Auto merge of #58592 - nikomatsakis:universe-leak-check, r=<try>

[WIP] Re-implement leak check in terms of universes

This PR temporarily restores the leak-check, but implemented in terms of universes. This is not because the leak check behavior was necessarily **correct**, but because (a) we may want to have a transition period and because (b) we want to have more breathing room to work through the full implications of handling higher-ranked types correctly. Note that this PR builds atop #58056.

Fixes #58451
Fixes #46989
Fixes #57639

r? @aturon
cc @arielb1, @lqd

~~Temporary note: I've not finished running `./x.py test` locally -- I'm confident a lot of error messages in tests will need updating. I sort of expect them to revert to the older, (imo) less good error messages, which is mildly unfortunate. There might be a way to preserve the new error messages, not sure.~~

src/librustc/infer/combine.rs

@@ -255,10 +255,24 @@ impl<'infcx, 'gcx, 'tcx> CombineFields<'infcx, 'gcx, 'tcx> {
             RelationDir::SupertypeOf => ty::Contravariant,
         };
 
+        debug!("generalize: ambient_variance = {:?}", ambient_variance);
+
+        let for_universe = match self.infcx.type_variables.borrow_mut().probe(for_vid) {
+            v @ TypeVariableValue::Known { .. } => panic!(
+                "instantiating {:?} which has a known value {:?}",
+                for_vid,
+                v,
+            ),
+            TypeVariableValue::Unknown { universe } => universe,
+        };
+
+        debug!("generalize: for_universe = {:?}", for_universe);
+
         let mut generalize = Generalizer {
             infcx: self.infcx,
             span: self.trace.cause.span,
             for_vid_sub_root: self.infcx.type_variables.borrow_mut().sub_root_var(for_vid),
+            for_universe,
             ambient_variance,
             needs_wf: false,
             root_ty: ty,
@@ -288,6 +302,11 @@ struct Generalizer<'cx, 'gcx: 'cx+'tcx, 'tcx: 'cx> {
     /// that means we would have created a cyclic type.
     for_vid_sub_root: ty::TyVid,
 
+    /// The universe of the type variable that is in the process of
+    /// being instantiated. Any fresh variables that we create in this
+    /// process should be in that same universe.
+    for_universe: ty::UniverseIndex,
+
     /// Track the variance as we descend into the type.
     ambient_variance: ty::Variance,
 
@@ -386,6 +405,8 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
     fn tys(&mut self, t: Ty<'tcx>, t2: Ty<'tcx>) -> RelateResult<'tcx, Ty<'tcx>> {
         assert_eq!(t, t2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 
+        debug!("generalize: t={:?}", t);
+
         // Check to see whether the type we are genealizing references
         // any other type variable related to `vid` via
         // subtyping. This is basically our "occurs check", preventing
@@ -403,12 +424,17 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                     match variables.probe(vid) {
                         TypeVariableValue::Known { value: u } => {
                             drop(variables);
+                            debug!("generalize: known value {:?}", u);
                             self.relate(&u, &u)
                         }
                         TypeVariableValue::Unknown { universe } => {
                             match self.ambient_variance {
                                 // Invariant: no need to make a fresh type variable.
-                                ty::Invariant => return Ok(t),
+                                ty::Invariant => {
+                                    if self.for_universe.can_name(universe) {
+                                        return Ok(t);
+                                    }
+                                }
 
                                 // Bivariant: make a fresh var, but we
                                 // may need a WF predicate. See
@@ -422,7 +448,7 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                             }
 
                             let origin = *variables.var_origin(vid);
-                            let new_var_id = variables.new_var(universe, false, origin);
+                            let new_var_id = variables.new_var(self.for_universe, false, origin);
                             let u = self.tcx().mk_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}",
                                    vid, u);
@@ -448,6 +474,8 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                -> RelateResult<'tcx, ty::Region<'tcx>> {
         assert_eq!(r, r2); // we are abusing TypeRelation here; both LHS and RHS ought to be ==
 
+        debug!("generalize: regions r={:?}", r);
+
         match *r {
             // Never make variables for regions bound within the type itself,
             // nor for erased regions.
@@ -456,37 +484,40 @@ impl<'cx, 'gcx, 'tcx> TypeRelation<'cx, 'gcx, 'tcx> for Generalizer<'cx, 'gcx, '
                 return Ok(r);
             }
 
