4 ms·

The RFC for that has been recently accepted[1] and the implementation is already available in nightly[2]. [1]: https://rust-lang.github.io/rfcs/3107-derive-enu

• conradludgate 5y ago
  Unfortunately there's still no guarantee for which variant will be represented by 0
  • erk__ 5y ago
    You may also have cases where no variant is 0 so this case is probably not as straightforward
    • estebank 5y ago
      The comment is fair. There have been some talks about being able to allow end users specify niches so that if you could have something like enum E<T> { Empty = 42, Some(NonFortyTwo<T>), } in the same way that the following is of size T today enum E<T> { Empty = 0, Some(NonZero<T>), } but that's not possible today.
      • tialaramex 5y ago
        Yes, I can see uses for this. Forty-two is obviously not something you'd actually want, but I can see needing numbers in the range 0-100 and being frustrated that since I need 0 I can't carve out a niche even though I don't mind losing 101 through 255 from a u8 instead of making my type wider. Still the existing tricks get a lot done for relatively little work. I have used Option<NonZeroUsize> to do roughly what I'd use a single integer for in C, but making explicit that zero isn't just zero, but "I dunno, invalid". Would I have done that even if it cost more space? Probably, but it was cool that I didn't even need to consider that, "Zero cost abstraction".