instantiate higher ranked goals in candidate selection

reverts #119820

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -64,20 +64,6 @@ mod _match;
 mod candidate_assembly;
 mod confirmation;
 
-/// Whether to consider the binder of higher ranked goals for the `leak_check` when
-/// evaluating higher-ranked goals. See #119820 for more info.
-///
-/// While this is a bit hacky, it is necessary to match the behavior of the new solver:
-/// We eagerly instantiate binders in the new solver, outside of candidate selection, so
-/// the leak check inside of candidates does not consider any bound vars from the higher
-/// ranked goal. However, we do exit the binder once we're completely finished with a goal,
-/// so the leak-check can be used in evaluate by causing nested higher-ranked goals to fail.
-#[derive(Debug, Copy, Clone)]
-enum LeakCheckHigherRankedGoal {
-    No,
-    Yes,
-}
-
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
 pub enum IntercrateAmbiguityCause<'tcx> {
     DownstreamCrate { trait_ref: ty::TraitRef<'tcx>, self_ty: Option<Ty<'tcx>> },
@@ -402,10 +388,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     let mut no_candidates_apply = true;
 
                     for c in candidate_set.vec.iter() {
-                        if self
-                            .evaluate_candidate(stack, c, LeakCheckHigherRankedGoal::No)?
-                            .may_apply()
-                        {
+                        if self.evaluate_candidate(stack, c)?.may_apply() {
                             no_candidates_apply = false;
                             break;
                         }
@@ -476,7 +459,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // is needed for specialization. Propagate overflow if it occurs.
         let mut candidates = candidates
             .into_iter()
-            .map(|c| match self.evaluate_candidate(stack, &c, LeakCheckHigherRankedGoal::No) {
+            .map(|c| match self.evaluate_candidate(stack, &c) {
                 Ok(eval) if eval.may_apply() => {
                     Ok(Some(EvaluatedCandidate { candidate: c, evaluation: eval }))
                 }
@@ -566,7 +549,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         obligation: &PredicateObligation<'tcx>,
     ) -> Result<EvaluationResult, OverflowError> {
         debug_assert!(!self.infcx.next_trait_solver());
-        self.evaluation_probe(|this, _outer_universe| {
+        self.evaluation_probe(|this| {
             let goal =
                 this.infcx.resolve_vars_if_possible((obligation.predicate, obligation.param_env));
             let mut result = this.evaluate_predicate_recursively(
@@ -589,11 +572,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// `op`, but this can be overwritten if necessary.
     fn evaluation_probe(
         &mut self,
-        op: impl FnOnce(&mut Self, &mut ty::UniverseIndex) -> Result<EvaluationResult, OverflowError>,
+        op: impl FnOnce(&mut Self) -> Result<EvaluationResult, OverflowError>,
     ) -> Result<EvaluationResult, OverflowError> {
         self.infcx.probe(|snapshot| -> Result<EvaluationResult, OverflowError> {
-            let mut outer_universe = self.infcx.universe();
-            let result = op(self, &mut outer_universe)?;
+            let outer_universe = self.infcx.universe();
+            let result = op(self)?;
 
