Rust is working out very well so far. If you look at the rope implementation, you'll see that it's high level and mathematical almost like a functional language, but performance is first-class. I'm also very much looking forward to retrofitting parallelism using libraries such as Rayon and crossbeam. I'll probably write a retrospective when I'm farther into the project.

My current development environment is just Sublime Text for the Rust parts, and Xcode for the parts written in Swift.