`derive` macros

[madsmtm]

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.

[madsmtm]

And my personal take: I really dislike the extra compilation cost and the complexity in implementing them; I'd any day rather use macro_rules!

[madsmtm]

objc2_foundation_derive is gone. I'll close this issue, but if someone wants to argue in favor of derive macros, feel free!

[madsmtm]

a derive macro wouldn't be able to uphold the required safety invariants.

Or *const u8 vs. c_void*; this is valid from a normal FFI-perspective, but Encode requires that the encodings match perfectly.

I'm finding, when converting huge amounts of code, that derived Encode and RefEncode might actually be useful... Maybe we can relax the safety requirements in them so that they don't have to be implemented correctly, only that they have to be correct FFI-wise? Or something...
Will have to match the objc_msgSend functions in message/apple/...!

[madsmtm]

We could maybe also add a "old-compat" feature flag, which also implements Encode from the objc v0.2.7 crate; then crates from e.g. core-foundation-rs wouldn't need to choose between objc and objc2 support:

unsafe impl objc::Encode for T {
    #[inline]
    fn encode() -> objc::Encoding {
        objc::Encoding::from_str(T::ENCODING)
    }
}