tests/ui/feature-gates/feature-gate-builtin_syntax.rs - toolchain/rustc - Git at Google

 struct Foo {
     v: u8,
     w: u8,
 }
 fn main() {
     builtin # offset_of(Foo, v); //~ ERROR `builtin #` syntax is unstable
 }
	struct Foo {
	v: u8,
	w: u8,
	}
	fn main() {
	builtin # offset_of(Foo, v); //~ ERROR `builtin #` syntax is unstable
	}