Add regression test for #81555

Author: hyd-dev

src/test/ui/proc-macro/issue-81555.rs

@@ -0,0 +1,15 @@
+// check-pass
+// aux-build:test-macros.rs
+#![feature(stmt_expr_attributes, proc_macro_hygiene)]
+
+extern crate test_macros;
+
+use test_macros::identity_attr;
+
+#[identity_attr]
+fn main() {
+    let _x;
+    let y = ();
+    #[identity_attr]
+    _x = y;
+}