Rust works pretty well with C, and you could use cbindgen[0] to generate c headers for rust functions marked as extern and it would work when linked together. However, rustls doesn't seems to be providing any such function. Most of its API is rust-based, using stuff that isn't directly available in C (Option, Vec, etc.). So you would have to write at least some rust code that calls the library, and have your C code calls the rust code that you write.

On the flip side, calling C code in rust is pretty easy - you have the entire libc available, and rust-bindgen[1] generate bindings for you, which in my experience works very well. So, you may also consider writing your program in rust… If that's possible.

  [0]: https://github.com/eqrion/cbindgen
  [1]: https://github.com/rust-lang/rust-bindgen