dont discard overflow from normalizes-to goals

compiler/rustc_middle/src/traits/solve.rs

@@ -248,6 +248,9 @@ pub enum GoalSource {
     /// This also impacts whether we erase constraints on overflow.
     /// Erasing constraints is generally very useful for perf and also
     /// results in better error messages by avoiding spurious errors.
+    /// We do not erase overflow constraints in `normalizes-to` goals unless
+    /// they are from an impl where-clause. This is necessary due to
+    /// backwards compatability, cc trait-system-refactor-initiatitive#70.
     ImplWhereBound,
 }
 

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

@@ -94,20 +94,6 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
         );
 
         let certainty = certainty.unify_with(goals_certainty);
-        if let Certainty::OVERFLOW = certainty {
-            // If we have overflow, it's probable that we're substituting a type
-            // into itself infinitely and any partial substitutions in the query
-            // response are probably not useful anyways, so just return an empty
-            // query response.
-            //
-            // This may prevent us from potentially useful inference, e.g.
-            // 2 candidates, one ambiguous and one overflow, which both
-            // have the same inference constraints.
-            //
-            // Changing this to retain some constraints in the future
-            // won't be a breaking change, so this is good enough for now.
-            return Ok(self.make_ambiguous_response_no_constraints(MaybeCause::Overflow));
-        }
 
         let var_values = self.var_values;
         let external_constraints = self.compute_external_query_constraints()?;

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

@@ -157,7 +157,7 @@ impl<'tcx> InferCtxtEvalExt<'tcx> for InferCtxt<'tcx> {
         Option<inspect::GoalEvaluation<'tcx>>,
     ) {
         EvalCtxt::enter_root(self, generate_proof_tree, |ecx| {
-            ecx.evaluate_goal(GoalEvaluationKind::Root, goal)
+            ecx.evaluate_goal(GoalEvaluationKind::Root, GoalSource::Misc, goal)
         })
     }
 }
@@ -335,6 +335,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
     fn evaluate_goal(
         &mut self,
         goal_evaluation_kind: GoalEvaluationKind,
+        source: GoalSource,
         goal: Goal<'tcx, ty::Predicate<'tcx>>,
     ) -> Result<(bool, Certainty, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
         let (orig_values, canonical_goal) = self.canonicalize_goal(goal);
@@ -354,10 +355,10 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
             Ok(response) => response,
         };
 
-        let has_changed = !canonical_response.value.var_values.is_identity_modulo_regions()
-            || !canonical_response.value.external_constraints.opaque_types.is_empty();
-        let (certainty, nested_goals) = match self.instantiate_and_apply_query_response(
+        let (certainty, has_changed, nested_goals) = match self
+            .instantiate_response_discarding_overflow(
                 goal.param_env,
+                source,
                 orig_values,
                 canonical_response,
             ) {
@@ -387,6 +388,30 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         Ok((has_changed, certainty, nested_goals))
     }
 
+    fn instantiate_response_discarding_overflow(
+        &mut self,
+        param_env: ty::ParamEnv<'tcx>,
+        source: GoalSource,
+        original_values: Vec<ty::GenericArg<'tcx>>,
+        response: CanonicalResponse<'tcx>,
+    ) -> Result<(Certainty, bool, Vec<Goal<'tcx, ty::Predicate<'tcx>>>), NoSolution> {
+        let keep_overflow_constraints = || {
+            self.search_graph.current_goal_is_normalizes_to()
+                && source != GoalSource::ImplWhereBound
+        };
+
+        if response.value.certainty == Certainty::OVERFLOW && !keep_overflow_constraints() {
+            Ok((Certainty::OVERFLOW, false, Vec::new()))
+        } else {
+            let has_changed = !response.value.var_values.is_identity_modulo_regions()
+                || !response.value.external_constraints.opaque_types.is_empty();
+
+            let (certainty, nested_goals) =
+                self.instantiate_and_apply_query_response(param_env, original_values, response)?;
+            Ok((certainty, has_changed, nested_goals))
+        }
+    }
+
     fn compute_goal(&mut self, goal: Goal<'tcx, ty::Predicate<'tcx>>) -> QueryResult<'tcx> {
         let Goal { param_env, predicate } = goal;
         let kind = predicate.kind();
@@ -509,6 +534,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
 
             let (_, certainty, instantiate_goals) = self.evaluate_goal(
                 GoalEvaluationKind::Nested { is_normalizes_to_hack: IsNormalizesToHack::Yes },
+                GoalSource::Misc,
                 unconstrained_goal,
             )?;
             self.nested_goals.goals.extend(with_misc_source(instantiate_goals));
@@ -544,6 +570,7 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
         for (source, goal) in goals.goals.drain(..) {
             let (has_changed, certainty, instantiate_goals) = self.evaluate_goal(
                 GoalEvaluationKind::Nested { is_normalizes_to_hack: IsNormalizesToHack::No },
+                source,
                 goal,
             )?;
             self.nested_goals.goals.extend(with_misc_source(instantiate_goals));

compiler/rustc_trait_selection/src/solve/search_graph.rs

@@ -8,6 +8,7 @@ use rustc_index::IndexVec;
 use rustc_middle::dep_graph::dep_kinds;
 use rustc_middle::traits::solve::CacheData;
 use rustc_middle::traits::solve::{CanonicalInput, Certainty, EvaluationCache, QueryResult};
+use rustc_middle::ty;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::Limit;
 use std::collections::hash_map::Entry;
@@ -111,6 +112,15 @@ impl<'tcx> SearchGraph<'tcx> {
         self.stack.is_empty()
     }
 
+    pub(super) fn current_goal_is_normalizes_to(&self) -> bool {
+        self.stack.raw.last().map_or(false, |e| {
+            matches!(
+                e.input.value.goal.predicate.kind().skip_binder(),
+                ty::PredicateKind::NormalizesTo(..)
+            )
+        })
+    }
+
     /// Returns the remaining depth allowed for nested goals.
     ///
     /// This is generally simply one less than the current depth.

tests/ui/traits/issue-90662-projection-caching.rs

@@ -1,7 +1,15 @@
+// revisions: old next
+//[next] compile-flags: -Znext-solver=coherence
 // check-pass
 
 // Regression test for issue #90662
-// Tests that projection caching does not cause a spurious error
+// Tests that projection caching does not cause a spurious error.
+// Coherence relies on the following overflowing goal to still constrain
+// `?0` to `dyn Service`.
+//
+//     Projection(<ServiceImpl as Provider<TestModule>>::Interface. ?0)
+//
+// cc https://github.com/rust-lang/trait-system-refactor-initiative/issues/70.
 
 trait HasProvider<T: ?Sized> {}
 trait Provider<M> {

tests/ui/traits/next-solver/overflow/recursion-limit-normalizes-to-constraints.rs

@@ -0,0 +1,25 @@
+// compile-flags: -Znext-solver=coherence
+// check-pass
+
+// A regression test for trait-system-refactor-initiative#70.
+
+trait Trait {
+    type Assoc;
+}
+
+struct W<T: ?Sized>(*mut T);
+impl<T: ?Sized> Trait for W<W<T>>
+where
+    W<T>: Trait,
+{
+    type Assoc = ();
+}
+
+trait NoOverlap {}
+impl<T: Trait<Assoc = u32>> NoOverlap for T {}
+// `Projection(<W<_> as Trait>::Assoc, u32)` should result in error even
+// though applying the impl results in overflow. This is necessary to match
+// the behavior of the old solver.
+impl<T: ?Sized> NoOverlap for W<T> {}
+
+fn main() {}