compute_trait_ref: Restore original order of predicates

This can matter for obligation causes that get reported as errors,
when there are more than one of the same obligation in the forest.

Author: tmandry

src/librustc/ty/wf.rs

@@ -465,7 +465,7 @@ impl<'a, 'gcx, 'tcx> WfPredicates<'a, 'gcx, 'tcx> {
             def_id: trait_ref.def_id,
             substs: Substs::identity_for_item(self.infcx.tcx, trait_ref.def_id)
         };
-        predicates.predicates.push(identity_trait_ref.to_poly_trait_ref().to_predicate());
+        predicates.predicates.insert(0, identity_trait_ref.to_poly_trait_ref().to_predicate());
 
         debug!("nominal_trait_obligations: trait_ref={:?} predicates={:?}",
                trait_ref, predicates.predicates);

src/librustc_typeck/check/wfcheck.rs

@@ -447,6 +447,7 @@ fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
     let predicates = predicates.instantiate_identity(fcx.tcx);
     let predicates = fcx.normalize_associated_types_in(span, &predicates);
 
+    debug!("check_where_clauses: predicates={:?}", predicates.predicates);
     let obligations =
         predicates.predicates
                     .iter()
@@ -457,6 +458,7 @@ fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
                                                                 span));
 
     for obligation in obligations {
+        debug!("next obligation cause: {:?}", obligation.cause);
         fcx.register_predicate(obligation);
     }
 }