Filter candidates when goal and impl polarity doesn't match

Author: spastorino

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

@@ -134,6 +134,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // candidate which assumes $0 == int, one that assumes `$0 ==
         // usize`, etc. This spells an ambiguity.
 
+        self.filter_impls(&mut candidates, stack);
+
         // If there is more than one candidate, first winnow them down
         // by considering extra conditions (nested obligations and so
         // forth). We don't winnow if there is exactly one
@@ -149,7 +151,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         // Instead, we select the right impl now but report "`Bar` does
         // not implement `Clone`".
         if candidates.len() == 1 {
-            return self.filter_impls(candidates.pop().unwrap(), stack.obligation);
+            return self.filter_reservation_impls(candidates.pop().unwrap(), stack.obligation);
         }
 
         // Winnow, but record the exact outcome of evaluation, which
@@ -223,7 +225,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         }
 
         // Just one candidate left.
-        self.filter_impls(candidates.pop().unwrap().candidate, stack.obligation)
+        self.filter_reservation_impls(candidates.pop().unwrap().candidate, stack.obligation)
     }
 
     #[instrument(skip(self, stack), level = "debug")]

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

@@ -1117,8 +1117,30 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         (result, dep_node)
     }
 
+    /// filter_impls filters candidates that have a positive impl for a negative goal and a
+    /// negative impl for a positive goal
     #[instrument(level = "debug", skip(self))]
     fn filter_impls(
+        &mut self,
+        candidates: &mut Vec<SelectionCandidate<'tcx>>,
+        stack: &TraitObligationStack<'o, 'tcx>,
+    ) {
+        let tcx = self.tcx();
+        candidates.retain(|candidate| {
+            if let ImplCandidate(def_id) = candidate {
+                ty::ImplPolarity::Reservation == tcx.impl_polarity(*def_id)
+                    || !self.allow_negative_impls
+                        && stack.obligation.predicate.skip_binder().polarity
+                            == tcx.impl_polarity(*def_id)
+            } else {
+                true
+            }
+        });
+    }
+
+    /// filter_reservation_impls filter reservation impl for any goal as ambiguous
+    #[instrument(level = "debug", skip(self))]
+    fn filter_reservation_impls(
         &mut self,
         candidate: SelectionCandidate<'tcx>,
         obligation: &TraitObligation<'tcx>,
@@ -1148,7 +1170,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
             }
         }
-        // Treat negative impls as unimplemented, and reservation impls as ambiguity.
+        // Treat reservation impls as ambiguity.
         if let ImplCandidate(def_id) = candidate {
             if let ty::ImplPolarity::Reservation = tcx.impl_polarity(def_id) {
                 if let Some(intercrate_ambiguity_clauses) = &mut self.intercrate_ambiguity_causes {
@@ -1170,12 +1192,6 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 }
                 return Ok(None);
             }
-
-            if !self.allow_negative_impls {
-                if obligation.predicate.skip_binder().polarity != tcx.impl_polarity(def_id) {
-                    return Err(Unimplemented);
-                }
-            }
         }
         Ok(Some(candidate))
     }