We have a few traits that users has to implement manually, namely Encode, RefEncode, Message and the INS... traits. It would be nice if these could be automatically derived, but in essence we can't allow that because they are unsafe, and a derive macro wouldn't be able to uphold the required safety invariants.

#[derive(Encode)] (requested upstream in SSheldon/rust-objc#52) specifically doesn't work because the user could create a #[repr(C)] struct which used e.g. [i8; 4] while the Objective-C side used int. Or *const u8 vs. c_void*; this is valid from a normal FFI-perspective, but Encode requires that the encodings match perfectly.

So I'm actually probably going to remove the existing objc2_foundation_derive helpers in this repo.