Post #1364932

@whitequark @wffl for context: https://docs.rs/objc/latest/objc/macro.msg_send.html relies on type inference for the arguments and return type of an FFI call. with insufficient annotations, the return type would correctly fall back to () but that stabilization attempt changed it to ! somehow, even though the call *did* return. lots of arguably-wrong but innocent-looking calls became UB in nightly