I imagine JSON serialisation overhead is pretty small when compared to parsing/typechecking a Rust program, which is probably what the Rust Language Server has

I imagine JSON serialisation overhead is pretty small when compared to parsing/typechecking a Rust program, which is probably what the Rust Language Server has to do whenever anything changes.. Not to mention that a lot of people capable of writing the tooling would struggle to export a C API. I write Scala in my day job and it would take me a while to learn how to do that - and I've done some programming in C/C++ before.