support const_evaluatable_checked across crate boundaries

compiler/rustc_privacy/src/lib.rs

@@ -97,6 +97,15 @@ where
                 ty.visit_with(self)
             }
             ty::PredicateAtom::RegionOutlives(..) => false,
+            ty::PredicateAtom::ConstEvaluatable(..)
+                if self.def_id_visitor.tcx().features().const_evaluatable_checked =>
+            {
+                // FIXME(const_evaluatable_checked): If the constant used here depends on a
+                // private function we may have to do something here...
+                //
+                // For now, let's just pretend that everything is fine.
+                false
+            }
             _ => bug!("unexpected predicate: {:?}", predicate),
         }
     }