Remove return type from variant specification

[ranjitjhala]

The only reason we need to check this is that we allow you to write the "return type" for a variant. If we instead had:

#[flux::refined_by(b: bool)]
enum Option<T> {
    #[flux::variant([false])]
    None,
    #[flux::variant((T) -> [true])]
    Some(T)
}

then, this wouldn't cause any problems. Not in this PR, but we should consider changing the syntax.

_Originally posted by @nilehmann in https://github.com/flux-rs/flux/pull/586#discussion_r1437115186_

[ranjitjhala]

I'm assuming there are no GADT shenanigans possible?

[nilehmann]

Yes, there are no GADTs in Rust and I doubt it will ever be possible because of monomorphization. Although, in theory, you could change the type if it has the same layout. I guess you could change the refinement in each variant.

If we do remove the type from the syntax, I suppose it looks weird to specify the indices by position and maybe we should write the names.

#[flux::refined_by(is_some: bool)]
enum Option<T> {
    #[flux::variant([is_some: false])]
    None,
    #[flux::variant((T) -> [is_some: true])]
    Some(T)
}

This starts to look awfully close to the "external" index/measure notation with the difference that you write all the indices/measures at once.