src/test/ui/macros/macro-match-nonterminal.rs - third_party/rust - Git at Google

 macro_rules! test {
     ($a, $b) => {
         //~^ ERROR missing fragment
         //~| ERROR missing fragment
         //~| WARN this was previously accepted
         ()
     };
 }

 fn main() {
     test!()
 }
	macro_rules! test {
	($a, $b) => {
	//~^ ERROR missing fragment
	//~\| ERROR missing fragment
	//~\| WARN this was previously accepted
	()
	};
	}

	fn main() {
	test!()
	}