Merge branch 'kb/userdiff-rust-macro-rules'

Userdiff for Rust update.

* kb/userdiff-rust-macro-rules:
  userdiff: recognize 'macro_rules!' as starting a Rust function block

t/t4018/rust-macro-rules

@@ -0,0 +1,6 @@
+macro_rules! RIGHT {
+    () => {
+        // a comment
+        let x = ChangeMe;
+    };
+}