mention `Gc` in the managed box feature gate

src/librustc/front/feature_gate.rs

@@ -141,11 +141,9 @@ impl Visitor<()> for Context {
             },
             ast::ty_box(_) => {
                 self.gate_feature("managed_boxes", t.span,
-                                  "The managed box syntax will be replaced \
-                                  by a library type, and a garbage \
-                                  collector is not yet implemented. \
-                                  Consider using the `std::rc::Rc` type \
-                                  for reference counted pointers.");
+                                  "The managed box syntax is being replaced by the `std::gc::Gc`
+                                  and `std::rc::Rc` types. Equivalent functionality to managed
+                                  trait objects will be implemented but is currently missing.");
             }
             _ => {}
         }