Add test for #2558

When run against invalid macro definitions, rustfmt should leave
them unchanged rather than panic.

Author: codeworm96

tests/source/macro_rules.rs

@@ -188,3 +188,9 @@ macro_rules! binary {
         }
     };
 }
+
+// #2558
+macro_rules! m {
+    ($x:) => {};
+    ($($foo:expr)()?) => {};
+}

tests/target/macro_rules.rs

@@ -220,3 +220,9 @@ macro_rules! binary {
         }
     };
 }
+
+// #2558
+macro_rules! m {
+    ($x:) => {};
+    ($($foo:expr)()?) => {};
+}