don't let const_fn feature flag affect impl-block-level trait bounds

Author: RalfJung

proc_macro/src/lib.rs

@@ -21,7 +21,8 @@
 #![feature(rustc_allow_const_fn_unstable)]
 #![feature(nll)]
 #![feature(staged_api)]
-#![feature(const_fn)]
+#![cfg_attr(bootstrap, feature(const_fn))]
+#![cfg_attr(not(bootstrap), feature(const_fn_trait_bound))]
 #![feature(const_fn_fn_ptr_basics)]
 #![feature(allow_internal_unstable)]
 #![feature(decl_macro)]