
             | ty::ReErased => {
                 // replace all free regions with 'erased
                 self.tcx().lifetimes.re_erased

src/librustc/infer/higher_ranked/mod.rs

@@ -128,6 +128,16 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
         placeholder_map: &PlaceholderMap<'tcx>,
         snapshot: &CombinedSnapshot<'_, 'tcx>,
     ) -> RelateResult<'tcx, ()> {
+        // If the user gave `-Zno-leak-check`, or we have been
+        // configured to skip the leak check, then skip the leak check
+        // completely. The leak check is deprecated. Any legitimate
+        // subtyping errors that it would have caught will now be
+        // caught later on, during region checking. However, we
+        // continue to use it for a transition period.
+        if self.tcx.sess.opts.debugging_opts.no_leak_check || self.skip_leak_check.get() {
+            return Ok(());
+        }
+
         self.borrow_region_constraints().leak_check(
             self.tcx,
             overly_polymorphic,