Specialize implementations

Implementations in stdlib are now optimized as they were before.

compiler/rustc_span/src/lib.rs

@@ -21,6 +21,7 @@
 #![feature(nll)]
 #![feature(min_specialization)]
 #![feature(thread_local_const_init)]
+#![feature(trusted_step)]
 
 #[macro_use]
 extern crate rustc_macros;