Add warn(unreachable_pub) to rustc_feature.

Author: nnethercote

compiler/rustc_feature/src/lib.rs

@@ -15,6 +15,7 @@
 #![allow(internal_features)]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 mod accepted;