PR author here, these are two great questions. The Joel blog post which you referenced is excellent, and is a major influence on my thinking. Joel identifies "

PR author here, these are two great questions. The Joel blog post which you referenced is excellent, and is a major influence on my thinking. Joel identifies "from scratch" rewrites as a major mistake. Don't throw away code! It convinced me. My proposal is a incremental translation of our C++ to Rust. Not throwing away code, but porting it: comments, warts, battle-scars, and all, incrementally on master. Any C++ changes concurrent with the port would either get merged first and then ported, or be translated to Rust. We use an FFI for interop. There is no long-lived branch which might fall behind. To your second question: fish-shell has a small all-volunteer set of devs, and we don't have the resources to actively develop both a C++ version and a Rust version. This would be all-in: we can do patches of past versions, but multiple implementations is not feasible.