I'm a fan of effects, and I think they would work great for Rust. (I/O just probably wouldn't be one of them, or else it would be "on by default" like `Sized`.)

I'm not sure how they'd give you a unified mechanism to implement these kinds of things, though. The Monad typeclass lets you implement new instances in libraries. Is there any work on defining effects like async/await in libraries? If so I imagine they'd still have to use something like continuations.