-            // Always make a fresh region variable for placeholder
-            // regions; the higher-ranked decision procedures rely on
-            // this.
-            ty::RePlaceholder(..) => { }
+            ty::ReClosureBound(..) => {
+                span_bug!(
+                    self.span,
+                    "encountered unexpected ReClosureBound: {:?}",
+                    r,
+                );
+            }
 
-            // For anything else, we make a region variable, unless we
-            // are *equating*, in which case it's just wasteful.
+            ty::RePlaceholder(..) |
+            ty::ReVar(..) |
             ty::ReEmpty |
             ty::ReStatic |
             ty::ReScope(..) |
-            ty::ReVar(..) |
             ty::ReEarlyBound(..) |
             ty::ReFree(..) => {
-                match self.ambient_variance {
-                    ty::Invariant => return Ok(r),
-                    ty::Bivariant | ty::Covariant | ty::Contravariant => (),
-                }
+                // see common code below
             }
+        }
 
-            ty::ReClosureBound(..) => {
-                span_bug!(
-                    self.span,
-                    "encountered unexpected ReClosureBound: {:?}",
-                    r,
-                );
+        // If we are in an invariant context, we can re-use the region
+        // as is, unless it happens to be in some universe that we
+        // can't name. (In the case of a region *variable*, we could
+        // use it if we promoted it into our universe, but we don't
+        // bother.)
+        if let ty::Invariant = self.ambient_variance {
+            let r_universe = self.infcx.universe_of_region(r);
+            if self.for_universe.can_name(r_universe) {
+                return Ok(r);
             }
         }
 
         // FIXME: This is non-ideal because we don't give a
         // very descriptive origin for this region variable.
-        Ok(self.infcx.next_region_var(MiscVariable(self.span)))
+        Ok(self.infcx.next_region_var_in_universe(MiscVariable(self.span), self.for_universe))
     }
 }
 

src/librustc/infer/higher_ranked/mod.rs

@@ -4,6 +4,7 @@
 use super::combine::CombineFields;
 use super::{HigherRankedType, InferCtxt, PlaceholderMap};
 
+use crate::infer::CombinedSnapshot;
 use crate::ty::relate::{Relate, RelateResult, TypeRelation};
 use crate::ty::{self, Binder, TypeFoldable};
 
@@ -29,27 +30,32 @@ impl<'a, 'gcx, 'tcx> CombineFields<'a, 'gcx, 'tcx> {
 
         let span = self.trace.cause.span;
 
-        // First, we instantiate each bound region in the supertype with a
-        // fresh placeholder region.
-        let (b_prime, _) = self.infcx.replace_bound_vars_with_placeholders(b);
+        return self.infcx.commit_if_ok(|snapshot| {
+            // First, we instantiate each bound region in the supertype with a
+            // fresh placeholder region.
+            let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
 
-        // Next, we instantiate each bound region in the subtype
-        // with a fresh region variable. These region variables --
-        // but no other pre-existing region variables -- can name
-        // the placeholders.
-        let (a_prime, _) =
-            self.infcx
-                .replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
+            // Next, we instantiate each bound region in the subtype
+            // with a fresh region variable. These region variables --
+            // but no other pre-existing region variables -- can name
+            // the placeholders.
+            let (a_prime, _) =
+                self.infcx
+                    .replace_bound_vars_with_fresh_vars(span, HigherRankedType, a);
+
+            debug!("a_prime={:?}", a_prime);
+            debug!("b_prime={:?}", b_prime);
 
-        debug!("a_prime={:?}", a_prime);
-        debug!("b_prime={:?}", b_prime);
+            // Compare types now that bound regions have been replaced.
+            let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
 
-        // Compare types now that bound regions have been replaced.
-        let result = self.sub(a_is_expected).relate(&a_prime, &b_prime)?;
+            self.infcx
+                .leak_check(!a_is_expected, &placeholder_map, snapshot)?;
 
-        debug!("higher_ranked_sub: OK result={:?}", result);
+            debug!("higher_ranked_sub: OK result={:?}", result);
 
-        Ok(ty::Binder::bind(result))
+            Ok(ty::Binder::bind(result))
+        });
     }
 }
 
