tests/ui/recursion_limit/invalid_macro.rs - third_party/github.com/rust-lang/rust - Git at Google

 #![recursion_limit = foo!()] //~ ERROR attribute value must be a literal

 macro_rules! foo {
     () => {"128"};
 }

 fn main() {}
	#![recursion_limit = foo!()] //~ ERROR attribute value must be a literal

	macro_rules! foo {
	() => {"128"};
	}

	fn main() {}