tests/ui/parser/doc-before-identifier.rs - third_party/github.com/rust-lang/rust - Git at Google

 fn /// document
 foo() {}
 //~^^ ERROR expected identifier, found doc comment `/// document`

 fn main() {
     foo();
 }
	fn /// document
	foo() {}
	//~^^ ERROR expected identifier, found doc comment `/// document`

	fn main() {
	foo();
	}