In a sense, Rust may have been a better choice for Mercurial overall, but it's hard to imagine how much of a pain the migration process would be. I don't think you could make much of any headway going Python 2 -> Rust with automated tools. That means the transition would look like, stop all Mercurial dev in its tracks, have all current contributors (who can and care to) learn Rust, bring on a couple of devs with experience in architecting large Rust projects, spend however long redesigning and rewriting in Rust, release a roughly feature-equal version a year or 2 later. Good way to move Mercurial from second-place to Git to barely known.
> That means the transition would look like, stop all Mercurial dev in its tracks
Not necessarily. One of the cool things of Rust is that it can easily expose and use a C-compatible API; one of the cool things of Python is that objects can somewhat easily be implemented or accessed through a C-compatible API. This allows for gradual replacement: the code can be piece by piece rewritten in C (actually, Rust pretending to be C), while still looking like Python objects to the rest of the code.
