mention Gc in the managed box feature gate

doc/tutorial.md

@@ -1320,7 +1320,8 @@ let z = x; // this moves `x` into `z`, rather than creating a new owner
 
 assert_eq!(*z.borrow(), [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]);
 
-let mut a = Rc::new([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]); // the variable is mutable, but not the box
+// the variable is mutable, but not the contents of the box
+let mut a = Rc::new([10, 9, 8, 7, 6, 5, 4, 3, 2, 1]);
 a = z;
 ~~~
 

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.");
             }
             _ => {}
         }