Don't add attribute to allow unused-qualifications to derive impl's

Currently `#![forbid(unused_qualifications)]` is incompatible with all
derive's because we add `#[allow(unused_qualifications)]` in all
generated impl's.

Author: samrat

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(

src/test/ui/lint/auxiliary/add-impl.rs

@@ -0,0 +1,22 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_derive(AddImpl)]
+// Unnecessary qualification `bar::foo`
+// https://github.com/rust-lang/rust/issues/71898
+pub fn derive(input: TokenStream) -> TokenStream {
+    "impl B {
+            fn foo(&self) { use bar::foo; bar::foo() }
+        }
+
+        fn foo() {}
+
+        mod bar { pub fn foo() {} }
+    ".parse().unwrap()
+}

src/test/ui/lint/unused-qualification-in-derive-expansion.rs

@@ -0,0 +1,16 @@
+// run-pass
+// aux-build:add-impl.rs
+
+#![forbid(unused_qualifications)]
+
+#[macro_use]
+extern crate add_impl;
+
+#[derive(AddImpl)]
+struct B;
+
+fn main() {
+    B.foo();
+    foo();
+    bar::foo();
+}