Convert libstd to use the Drop trait

src/libstd/future.rs

@@ -23,10 +23,12 @@ use cast::copy_lifetime;
 #[doc = "The future type"]
 pub struct Future<A> {
     /*priv*/ mut state: FutureState<A>,
+}
 
-    // FIXME(#2829) -- futures should not be copyable, because they close
-    // over fn~'s that have pipes and so forth within!
-    drop {}
+// FIXME(#2829) -- futures should not be copyable, because they close
+// over fn~'s that have pipes and so forth within!
+impl<A> Future<A> : Drop {
+    fn finalize() {}
 }
 
 priv enum FutureState<A> {