introduce evaluate_obligation_conservatively and use it

src/librustc/middle/traits/mod.rs

@@ -340,6 +340,21 @@ pub fn type_known_to_meet_builtin_bound<'a,'tcx>(infcx: &InferCtxt<'a,'tcx>,
            ty,
            bound);
 
+    if !ty.has_infer_types() && !ty.has_closure_types() {
+        let cause = ObligationCause::misc(span, ast::DUMMY_NODE_ID);
+        let obligation =
+            util::predicate_for_builtin_bound(infcx.tcx, cause, bound, 0, ty);
+        let obligation = match obligation {
+            Ok(o) => o,
+            Err(..) => return false
+        };
+        let result = SelectionContext::new(infcx)
+            .evaluate_obligation_conservatively(&obligation);
+        debug!("type_known_to_meet_builtin_bound: ty={:?} bound={:?} => {:?}",
+               ty, bound, result);
+        return result;
+    }
+
     let mut fulfill_cx = FulfillmentContext::new(false);
 
     // We can use a dummy node-id here because we won't pay any mind

src/librustc/middle/traits/select.rs

@@ -407,6 +407,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         })
     }
 
+    /// Evaluates whether the obligation `obligation` can be satisfied,
+    /// and returns `false` if not certain. However, this is not entirely
+    /// accurate if inference variables are involved.
+    pub fn evaluate_obligation_conservatively(&mut self,
+                                              obligation: &PredicateObligation<'tcx>)
+                               -> bool
+    {
+        debug!("evaluate_obligation_conservatively({:?})",
+               obligation);
+
+        self.infcx.probe(|_| {
+            self.evaluate_predicate_recursively(TraitObligationStackList::empty(), obligation)
+                == EvaluatedToOk
+        })
+    }
+
+
     fn evaluate_predicates_recursively<'a,'o,I>(&mut self,
                                                 stack: TraitObligationStackList<'o, 'tcx>,
                                                 predicates: I)