Nightly requirements

[inquisitivecrystal]

I would like to be able to use this without nightly. I think it is now possible to remove the fnbox feature. This means the only feature I don't know how to remove is the untagged_unions feature. @sdleffler, can you explain why unions are necessary?

[sdleffler]

Hi! Frankly I can't remember any exact reasons why I decided against enums for the unsync implementation. I think it had to do with a combination of safety and efficiency, and a desire to keep its implementation similar to the synchronous one. For the synchronous thunk implementation, I definitely did it because I needed an atomic enum discriminant; so I separated what would have been a regular Rust enum it into an AtomicUsize and then the untagged union. Again, you could rewrite it with an enum, but it would add the space for a discriminant which is redundant with the atomic discriminant.

[inquisitivecrystal]

If I'm reading rust-lang/rfcs#2514 correctly, there's likely to be a solution for this in the near- to mid- term future. So it probably makes sense just to wait for that. I may have some other PRs for you though. Thanks for being so responsive!