Add a macro to conveniently implement marker traits

library/core/src/lib.rs

@@ -162,6 +162,7 @@
 #![feature(const_waker)]
 #![feature(core_panic)]
 #![feature(duration_consts_float)]
+#![feature(internal_impls_macro)]
 #![feature(ip)]
 #![feature(is_ascii_octdigit)]
 #![feature(maybe_uninit_uninit_array)]