Hello all! I've been fascinated by Rust's poll-based approach  to async programming so I've been working on a library for quite a while now, which you'll find here: https://github.com/dfunckt/swift-futures.
What's so nice about this? Turning the traditional callback-based approach of "I'll call you when I'm ready" into "let me know if I should check back next time" simplifies semantics a lot -- eg. you always know which thread your code is going to be called on, you can avoid memory allocations and locks at runtime, you can get firm control over cancellation and back-pressure, and more. What's also interesting is you also get to use value types everywhere which the compiler happily optimises out, à la SwiftUI, which means extremely low overhead and great performance.
It's still early days for the project. Ultimately, I hope it'll be a useful alternative to reactive libraries out there, but also a playground to explore language features (such as move-only and opaque types) as they arrive.
 interesting read: http://aturon.github.io/tech/2016/09/07/futures-design/