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

Add `unreachable_pub`, round 4

A follow-up to #129732.

r? `@Urgau`

compiler/rustc_span/src/lib.rs

@@ -30,6 +30,7 @@
 #![feature(round_char_boundary)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
+#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 // The code produced by the `Encodable`/`Decodable` derive macros refer to