Add warn(unreachable_pub) to rustc_parse_format.

Author: nnethercote

compiler/rustc_parse_format/src/lib.rs

@@ -13,6 +13,7 @@
     html_playground_url = "https://play.rust-lang.org/",
     test(attr(deny(warnings)))
 )]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 use std::{iter, str, string};