It is not actually possible. Or rather unsafe stuff in C will remain unsafe in Rust. In addition, most/all other bugs (like logic errors, wrong use of system APIs, etc.) in the C code will remain. Rust does not magically make code secure. Coding directly in Rust has some advantages, as it is hard to learn and that keeps the prevalent low-skill coders out. Rust can also avoid some problems (not that many) when the code is designed for Rust. C code is obviously not.
That said, there is an application scenario for this: If you C code is in good shape, but you want to do additional development and maintenance in Rust from a certain point onwards. That does not improve the old C/Rust code, but it may have some benefits for new code and for changes.