You can't just write Rust in a part of the codebase that's all C/C++. Tools for checking the newly written C/C++ code for issues will still be valuable for a very long time.
You actually can? A Rust-written function that exports a C ABI and calls C ABI functions interops just fine with C. Of course that's all unsafe (unless you're doing pure value-based programming and not calling any foreign code), so you don't get much of a safety gain at the single-function level.
If you're going to swap out one function in a chain of functions for a Rust version, you're destroying your codebase. You simply can't replace one tiny piece of code in a large codebase with a version in a different language. Doing so would be insane.
C ABI is not C++ ABI. People often write C/C++ but they're completely different languages. C++ is much higher level and modern. C++ is closer to Rust than it is to C.
