Auto merge of #120393 - Urgau:rfc3373-non-local-defs, r=WaffleLapkin

Implement RFC 3373: Avoid non-local definitions in functions

This PR implements [RFC 3373: Avoid non-local definitions in functions](https://github.com/rust-lang/rust/issues/120363).

src/source_file.rs

@@ -66,6 +66,7 @@ fn ensure_real_path(filename: &FileName) -> &Path {
         }
     }
 
+    #[cfg_attr(not(bootstrap), allow(non_local_definitions))]
     impl From<&FileName> for rustc_span::FileName {
         fn from(filename: &FileName) -> rustc_span::FileName {
             match filename {