Rollup merge of #99485 - mdholloway:unused-qualifications-in-derive, …

…r=oli-obk

Stop injecting `#[allow(unused_qualifications)]` in generated `derive` implementations

Currently, the `#[derive]` attribute always injects an `#[allow(unused_qualifications)]` attribute in the generated implementation. This results in an error when a derive is used in combination with `#![forbid(unused_qualifications)]`, because the `forbid` rule by definition cannot be overridden by `allow`.

It appears that the original issue that prompted the inclusion of `#[allow(unused_qualifications)]` (#19102) is no longer present in the current stable release, and the associated [test case](https://github.com/rust-lang/rust/blob/master/src/test/ui/issues/issue-19102.rs) still passes, so the `allow` is simply removed here.

Fixes #71898.

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -727,16 +727,8 @@ impl<'a> TraitDef<'a> {
 
         let attr = cx.attribute(cx.meta_word(self.span, sym::automatically_derived));
         let opt_trait_ref = Some(trait_ref);
-        let unused_qual = {
-            let word = rustc_ast::attr::mk_nested_word_item(Ident::new(
-                sym::unused_qualifications,
-                self.span,
-            ));
-            let list = rustc_ast::attr::mk_list_item(Ident::new(sym::allow, self.span), vec![word]);
-            cx.attribute(list)
-        };
 
-        let mut a = vec![attr, unused_qual];
+        let mut a = vec![attr];
         a.extend(self.attributes.iter().cloned());
 
         cx.item(