sirpalee

I had success with it on a large, established project when using it for refactoring, moving around functions, implementing simple things and writing documentation. It failed when implementing complex new features and horribly went off the rails when trying to debug issues. Almost all its recommendations were wrong, and it kept trying to change things that certainly weren't the problem.

A|B Street does some of that, but it is not a game: https://github.com/a-b-street/abstreet

Neither C++ or Rust has any characteristic that would make it slower than C. It's all up to what is your existing codebase, and what kind of experts you have. Also, both languages can mix with C. For example, the Linux Kernel could also accept C++ code easily if they wanted, but it's a conscious decision not to do so. So, yeah, C is not the only option.