Auto merge of #129777 - nnethercote:unreachable_pub-4, r=Urgau

Add `unreachable_pub`, round 4

A follow-up to #129732.

r? `@Urgau`

compiler/rustc_serialize/src/lib.rs

@@ -16,6 +16,7 @@
 #![feature(never_type)]
 #![feature(ptr_sub_ptr)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub use self::serialize::{Decodable, Decoder, Encodable, Encoder};