             match self.infcx.leak_check(outer_universe, Some(snapshot)) {
                 Ok(()) => {}
@@ -1254,7 +1237,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         match self.candidate_from_obligation(stack) {
-            Ok(Some(c)) => self.evaluate_candidate(stack, &c, LeakCheckHigherRankedGoal::Yes),
+            Ok(Some(c)) => self.evaluate_candidate(stack, &c),
             Ok(None) => Ok(EvaluatedToAmbig),
             Err(Overflow(OverflowError::Canonical)) => Err(OverflowError::Canonical),
             Err(..) => Ok(EvaluatedToErr),
@@ -1279,10 +1262,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
     /// Further evaluates `candidate` to decide whether all type parameters match and whether nested
     /// obligations are met. Returns whether `candidate` remains viable after this further
     /// scrutiny.
-    ///
-    /// Depending on the value of [LeakCheckHigherRankedGoal], we may ignore the binder of the goal
-    /// when eagerly detecting higher ranked region errors via the `leak_check`. See that enum for
-    /// more info.
     #[instrument(
         level = "debug",
         skip(self, stack),
@@ -1293,25 +1272,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
         candidate: &SelectionCandidate<'tcx>,
-        leak_check_higher_ranked_goal: LeakCheckHigherRankedGoal,
     ) -> Result<EvaluationResult, OverflowError> {
-        let mut result = self.evaluation_probe(|this, outer_universe| {
-            // We eagerly instantiate higher ranked goals to prevent universe errors
-            // from impacting candidate selection. This matches the behavior of the new
-            // solver. This slightly weakens type inference.
-            //
-            // In case there are no unresolved type or const variables this
-            // should still not be necessary to select a unique impl as any overlap
-            // relying on a universe error from higher ranked goals should have resulted
-            // in an overlap error in coherence.
-            let p = self.infcx.enter_forall_and_leak_universe(stack.obligation.predicate);
-            let obligation = stack.obligation.with(this.tcx(), ty::Binder::dummy(p));
-            match leak_check_higher_ranked_goal {
-                LeakCheckHigherRankedGoal::No => *outer_universe = self.infcx.universe(),
-                LeakCheckHigherRankedGoal::Yes => {}
-            }
-
-            match this.confirm_candidate(&obligation, candidate.clone()) {
+        let mut result = self.evaluation_probe(|this| {
+            match this.confirm_candidate(stack.obligation, candidate.clone()) {
                 Ok(selection) => {
                     debug!(?selection);
                     this.evaluate_predicates_recursively(
@@ -1731,19 +1694,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             })
             .map_err(|_| ())
     }
+
     fn where_clause_may_apply<'o>(
         &mut self,
         stack: &TraitObligationStack<'o, 'tcx>,
         where_clause_trait_ref: ty::PolyTraitRef<'tcx>,
     ) -> Result<EvaluationResult, OverflowError> {
-        self.evaluation_probe(|this, outer_universe| {
-            // Eagerly instantiate higher ranked goals.
-            //
-            // See the comment in `evaluate_candidate` to see why.
-            let p = self.infcx.enter_forall_and_leak_universe(stack.obligation.predicate);
-            let obligation = stack.obligation.with(this.tcx(), ty::Binder::dummy(p));
-            *outer_universe = self.infcx.universe();
-            match this.match_where_clause_trait_ref(&obligation, where_clause_trait_ref) {
+        self.evaluation_probe(|this| {
+            match this.match_where_clause_trait_ref(stack.obligation, where_clause_trait_ref) {
                 Ok(obligations) => this.evaluate_predicates_recursively(stack.list(), obligations),
                 Err(()) => Ok(EvaluatedToErr),
             }

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-1.next.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the trait bound `for<'a> &'a &T: Trait` is not satisfied
+  --> $DIR/candidate-from-env-universe-err-1.rs:27:16
+   |
+LL |     hr_bound::<&T>();
+   |                ^^ the trait `for<'a> Trait` is not implemented for `&'a &T`
+   |
+note: required by a bound in `hr_bound`
+  --> $DIR/candidate-from-env-universe-err-1.rs:14:20
+   |
+LL | fn hr_bound<T>()
+   |    -------- required by a bound in this function
+LL | where
+LL |     for<'a> &'a T: Trait,
+   |                    ^^^^^ required by this bound in `hr_bound`
+help: consider removing the leading `&`-reference
+   |
+LL -     hr_bound::<&T>();
+LL +     hr_bound::<T>();
+   |
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-1.rs

@@ -1,3 +1,7 @@
+//@ revisions: old next
+//@[next] compile-flags: -Znext-solver
+//@[old] check-pass
+
 // cc #119820
 
 trait Trait {}
@@ -21,8 +25,7 @@ where
     // the leak check both candidates may apply and we prefer the
     // `param_env` candidate in winnowing.
     hr_bound::<&T>();
-    //~^ ERROR the parameter type `T` may not live long enough
-    //~| ERROR implementation of `Trait` is not general enough
+    //[next]~^ ERROR the trait bound `for<'a> &'a &T: Trait` is not satisfied
 }
 
 fn main() {}

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-2.next.stderr

@@ -1,11 +1,11 @@
 error[E0277]: the trait bound `for<'a> T: Trait<'a, '_>` is not satisfied
-  --> $DIR/candidate-from-env-universe-err-2.rs:14:5
+  --> $DIR/candidate-from-env-universe-err-2.rs:15:5
    |
 LL |     impl_hr::<T>();
    |     ^^^^^^^^^^^^^^ the trait `for<'a> Trait<'a, '_>` is not implemented for `T`
    |
 note: required by a bound in `impl_hr`
-  --> $DIR/candidate-from-env-universe-err-2.rs:11:19
+  --> $DIR/candidate-from-env-universe-err-2.rs:12:19
    |
 LL | fn impl_hr<'b, T: for<'a> Trait<'a, 'b>>() {}
    |                   ^^^^^^^^^^^^^^^^^^^^^ required by this bound in `impl_hr`

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-2.rs

@@ -1,5 +1,6 @@
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
+//@[current] check-pass
 
