very buggy

Author: Ariel Ben-Yehuda

src/librustc/middle/traits/select.rs

@@ -678,9 +678,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             candidates.retain(|c| self.winnow_candidate(stack, c).may_apply())
         }
 
-        // If there are STILL multiple candidate, we can further reduce
-        // the list by dropping duplicates.
-        if candidates.len() > 1 {
+        // If the predicate is fully known, there should be only one
+        // matching candidate by coherence. However, there may also be
+        // other candidates from impls (see the doc-comment for
+        // Candidate). Pick one candidate to use. This is somewhat
+        // wrong around regions.
+        if candidates.len() > 1 && !stack.obligation.predicate.has_ty_infer() {
             let mut i = 0;
             while i < candidates.len() {
                 let is_dup =
@@ -817,9 +820,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         match *candidate {
             Ok(Some(_)) | Err(_) => true,
-            Ok(None) => {
-                cache_fresh_trait_pred.0.input_types().iter().any(|&t| ty::type_has_ty_infer(t))
-            }
+            Ok(None) => cache_fresh_trait_pred.has_ty_infer()
         }
     }
 

src/librustc/middle/ty.rs

@@ -1986,6 +1986,10 @@ impl<'tcx> PolyTraitPredicate<'tcx> {
     pub fn def_id(&self) -> ast::DefId {
         self.0.def_id()
     }
+
+    pub fn has_ty_infer(&self) -> bool {
+        self.0.input_types().iter().any(|&t| ty::type_has_ty_infer(t))
+    }
 }
 
 #[derive(Clone, PartialEq, Eq, Hash, Debug)]