eagerly normalize when adding goals

Author: lcnr

compiler/rustc_middle/src/ty/predicate.rs

@@ -113,17 +113,14 @@ impl<'tcx> Predicate<'tcx> {
     #[inline]
     pub fn allow_normalization(self) -> bool {
         match self.kind().skip_binder() {
-            PredicateKind::Clause(ClauseKind::WellFormed(_)) => false,
-            // `NormalizesTo` is only used in the new solver, so this shouldn't
-            // matter. Normalizing `term` would be 'wrong' however, as it changes whether
-            // `normalizes-to(<T as Trait>::Assoc, <T as Trait>::Assoc)` holds.
-            PredicateKind::NormalizesTo(..) => false,
+            PredicateKind::Clause(ClauseKind::WellFormed(_))
+            | PredicateKind::AliasRelate(..)
+            | PredicateKind::NormalizesTo(..) => false,
             PredicateKind::Clause(ClauseKind::Trait(_))
             | PredicateKind::Clause(ClauseKind::RegionOutlives(_))
             | PredicateKind::Clause(ClauseKind::TypeOutlives(_))
             | PredicateKind::Clause(ClauseKind::Projection(_))
             | PredicateKind::Clause(ClauseKind::ConstArgHasType(..))
-            | PredicateKind::AliasRelate(..)
             | PredicateKind::ObjectSafe(_)
             | PredicateKind::Subtype(_)
             | PredicateKind::Coerce(_)

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -16,12 +16,15 @@ use rustc_middle::traits::solve::{
     inspect, CanonicalInput, CanonicalResponse, Certainty, PredefinedOpaquesData, QueryResult,
 };
 use rustc_middle::traits::specialization_graph;
+use rustc_middle::ty::AliasRelationDirection;
+use rustc_middle::ty::TypeFolder;
 use rustc_middle::ty::{
     self, InferCtxtLike, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
     TypeVisitable, TypeVisitableExt, TypeVisitor,
 };
 use rustc_session::config::DumpSolverProofTree;
 use rustc_span::DUMMY_SP;
+use rustc_type_ir::fold::TypeSuperFoldable;
 use rustc_type_ir::{self as ir, CanonicalVarValues, Interner};
 use rustc_type_ir_macros::{Lift_Generic, TypeFoldable_Generic, TypeVisitable_Generic};
 
@@ -468,13 +471,23 @@ impl<'a, 'tcx> EvalCtxt<'a, InferCtxt<'tcx>> {
     }
 
     #[instrument(level = "trace", skip(self))]
-    pub(super) fn add_normalizes_to_goal(&mut self, goal: Goal<'tcx, ty::NormalizesTo<'tcx>>) {
+    pub(super) fn add_normalizes_to_goal(&mut self, mut goal: Goal<'tcx, ty::NormalizesTo<'tcx>>) {
+        goal.predicate = goal
+            .predicate
+            .fold_with(&mut ReplaceAliasWithInfer { ecx: self, param_env: goal.param_env });
         self.inspect.add_normalizes_to_goal(self.infcx, self.max_input_universe, goal);
         self.nested_goals.normalizes_to_goals.push(goal);
     }
 
