I suspect the use of Rust rather than C++ might have something to do with the fact that Simon Peyton Jones and Lennart Augustsson (both of Haskell fame) both work at Epic and there would have been a strong internal push to do this in a language with some functional programming features. Rust rather than Verse because that would probably not be the right tool for the job (even if Simon works on it). Rust rather than Haskell probably because of performance -- DARCS never caught on partly for performance reasons.