added back bindings_after_at as a cfg_attr

Author: MarcusDunn

Diff for: alloc/src/lib.rs

@@ -82,6 +82,7 @@
 #![feature(allow_internal_unstable)]
 #![feature(arbitrary_self_types)]
 #![feature(async_stream)]
+#![cfg_attr(bootstrap, feature(bindings_after_at))]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
 #![feature(cfg_sanitize)]