add a test

Author: PonasKovas

tests/ui/proc-macro/auxiliary/helper-attr.rs

@@ -0,0 +1,12 @@
+//@ force-host
+//@ no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+
+// Doesn't do anything, but has a helper attribute.
+#[proc_macro_derive(WithHelperAttr, attributes(x))]
+pub fn derive(_input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    proc_macro::TokenStream::new()
+}

tests/ui/proc-macro/helper-attr-builtin-derive.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+//@ aux-build:helper-attr.rs
+//@ edition:2021
+
+// This test checks that helper attributes of a derive proc macro can be used together with
+// other built-in derive macros.
+
+#[macro_use]
+extern crate helper_attr;
+
+use helper_attr::WithHelperAttr;
+
+#[derive(WithHelperAttr, Debug, Clone, PartialEq)]
+struct MyStruct<#[x] 'a, #[x] const A: usize, #[x] B> {
+    #[x]
+    field: &'a [B; A],
+}
+
+fn main() {}