WIP patch new user of evaluate_obligation

Author: nikomatsakis

src/librustc/traits/error_reporting.rs

@@ -871,7 +871,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                                          obligation.param_env,
                                                          new_trait_ref.to_predicate());
 
-                    if selcx.evaluate_obligation(&new_obligation) {
+                    if selcx.evaluate_obligation(&new_obligation).must_apply() {
                         let sp = self.tcx.sess.codemap()
                             .span_take_while(span, |c| c.is_whitespace() || *c == '&');
 

src/librustc/traits/select.rs

@@ -394,6 +394,17 @@ impl EvaluationResult {
         }
     }
 
+    pub fn must_apply(self) -> bool {
+        match self {
+            EvaluatedToOk => true,
+
+            EvaluatedToAmbig |
+            EvaluatedToUnknown |
+            EvaluatedToErr |
+            EvaluatedToRecur => false
+        }
+    }
+
     fn is_stack_dependent(self) -> bool {
         match self {
             EvaluatedToUnknown |