Add test for derive helper compat collisions

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 +1,23 @@
+//@ 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;
+
+// Testing the behavior of derive attributes with helpers that share the same name.
+//
+// Normally if the first derive below were absent the call to #[empty_helper] before it it
+// introduced by its own derive would produce a future incompat error.
+//
+// With the extra derive also introducing that attribute in advanced the warning gets supressed.
+// Demonstrates a lack of identity to helper attributes, the compiler does not track which derive
+// introduced a helper, just that a derive introduced the helper.
+#[derive(Empty)]
+#[empty_helper]
+#[derive(Empty2)]
+struct S;
+
+fn main() {}