dont assemble shadowed impl candidates

compiler/rustc_next_trait_solver/src/solve/assembly/mod.rs

@@ -21,7 +21,7 @@ use crate::delegate::SolverDelegate;
 use crate::solve::inspect::ProbeKind;
 use crate::solve::{
     BuiltinImplSource, CandidateSource, CanonicalResponse, Certainty, EvalCtxt, Goal, GoalSource,
-    MaybeCause, NoSolution, ParamEnvSource, QueryResult,
+    MaybeCause, NoSolution, ParamEnvSource, QueryResult, has_no_inference_or_external_constraints,
 };
 
 enum AliasBoundKind {
@@ -395,9 +395,30 @@ where
 
         match assemble_from {
             AssembleCandidatesFrom::All => {
-                self.assemble_impl_candidates(goal, &mut candidates);
                 self.assemble_builtin_impl_candidates(goal, &mut candidates);
-                self.assemble_object_bound_candidates(goal, &mut candidates);
+                // For performance we only assemble impls if there are no candidates
+                // which would shadow them. This is necessary to avoid hangs in rayon,
+                // see trait-system-refactor-initiative#109 for more details.
+                //
+                // We always assemble builtin impls as trivial builtin impls have a higher
+                // priority than where-clauses.
+                //
+                // We only do this if any such candidate applies without any constraints
+                // as we may want to weaken inference guidance in the future and don't want
+                // to worry about causing major performance regressions when doing so.
+                // See trait-system-refactor-initiative#226 for some ideas here.
+                if TypingMode::Coherence == self.typing_mode()
+                    || !candidates.iter().any(|c| {
+                        matches!(
+                            c.source,
+                            CandidateSource::ParamEnv(ParamEnvSource::NonGlobal)
+                                | CandidateSource::AliasBound
+                        ) && has_no_inference_or_external_constraints(c.result)
+                    })
+                {
+                    self.assemble_impl_candidates(goal, &mut candidates);
+                    self.assemble_object_bound_candidates(goal, &mut candidates);
+                }
             }
             AssembleCandidatesFrom::EnvAndBounds => {}
         }

tests/ui/traits/next-solver/cycles/rayon-hang-1.rs

@@ -0,0 +1,32 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// A regression test for trait-system-refactor-initiative#109.
+
+trait ParallelIterator: Sized {
+    type Item;
+}
+trait IntoParallelIterator {
+    type Iter: ParallelIterator<Item = Self::Item>;
+    type Item;
+}
+impl<T: ParallelIterator> IntoParallelIterator for T {
+    type Iter = T;
+    type Item = T::Item;
+}
+
+macro_rules! multizip_impls {
+    ($($T:ident),+) => {
+       fn foo<$( $T, )+>() where
+        $(
+            $T: IntoParallelIterator,
+            $T::Iter: ParallelIterator,
+        )+
+            ($( $T, )+): IntoParallelIterator<Item = ($( $T::Item, )+)>,
+        {}
+    }
+}
+
+multizip_impls! { A, B, C, D, E, F, G, H, I, J, K, L }
+
+fn main() {}

tests/ui/traits/next-solver/cycles/rayon-hang-2.rs

@@ -0,0 +1,49 @@
+//@ compile-flags: -Znext-solver
+//@ check-pass
+
+// A regression test for trait-system-refactor-initiative#109.
+// Unlike `rayon-hang-1.rs` the cycles in this test are not
+// unproductive, which causes the `AliasRelate` goal when trying
+// to apply where-clauses to only error in the second iteration.
+//
+// This makes the exponential blowup to be significantly harder
+// to avoid.
+
+trait ParallelIterator: Sized {
+    type Item;
+}
+
+trait IntoParallelIteratorIndir {
+    type Iter: ParallelIterator<Item = Self::Item>;
+    type Item;
+}
+impl<I> IntoParallelIteratorIndir for I
+where
+    Box<I>: IntoParallelIterator,
+{
+    type Iter = <Box<I> as IntoParallelIterator>::Iter;
+    type Item = <Box<I> as IntoParallelIterator>::Item;
+}
+trait IntoParallelIterator {
+    type Iter: ParallelIterator<Item = Self::Item>;
+    type Item;
+}
+impl<T: ParallelIterator> IntoParallelIterator for T {
+    type Iter = T;
+    type Item = T::Item;
+}
+
+macro_rules! multizip_impls {
+    ($($T:ident),+) => {
+       fn foo<'a, $( $T, )+>() where
+        $(
+            $T: IntoParallelIteratorIndir,
+            $T::Iter: ParallelIterator,
+        )+
+        {}
+    }
+}
+
+multizip_impls! { A, B, C, D, E, F, G, H, I, J, K, L }
+
+fn main() {}