Console games frequently use alternate languages, for example UnrealScript, Lua, and JavaScript. Of course, these are interpreted instead of compiled, but I'm told that AOT compilation is not a deal-breaker.
I suspect that any integration would be gradual, with some parts written in C++ and some written in Rust. This is the path we're following with Servo, after all.
Which parts of what? Modern game engines? Nearly all of it, minus often gameplay scripting.
In the hypothetical C++/Rust fusion engine? Whatever hasn't been rewritten yet in Rust, and specifically things that interop with other C++ libraries will remain in C++ for longer times.
Edit: Ohh Servo? Similarly, things that haven't been rewritten yet, specifically javascript interpertation/compiliation. AFAIK Servo is focusing on rewriting layout.
I suspect that any integration would be gradual, with some parts written in C++ and some written in Rust. This is the path we're following with Servo, after all.