Register new proto in runtime

[pronebird]

Hi,

Trying to register my own protocol for use with NSXPCInterface. For some reason, the protocol is never registered and <dyn NSConnectionInterface>::protocol() returns None.

Trying to register protocol manually but I arrive at objc exception:

thread '' (1637226) panicked at crates/nym-ipc/src/xpc/client.rs:71:6:
objc exception!: Some(exception <NSException: 0x106227730> 'NSInvalidArgumentException' reason: NSXPCInterface: Unable to get extended method signature from Protocol data (NSConnectionInterface / write:). Use of clang is required for NSXPCInterface.)

Some good reads too: https://pyobjc.readthedocs.io/en/latest/notes/using-nsxpcinterface.html

Besides xpc shenanigans which I don't think we can workaround without using objc. My first question is, how do we ensure that protocols defined in Rust are registered with runtime? This seems to automatic in objc right?

Example

extern_protocol!(
    #[allow(clippy::missing_safety_doc)]
    pub unsafe trait NSConnectionInterface {
        #[unsafe(method(write:))]
        fn write(&self, buf: &NSData);
    }
);

const CONNECTION_INTERFACE_PROTOCOL: &CStr = c"NSConnectionInterface";
pub(crate) fn connection_interface() -> Retained<NSXPCInterface> {
    let proto = match <dyn NSConnectionInterface>::protocol() {
        Some(proto) => proto,
        None => {
            let mut builder = ProtocolBuilder::new(CONNECTION_INTERFACE_PROTOCOL)
                .expect("Failed to create protocol builder");
            builder.add_method_description::<(&NSData,), ()>(objc2::sel!(write:), true);
            let proto = builder.register();
            proto
        }
    };

    unsafe { NSXPCInterface::interfaceWithProtocol(proto) }
}

[madsmtm]

I haven't tested this specific class myself, but the pyobjc docs are probably correct here, we ideally want to emit the information about the protocol as a special static, similar to what objrs does. The problem is, that requires further compiler support to do correctly, see rust-lang/rust#145496 for some recent work in this space.

See also #417 for a workaround (same as what's discussed in the pyobjc docs, you need to use Clang for now).