Implementing protocol methods with Id<_, _> return types

[ericmarkmartin]

Suppose we have a protocol P with method f(&self) -> Id<NSObject, Shared>. If we use declare_class! to implement this protocol for a new type...

declare_class!(
  struct C {
    // This is unrelated to this ticket but I can't figure out how to do empty structs in `declare_class`
    filler: u8
  }

  unsafe impl ConformsTo<P> for C {
    #[method(f)]
    fn f(&self) -> Id<NSObject, Shared> {
      unimplemented!()
    }
  }
)

We see

the trait Encodeis not implemented foricrate::objc2::rc::Id<NSObject, Shared>`

in the macro expansion because it generates

<Id<NSObject, Shared> as $crate::encode::EncodeConvert>::__into_inner({
  $crate::panicking::panic("not implemented")
})

And Id<_, _> doesn't implement EncodeConvert

[madsmtm]

You can't use #[method(f)], because returning Id requires doing extra work based on the selector, akin to msg_send! vs. msg_send_id!.

Allowing this is indeed on my todolist, see #282.

Until the above issue is resolved, you can return *mut NSObject instead.

[madsmtm]

This is unrelated to this ticket but I can't figure out how to do empty structs in declare_class

This should work:

declare_class!(
  struct C {}

  // ...
)

Though you're right, declare_class! should allow an empty struct C;, I've fixed that in #318.