Prefer param-env candidates that do no inference

Author: compiler-errors

compiler/rustc_trait_selection/src/solve/assembly.rs

@@ -565,6 +565,8 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
     ) -> bool {
         // FIXME: implement this
         match (candidate.source, other.source) {
+            (CandidateSource::ParamEnv(_), CandidateSource::ParamEnv(_)) => false,
+            (_, CandidateSource::ParamEnv(_)) if other.result.has_only_region_constraints() => true,
             (CandidateSource::Impl(_), _)
             | (CandidateSource::ParamEnv(_), _)
             | (CandidateSource::AliasBound, _)

compiler/rustc_trait_selection/src/solve/mod.rs

@@ -52,6 +52,8 @@ enum SolverMode {
 
 trait CanonicalResponseExt {
     fn has_no_inference_or_external_constraints(&self) -> bool;
+
+    fn has_only_region_constraints(&self) -> bool;
 }
 
 impl<'tcx> CanonicalResponseExt for Canonical<'tcx, Response<'tcx>> {
@@ -60,6 +62,11 @@ impl<'tcx> CanonicalResponseExt for Canonical<'tcx, Response<'tcx>> {
             && self.value.var_values.is_identity()
             && self.value.external_constraints.opaque_types.is_empty()
     }
+
+    fn has_only_region_constraints(&self) -> bool {
+        self.value.var_values.is_identity()
+            && self.value.external_constraints.opaque_types.is_empty()
+    }
 }
 
 impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {

tests/ui/traits/new-solver/prefer-param-env-on-ambiguity.rs

@@ -0,0 +1,10 @@
+// compile-flags: -Ztrait-solver=next
+// check-pass
+
+trait Foo<'a> {}
+trait Bar<'a> {}
+
+impl<'a, T: Bar<'a>> Foo<'a> for T {}
+impl<T> Bar<'static> for T {}
+
+fn main() {}