In the medium-long term I want to do this for Rust as "Pattern types" because the thing I actually want (custom types with niches) is gated on Pattern types, as the way to explain to the type system where the niche goes is a Pattern. I was persuaded that we can't/ shouldn't just say we'll half ass it, we must do it properly if we're doing it.

e.g. I don't necessarily have a use for an integer from 0 to 11, but I do see a use for BalancedI8, a one byte type with values -127 to +127 via 0, thus omitting -128. I reckon lots of people don't need -128, whereas a niche is very useful. Rust provides NonZeroI8, which has -128 through +127 but no zero, but I find that's less often what you want, and it's not today possible to make your own in stable Rust (and in nightly Rust you need a not-for-mortals perma-unstable attribute today).