separate feature flag for unsizing casts in const fn

Author: RalfJung

alloc/src/lib.rs

@@ -89,7 +89,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![feature(const_btree_new)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
 #![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]

core/src/lib.rs

@@ -81,7 +81,7 @@
 #![feature(const_refs_to_cell)]
 #![feature(const_panic)]
 #![feature(const_pin)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
 #![feature(const_fn_union)]
 #![feature(const_impl_trait)]
 #![feature(const_fn_floating_point_arithmetic)]