[enhancement] variant_count!<> builtin

[grebe]

What's hard to do? (limit 100 words)

Similar to u32::MAX, it would be nice to have a way to get the number of elements in an enum. Similar to rust's std::mem::variant_count.

Current best alternative workaround (limit 100 words)

Make a const yourself and hope you keep it up to date.

Your view of the "best case XLS enhancement" (limit 100 words)

A builtin std::variant_count!<> or a colon-ref (like u32::MAX), e.g. EnumType::NUM_VARIANTS.

[mikex-oss]

Surprisingly, it seems Rust does not yet have a robust story for this. variant_count is still not stabilized and the latest comments in the issue seem to indicate going in a different direction with magic constants like MyEnum::enum#VARIANT_COUNT .

[cdleary]

I think this also gets more urgent if we had iota-assigning enums -- right now you have to name all the values by hand so making COUNT with the underlying type is comparatively not so burdensome. I'm not sure if we have an issue for that? We do have #1306 for getting proper sum types.