Add test for derive helper compat collisions

Author: yaahc

tests/ui/proc-macro/auxiliary/extra-empty-derive.rs

@@ -0,0 +1,7 @@
+extern crate proc_macro;
+use proc_macro::{TokenStream, TokenTree};
+
+#[proc_macro_derive(Empty2, attributes(empty_helper))]
+pub fn empty_derive2(_: TokenStream) -> TokenStream {
+    TokenStream::new()
+}

tests/ui/proc-macro/helper-attr-compat-collision.rs

@@ -0,0 +1,15 @@
+//@ proc-macro: test-macros.rs
+//@ proc-macro: extra-empty-derive.rs
+//@ build-pass
+
+#[macro_use(Empty)]
+extern crate test_macros;
+#[macro_use(Empty2)]
+extern crate extra_empty_derive;
+
+#[derive(Empty)]
+#[empty_helper]
+#[derive(Empty2)]
+struct S;
+
+fn main() {}