I've been working on integrating Rust with Firefox's C++ codebase and while it is challenging it's not prohibitively so, tooling like bindgen makes things easie

I've been working on integrating Rust with Firefox's C++ codebase and while it is challenging it's not prohibitively so, tooling like bindgen makes things easier. Nightly Rust shouldn't be breaking your code. Use stable features, use repr(C) for stuff that's crossing the boundary (we sloppily didn't in some places, this did cause a breakage on a rust up, but that was our fault), and you should be good. Avoid unsafe-code-UB.