src/test/ui/borrowck/immutable-arg.rs - third_party/rust - Git at Google

 fn foo(_x: u32) {
     _x = 4;
     //~^ ERROR cannot assign to immutable argument `_x`
 }

 fn main() {}
	fn foo(_x: u32) {
	_x = 4;
	//~^ ERROR cannot assign to immutable argument `_x`
	}

	fn main() {}