tests/ui/panic-runtime/bad-panic-flag1.rs - third_party/github.com/rust-lang/rust - Git at Google

 //@ compile-flags:-C panic=foo

 fn main() {}

 //~? ERROR incorrect value `foo` for codegen option `panic` - either `unwind`, `abort`, or `immediate-abort` was expected
	//@ compile-flags:-C panic=foo

	fn main() {}

	//~? ERROR incorrect value `foo` for codegen option `panic` - either `unwind`, `abort`, or `immediate-abort` was expected