The full-blown version that guarantees no bounds-check errors at runtime requires dependent types (and consequently requires programmers to work with a proof assistant, which is why it's not very popular). You could have a more lightweight version that instead just crashes the program at runtime if an out-of-range assignment is attempted, and optionally requires such fallible assignments to be marked as such in the code. Rust can do this today with const generics, though it's rather clunky as there's very little syntactic sugar and no implicit widening.
AIUI WUFFS doesn't need a full blown proof assistant because instead of attempting the difficult problem "Can we prove this code is safe?" it has the programmer provide elements of such a proof as they write their program so it can merely ask "Is this a proof that the program is safe?" instead.
This is also approximately true of Idris. The thing that really helps Wuffs is that it's a pretty simple language without a lot of language features (e.g., no memory allocation and only very limited pointers) that complicate proofs. Also, nobody is particularly tempted to use it and then finds it unexpectedly forbidding, because most programmers don't ever have to write high-performance codecs; Wuffs's audience is people who are already experts.
Also, Wuffs doesn't let you prove arbitrary correctness properties, it aims only to prove the absence of memory corruption. That reduces how expressive the proof system has to be.
