Self-documenting `offer!()` and `choose!()`

[yaymukund]

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Could we do something like:

offer!(in chan {
    End => break chan.close(),
    Push => {
        // push a value
    }
})?;

End and Push would reduce to enums:

enum Choice {
    End,
    Push
}

offer!(in chan {
    Choice::End => ...,
    Choice::Push => ...,
})?;

[sdleffler]

I've been hoping for something like this for a while; unfortunately, there are some nasty reasons this is not currently doable. Right now, the numeric indices are used to index a trait as const generics (trait Case<const N: usize> in Vesta.) I have some ideas on how we could potentially get around this with overly clever use of macros but they can't currently be anything other than usize or something that becomes usize because of how early in compilation they have to be used.

[sdleffler]

Also... just realized you're probably looking at main while writing this. :D The carrier-type branch is the current state of enhancements for offer!(). main doesn't use Vesta yet.

[yaymukund]

This isn't presently possible because at macro expansion time, there's no way to go from Push to Choice::Push because name resolution happens after macro expansion.

If you can access the enum, then you can turn that into an integer using a sweet hack™ which would involve having a module by the same name (so it gets imported automatically with the Choice enum) that contains an enum that maps to the integer index. I probably messed up some part of that explanation, but hopefully it offers (eyyy) a starting point.

Both of these issues could be solved a lot nicer with const generic enums, but so could a lot of dialectic.