     #[instrument(level = "debug", skip(self))]
-    pub(super) fn add_goal(&mut self, source: GoalSource, goal: Goal<'tcx, ty::Predicate<'tcx>>) {
+    pub(super) fn add_goal(
+        &mut self,
+        source: GoalSource,
+        mut goal: Goal<'tcx, ty::Predicate<'tcx>>,
+    ) {
+        goal.predicate = goal
+            .predicate
+            .fold_with(&mut ReplaceAliasWithInfer { ecx: self, param_env: goal.param_env });
         self.inspect.add_goal(self.infcx, self.max_input_universe, source, goal);
         self.nested_goals.goals.push((source, goal));
     }
@@ -1101,3 +1114,63 @@ impl<'tcx> EvalCtxt<'_, InferCtxt<'tcx>> {
         });
     }
 }
+
+/// Eagerly replace aliases with inference variables, emitting `AliasRelate`
+/// goals, used when adding goals to the `EvalCtxt`. We compute the
+/// `AliasRelate` goals before evaluating the actual goal to get all the
+/// constraints we can.
+///
+/// This is a performance optimization to more eagerly detect cycles during trait
+/// solving. See tests/ui/traits/next-solver/cycles/cycle-modulo-ambig-aliases.rs.
+struct ReplaceAliasWithInfer<'me, 'a, 'tcx> {
+    ecx: &'me mut EvalCtxt<'a, InferCtxt<'tcx>>,
+    param_env: ty::ParamEnv<'tcx>,
+}
+
+impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ReplaceAliasWithInfer<'_, '_, 'tcx> {
+    fn interner(&self) -> TyCtxt<'tcx> {
+        self.ecx.tcx()
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        match *ty.kind() {
+            ty::Alias(..) if !ty.has_escaping_bound_vars() => {
+                let infer_ty = self.ecx.next_ty_infer();
+                let normalizes_to = ty::PredicateKind::AliasRelate(
+                    ty.into(),
+                    infer_ty.into(),
+                    AliasRelationDirection::Equate,
+                );
+                self.ecx.add_goal(
+                    GoalSource::Misc,
+                    Goal::new(self.interner(), self.param_env, normalizes_to),
+                );
+                infer_ty
+            }
+            _ => ty.super_fold_with(self),
+        }
+    }
+
+    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        match ct.kind() {
+            ty::ConstKind::Unevaluated(..) if !ct.has_escaping_bound_vars() => {
+                let infer_ct = self.ecx.next_const_infer(ct.ty());
+                let normalizes_to = ty::PredicateKind::AliasRelate(
+                    ct.into(),
+                    infer_ct.into(),
+                    AliasRelationDirection::Equate,
+                );
+                self.ecx.add_goal(
+                    GoalSource::Misc,
+                    Goal::new(self.interner(), self.param_env, normalizes_to),
+                );
+                infer_ct
+            }
+            _ => ct.super_fold_with(self),
+        }
+    }
+
+    fn fold_predicate(&mut self, predicate: ty::Predicate<'tcx>) -> ty::Predicate<'tcx> {
+        if predicate.allow_normalization() { predicate.super_fold_with(self) } else { predicate }
+    }
+}

tests/ui/coherence/coherence-overlap-unnormalizable-projection-1.next.stderr

@@ -12,7 +12,7 @@ LL |   impl<T> Trait for Box<T> {}
    |   ^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `Box<_>`
    |
    = note: downstream crates may implement trait `WithAssoc<'a>` for type `std::boxed::Box<_>`
-   = note: downstream crates may implement trait `WhereBound` for type `std::boxed::Box<<std::boxed::Box<_> as WithAssoc<'a>>::Assoc>`
+   = note: downstream crates may implement trait `WhereBound` for type `std::boxed::Box<_>`
 
 error: aborting due to 1 previous error
 

tests/ui/coherence/occurs-check/opaques.next.stderr

@@ -11,7 +11,7 @@ error[E0282]: type annotations needed
   --> $DIR/opaques.rs:13:20
    |
 LL |     pub fn cast<T>(x: Container<Alias<T>, T>) -> Container<T, T> {
-   |                    ^ cannot infer type for associated type `<T as Trait<T>>::Assoc`
+   |                    ^ cannot infer type
 
 error: aborting due to 2 previous errors
 

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.next.stderr

@@ -16,6 +16,22 @@ LL |     where
 LL |         T: AsExpression<Self::SqlType>,
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Foo::check`
 
-error: aborting due to 1 previous error
+error[E0277]: the trait bound `&str: AsExpression<Integer>` is not satisfied
+  --> $DIR/as_expression.rs:57:15
+   |
+LL |     SelectInt.check("bar");
+   |               ^^^^^ the trait `AsExpression<Integer>` is not implemented for `&str`
+   |
+   = help: the trait `AsExpression<Text>` is implemented for `&str`
+   = help: for that trait implementation, expected `Text`, found `Integer`
+
+error[E0271]: type mismatch resolving `<&str as AsExpression<<SelectInt as Expression>::SqlType>>::Expression == _`
+  --> $DIR/as_expression.rs:57:5
+   |
+LL |     SelectInt.check("bar");
+   |     ^^^^^^^^^^^^^^^^^^^^^^ types differ
+
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0271, E0277.
+For more information about an error, try `rustc --explain E0271`.

tests/ui/diagnostic_namespace/do_not_recommend/as_expression.rs

@@ -55,6 +55,7 @@ impl<T> Foo for T where T: Expression {}
 
 fn main() {
     SelectInt.check("bar");
-    //[next]~^ ERROR the trait bound `&str: AsExpression<<SelectInt as Expression>::SqlType>` is not satisfied
-    //[current]~^^ ERROR the trait bound `&str: AsExpression<Integer>` is not satisfied
+    //~^ ERROR the trait bound `&str: AsExpression<Integer>` is not satisfied
+    //[next]~| the trait bound `&str: AsExpression<<SelectInt as Expression>::SqlType>` is not satisfied
+    //[next]~| type mismatch
 }

tests/ui/traits/next-solver/canonical/const-region-infer-to-static-in-binder.stderr

