`async` support

[madsmtm]

Objective-C uses "completion handlers" for concurrency.

Swift automatically translates these to async functions, we could consider doing something similar after #264.

Also would be a good step forwards for async in winit.

See also blockr's async support, and cidre's async support.

See also this blog post on how Zed uses async and GCD.

Finally, see the equivalent in the Python space, objc_asyncio.

[madsmtm]

Would probably also be good to look at what is actually required on the runtime-side of this? Do we need to do stuff with NSRunLoop?

[madsmtm]

Instead of async fn method(), we could use fn method() -> CompletionHandler<()>, and then impl IntoFuture for CompletionHandler

[notgull]

Would probably also be good to look at what is actually required on the runtime-side of this? Do we need to do stuff with NSRunLoop?

There is actually a way to make NSRunLoop fully async, since:

1. NSRunLoop is a wrapper around CFRunLoop.
2. CFRunLoop is a wrapper around Grand Central Dispatch.
3. Grand Central Dispatch uses a Mach port to coordinate itself.
4. Mach ports can be registered in kqueue.

So if we can register the global GCD Mach port into a kqueue, we can then put that into, say, a async-io::Async and then use the readable() function to tell when it's available. The main downside of this approach is that steps 2 and 3 is that gaining access to the GCD Mach port requires access to unstable OS APIs (namely, _dispatch_get_main_queue_port_4CF()), but it's not like the Rust async stack isn't already built on top of unstable OS APIs anyways.

[madsmtm]

Interesting to know, thanks!

I don't think I would have that much against using unstable APIs like _dispatch_get_main_queue_port_4CF, but would ideally really like to avoid it (since it can have consequences for submitting apps to the App Store, as it is not public).

[madsmtm]

Related Kotlin issue about their unfinished async support: https://youtrack.jetbrains.com/issue/KT-47610

[madsmtm]

A necessary prerequisite is #168, since we need some way to tell that the completion handlers will only be run once.

[madsmtm]

To take the example from the linked Swift proposal:

- (void)fetchShareParticipantWithUserRecordID:(CKRecordID *)userRecordID 
    completionHandler:(void (^)(CKShareParticipant * _Nullable, NSError * _Nullable))completionHandler;

And what that would (ideally) be in Rust, pre-async translation:

fn method_name_bikeshed(
    &self,
    userRecordID: &CKRecordID, 
    completionHandler: BlockOnceSendSync<(Option<&CKShareParticipant>, Option<&NSError>), ()>,
);

It would probably be prudent to first convert the Objective-C style error into the usual Rust error:

fn method_name_bikeshed(
    &self,
    userRecordID: &CKRecordID, 
    completionHandler: BlockOnceSendSync<(Result<&CKShareParticipant, &NSError>,), ()>,
);

Note that this has a different ABI, but I might be able to handle such things directly in block2.

Now, we could subsequently convert it to:

fn method_name_bikeshed(
    &self,
    userRecordID: &CKRecordID,
) -> BikeshedHelper<Result<&CKShareParticipant, &NSError>, impl FnOnce(...)>;

struct BikeshedHelper<T, F: FnOnce(BlockOnceSendSync<(T,), ()>)> { ... }
impl BikeshedHelper<T, F> {
    pub fn run_with_block(self, block: BlockOnceSendSync<(T,), ()>) { ... }
}
impl IntoFuture for BikeshedHelper<T, F> {
    type Output = T; // TODO: Convert &MyClass to Id<MyClass> here
    type Future = BlockFuture<T>;
}

struct BlockFuture<T> { ... }

Immediately we see a few issues:

1. Ideally we'd be able to choose to do the call manually using blocks, instead of the async functionality. This is attempted made possible by the run_with_block method, but that does complicate the signature in a way I'm not sure will be possible to support easily. Alternatively we will have to emit both the async and the non-asynv version, which also sounds bad.
2. We need some way to generically convert reference arguments given in the block, to retained arguments that are safe to return to the surrounding context.
3. There is a performance cost to this, since we have to do said retain.

[madsmtm]

Note: I think it makes sense to make separate adapters depending on whether the block can receive an error or not (one parameter is Option<&NSError> (though not necessarily the last?)), since the Swift design document seems to treat them differently.

And we also need to distinguish between blocks with one parameter vs. multiple parameters (the former should be a simple output, while the latter should return a tuple).

[madsmtm]

Linking clang's _Nullable_result attribute, which is useful for determining whether the async function should return an Option or not.