 // cc #119820
 
@@ -13,8 +14,6 @@ fn impl_hr<'b, T: for<'a> Trait<'a, 'b>>() {}
 fn not_hr<'a, T: for<'b> Trait<'a, 'b> + OtherTrait<'static>>() {
     impl_hr::<T>();
     //[next]~^ ERROR the trait bound `for<'a> T: Trait<'a, '_>` is not satisfied
-    //[current]~^^ERROR lifetime may not live long enough
-    //[current]~| ERROR implementation of `Trait` is not general enough
 }
 
 fn main() {}

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-project.current.stderr

@@ -1,23 +1,5 @@
-error: implementation of `Trait` is not general enough
-  --> $DIR/candidate-from-env-universe-err-project.rs:28:5
-   |
-LL |     trait_bound::<T>();
-   |     ^^^^^^^^^^^^^^^^^^ implementation of `Trait` is not general enough
-   |
-   = note: `T` must implement `Trait<'0>`, for any lifetime `'0`...
-   = note: ...but it actually implements `Trait<'static>`
-
-error: implementation of `Trait` is not general enough
-  --> $DIR/candidate-from-env-universe-err-project.rs:39:5
-   |
-LL |     projection_bound::<T>();
-   |     ^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Trait` is not general enough
-   |
-   = note: `T` must implement `Trait<'0>`, for any lifetime `'0`...
-   = note: ...but it actually implements `Trait<'static>`
-
 error[E0308]: mismatched types
-  --> $DIR/candidate-from-env-universe-err-project.rs:39:5
+  --> $DIR/candidate-from-env-universe-err-project.rs:38:5
    |
 LL |     projection_bound::<T>();
    |     ^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
@@ -31,7 +13,7 @@ LL | fn projection_bound<T: for<'a> Trait<'a, Assoc = usize>>() {}
    |                                          ^^^^^^^^^^^^^
 
 error[E0308]: mismatched types
-  --> $DIR/candidate-from-env-universe-err-project.rs:55:30
+  --> $DIR/candidate-from-env-universe-err-project.rs:53:30
    |
 LL |     let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
@@ -40,7 +22,7 @@ LL |     let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
               found associated type `<T as Trait<'a>>::Assoc`
 
 error[E0308]: mismatched types
-  --> $DIR/candidate-from-env-universe-err-project.rs:55:30
+  --> $DIR/candidate-from-env-universe-err-project.rs:53:30
    |
 LL |     let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ one type is more general than the other
@@ -49,6 +31,6 @@ LL |     let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
               found associated type `<T as Trait<'a>>::Assoc`
    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0308`.

tests/ui/higher-ranked/leak-check/candidate-from-env-universe-err-project.next.stderr

@@ -15,7 +15,7 @@ LL | fn function1<T: Trait<'static> + for<'a> Trait<'a>>() {
    |                                +++++++++++++++++++
 
 error[E0277]: the trait bound `for<'a> T: Trait<'a>` is not satisfied
-  --> $DIR/candidate-from-env-universe-err-project.rs:39:24
+  --> $DIR/candidate-from-env-universe-err-project.rs:38:24
    |
 LL |     projection_bound::<T>();
    |                        ^ the trait `for<'a> Trait<'a>` is not implemented for `T`
@@ -31,7 +31,7 @@ LL | fn function2<T: Trait<'static, Assoc = usize> + for<'a> Trait<'a>>() {
    |                                               +++++++++++++++++++
 
 error[E0271]: type mismatch resolving `<T as Trait<'a>>::Assoc == usize`
-  --> $DIR/candidate-from-env-universe-err-project.rs:39:24
+  --> $DIR/candidate-from-env-universe-err-project.rs:38:24
    |
 LL |     projection_bound::<T>();
    |                        ^ type mismatch resolving `<T as Trait<'a>>::Assoc == usize`
@@ -48,13 +48,13 @@ LL | fn projection_bound<T: for<'a> Trait<'a, Assoc = usize>>() {}
    |                                          ^^^^^^^^^^^^^ required by this bound in `projection_bound`
 
 error: higher-ranked subtype error
-  --> $DIR/candidate-from-env-universe-err-project.rs:55:30
+  --> $DIR/candidate-from-env-universe-err-project.rs:53:30
    |
 LL |     let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: higher-ranked subtype error
-  --> $DIR/candidate-from-env-universe-err-project.rs:55:30
+  --> $DIR/candidate-from-env-universe-err-project.rs:53:30
    |
 LL |     let _higher_ranked_norm: for<'a> fn(<T as Trait<'a>>::Assoc) = |_| ();
    |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^