tests/ui/underscore-ident-matcher.rs - toolchain/rustc - Git at Google

 macro_rules! identity {
     ($i: ident) => (
         $i
     )
 }

 fn main() {
     let identity!(_) = 10; //~ ERROR no rules expected the token `_`
 }
	macro_rules! identity {
	($i: ident) => (
	$i
	)
	}

	fn main() {
	let identity!(_) = 10; //~ ERROR no rules expected the token `_`
	}