@@ -72,10 +78,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     /// [rustc guide]: https://rust-lang.github.io/rustc-guide/traits/hrtb.html
     pub fn replace_bound_vars_with_placeholders<T>(
         &self,
-        binder: &ty::Binder<T>
+        binder: &ty::Binder<T>,
     ) -> (T, PlaceholderMap<'tcx>)
     where
-        T: TypeFoldable<'tcx>
+        T: TypeFoldable<'tcx>,
     {
         let next_universe = self.create_next_universe();
 
@@ -96,12 +102,25 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         let (result, map) = self.tcx.replace_bound_vars(binder, fld_r, fld_t);
 
         debug!(
-            "replace_bound_vars_with_placeholders(binder={:?}, result={:?}, map={:?})",
-            binder,
-            result,
-            map
+            "replace_bound_vars_with_placeholders(\
+             next_universe={:?}, \
+             binder={:?}, \
+             result={:?}, \
+             map={:?})",
+            next_universe, binder, result, map,
         );
 
         (result, map)
     }
+
+    /// See `infer::region_constraints::RegionConstraintCollector::leak_check`.
+    pub fn leak_check(
+        &self,
+        overly_polymorphic: bool,
+        placeholder_map: &PlaceholderMap<'tcx>,
+        snapshot: &CombinedSnapshot<'_, 'tcx>,
+    ) -> RelateResult<'tcx, ()> {
+        self.borrow_region_constraints()
+            .leak_check(self.tcx, overly_polymorphic, placeholder_map, snapshot)
+    }
 }

src/librustc/infer/mod.rs

@@ -937,32 +937,41 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             return None;
         }
 
-        let (
-            ty::SubtypePredicate {
-                a_is_expected,
-                a,
-                b,
-            },
-            _,
-        ) = self.replace_bound_vars_with_placeholders(predicate);
+        Some(self.commit_if_ok(|snapshot| {
+            let (
+                ty::SubtypePredicate {
+                    a_is_expected,
+                    a,
+                    b,
+                },
+                placeholder_map,
+            ) = self.replace_bound_vars_with_placeholders(predicate);
 
-        Some(
-            self.at(cause, param_env)
-                .sub_exp(a_is_expected, a, b)
-                .map(|ok| ok.unit()),
-        )
+            let ok = self.at(cause, param_env)
+                .sub_exp(a_is_expected, a, b)?;
+
+            self.leak_check(false, &placeholder_map, snapshot)?;
+
+            Ok(ok.unit())
+        }))
     }
 
     pub fn region_outlives_predicate(
         &self,
         cause: &traits::ObligationCause<'tcx>,
         predicate: &ty::PolyRegionOutlivesPredicate<'tcx>,
-    ) {
-        let (ty::OutlivesPredicate(r_a, r_b), _) =
-            self.replace_bound_vars_with_placeholders(predicate);
-        let origin =
-            SubregionOrigin::from_obligation_cause(cause, || RelateRegionParamBound(cause.span));
-        self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
+    ) -> UnitResult<'tcx> {
+        self.commit_if_ok(|snapshot| {
+            let (ty::OutlivesPredicate(r_a, r_b), placeholder_map) =
+                self.replace_bound_vars_with_placeholders(predicate);
+            let origin = SubregionOrigin::from_obligation_cause(
+                cause,
+                || RelateRegionParamBound(cause.span),
+            );
+            self.sub_regions(origin, r_b, r_a); // `b : a` ==> `a <= b`
+            self.leak_check(false, &placeholder_map, snapshot)?;
+            Ok(())
+        })
     }
 
     pub fn next_ty_var_id(&self, diverging: bool, origin: TypeVariableOrigin) -> TyVid {
@@ -1018,6 +1027,18 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         self.tcx.mk_region(ty::ReVar(region_var))
     }
 
+    /// Return the universe that the region `r` was created in.  For
+    /// most regions (e.g., `'static`, named regions from the user,
+    /// etc) this is the root universe U0. For inference variables or
+    /// placeholders, however, it will return the universe which which
+    /// they are associated.
+    fn universe_of_region(
+        &self,
+        r: ty::Region<'tcx>,
+    ) -> ty::UniverseIndex {
+        self.borrow_region_constraints().universe(r)
+    }
+
     /// Number of region variables created so far.
     pub fn num_region_vars(&self) -> usize {
         self.borrow_region_constraints().num_region_vars()