src/test/ui/hygiene/hygienic-label-1.rs - third_party/rust - Git at Google

 macro_rules! foo {
     () => { break 'x; } //~ ERROR use of undeclared label `'x`
 }

 pub fn main() {
     'x: loop { foo!() }
 }
	macro_rules! foo {
	() => { break 'x; } //~ ERROR use of undeclared label `'x`
	}

	pub fn main() {
	'x: loop { foo!() }
	}