libsyntax: add some explicit copies

src/libsyntax/fold.rs

@@ -280,7 +280,7 @@ pub fn noop_fold_item_underscore(i: &item_, fld: ast_fold) -> item_ {
         }
         item_mac(ref m) => {
             // FIXME #2888: we might actually want to do something here.
-            item_mac((*m))
+            item_mac(copy *m)
         }
     }
 }