Feature-gate associated constants.

src/test/compile-fail/impl-wrong-item-for-trait.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_consts)]
+
 trait Foo {
     fn bar(&self);
     const MY_CONST: u32;