cool beans

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

@@ -461,7 +461,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .flat_map(Result::transpose)
             .collect::<Result<Vec<_>, _>>()?;
 
-        debug!(?stack, ?candidates, "winnowed to {} candidates", candidates.len());
+        debug!(?stack, ?candidates, "{} potentially applicable candidates", candidates.len());
         // If there are *NO* candidates, then there are no impls --
         // that we know of, anyway. Note that in the case where there
         // are unbound type variables within the obligation, it might
@@ -1896,7 +1896,33 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             }
         }
         if let Some((def_id, _evaluation)) = impl_candidate {
-            return Some(ImplCandidate(def_id));
+            // Don't use impl candidates which overlap with other candidates.
+            // This should pretty much only ever happen with malformed impls.
+            if candidates.iter().all(|c| match c.candidate {
+                BuiltinCandidate { has_nested: _ }
+                | TransmutabilityCandidate
+                | AutoImplCandidate
+                | ClosureCandidate { .. }
+                | AsyncClosureCandidate
+                | AsyncFnKindHelperCandidate
+                | CoroutineCandidate
+                | FutureCandidate
+                | IteratorCandidate
+                | AsyncIteratorCandidate
+                | FnPointerCandidate
+                | TraitAliasCandidate
+                | TraitUpcastingUnsizeCandidate(_)
+                | BuiltinObjectCandidate
+                | BuiltinUnsizeCandidate => false,
+                // Non-global param candidates have already been handled, global
+                // where-bounds get ignored.
+                ParamCandidate(_) | ImplCandidate(_) => true,
+                ProjectionCandidate(_) | ObjectCandidate(_) => unreachable!(),
+            }) {
+                return Some(ImplCandidate(def_id));
+            } else {
+                return None;
+            }
         }
 
         // Also try ignoring all global where-bounds and check whether we end