Comment by joz1-k

Correct. And if those projects exposed a C++ API, then rewriting them in Rust would be highly problematic, because Rust did not prioritize C++/Rust interoperability (for well-understood reasons). So, you can either have a C API and move to Rust or have a C++ API and create a Rust version of the library for Rust developers, but your original users of the C++ library will stick with the C++ version.