move 'trait bounds on const fn' to separate feature gate

library/alloc/src/lib.rs

@@ -90,6 +90,7 @@
 #![feature(coerce_unsized)]
 #![feature(const_btree_new)]
 #![feature(const_fn)]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]
 #![feature(destructuring_assignment)]