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

Author: RalfJung

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)]

core/src/lib.rs

@@ -86,6 +86,7 @@
 #![feature(const_impl_trait)]
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_fn_fn_ptr_basics)]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(const_option)]
 #![feature(const_precise_live_drops)]
 #![feature(const_ptr_offset)]