Sorry if that wasn't clear, it's been a rather long time since I checked on the Future story. There seem to be more options now than when I looked into it. When I did my research over half a year ago tokio seemed to be the only option. Glad to see that's not the case any more.
It’s all good! To elaborate a bit, there’s a trait, Executor, that people can implement to build these kinds of things. This stuff, along with the final version of Futures, stabilized a few days ago. But it’s been baking a while.