improve comments

src/librustc/traits/select.rs

@@ -1326,7 +1326,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         (result, dep_node)
     }
 
-    // Treat negative impls as unimplemented, and reservation impls as Ok(None)
+    // Treat negative impls as unimplemented, and reservation impls as ambiguity.
     fn filter_negative_and_reservation_impls(
         &self,
         candidate: SelectionCandidate<'tcx>,

src/librustc/ty/mod.rs

@@ -2931,7 +2931,7 @@ impl<'tcx> TyCtxt<'tcx> {
             }
             (ImplPolarity::Positive, ImplPolarity::Negative) |
             (ImplPolarity::Negative, ImplPolarity::Positive) => {
-                // FIXME: when can this happen?
+                // `impl AutoTrait for Type` + `impl !AutoTrait for Type`
                 debug!("impls_are_allowed_to_overlap({:?}, {:?}) - None (differing polarities)",
                        def_id1, def_id2);
                 return None;