@@ -1,8 +1,8 @@
-error[E0284]: type annotations needed: cannot satisfy `the constant `{ || {} }` can be evaluated`
+error[E0284]: type annotations needed: cannot satisfy `{ || {} } == _`
   --> $DIR/const-region-infer-to-static-in-binder.rs:4:10
    |
 LL | struct X<const FN: fn() = { || {} }>;
-   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `the constant `{ || {} }` can be evaluated`
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^ cannot satisfy `{ || {} } == _`
 
 error: using function pointers as const generic parameters is forbidden
   --> $DIR/const-region-infer-to-static-in-binder.rs:4:20

tests/ui/traits/next-solver/cycles/cycle-modulo-ambig-aliases.rs

@@ -0,0 +1,89 @@
+//@ compile-flags: -Znext-solver
+
+// A regression test for #125269. We previously ended up
+// recursively proving `&<_ as SpeciesPackedElem>::Assoc: Typed`
+// for all aliases which ended up causing exponential blowup.
+//
+// This has been fixed by eagerly normalizing the associated
+// type before computing the nested goals, resulting in an
+// immediate inductive cycle.
+
+pub trait Typed {}
+
+pub struct SpeciesCases<E>(E);
+
+pub trait SpeciesPackedElim {
+    type Ogre;
+    type Cyclops;
+    type Wendigo;
+    type Cavetroll;
+    type Mountaintroll;
+    type Swamptroll;
+    type Dullahan;
+    type Werewolf;
+    type Occultsaurok;
+    type Mightysaurok;
+    type Slysaurok;
+    type Mindflayer;
+    type Minotaur;
+    type Tidalwarrior;
+    type Yeti;
+    type Harvester;
+    type Blueoni;
+    type Redoni;
+    type Cultistwarlord;
+    type Cultistwarlock;
+    type Huskbrute;
+    type Tursus;
+    type Gigasfrost;
+    type AdletElder;
+    type SeaBishop;
+    type HaniwaGeneral;
+    type TerracottaBesieger;
+    type TerracottaDemolisher;
+    type TerracottaPunisher;
+    type TerracottaPursuer;
+    type Cursekeeper;
+}
+
+impl<'b, E: SpeciesPackedElim> Typed for &'b SpeciesCases<E>
+where
+    &'b E::Ogre: Typed,
+    &'b E::Cyclops: Typed,
+    &'b E::Wendigo: Typed,
+    &'b E::Cavetroll: Typed,
+    &'b E::Mountaintroll: Typed,
+    &'b E::Swamptroll: Typed,
+    &'b E::Dullahan: Typed,
+    &'b E::Werewolf: Typed,
+    &'b E::Occultsaurok: Typed,
+    &'b E::Mightysaurok: Typed,
+    &'b E::Slysaurok: Typed,
+    &'b E::Mindflayer: Typed,
+    &'b E::Minotaur: Typed,
+    &'b E::Tidalwarrior: Typed,
+    &'b E::Yeti: Typed,
+    &'b E::Harvester: Typed,
+    &'b E::Blueoni: Typed,
+    &'b E::Redoni: Typed,
+    &'b E::Cultistwarlord: Typed,
+    &'b E::Cultistwarlock: Typed,
+    &'b E::Huskbrute: Typed,
+    &'b E::Tursus: Typed,
+    &'b E::Gigasfrost: Typed,
+    &'b E::AdletElder: Typed,
+    &'b E::SeaBishop: Typed,
+    &'b E::HaniwaGeneral: Typed,
+    &'b E::TerracottaBesieger: Typed,
+    &'b E::TerracottaDemolisher: Typed,
+    &'b E::TerracottaPunisher: Typed,
+    &'b E::TerracottaPursuer: Typed,
+    &'b E::Cursekeeper: Typed,
+{}
+
+fn foo<T: Typed>() {}
+
+fn main() {
+    foo::<&_>();
+    //~^ ERROR overflow evaluating the requirement `&_: Typed`
+}

tests/ui/traits/next-solver/cycles/cycle-modulo-ambig-aliases.stderr

@@ -0,0 +1,15 @@
+error[E0275]: overflow evaluating the requirement `&_: Typed`
+  --> $DIR/cycle-modulo-ambig-aliases.rs:87:11
+   |
+LL |     foo::<&_>();
+   |           ^^
+   |
+note: required by a bound in `foo`
+  --> $DIR/cycle-modulo-ambig-aliases.rs:84:11
+   |
+LL | fn foo<T: Typed>() {}
+   |           ^^^^^ required by this bound in `foo`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0275`.