The only time I’ve ever needed a Mutex<()> so far with Rust is when I had to interop with a C library which itself was not thread safe (unprotected use of global variables), so I needed to lock the placeholder mutex each time I called one of the C functions.

Actually I think in this case you're still better off using a Mutex with "data" inside. I've done this before. The idea is that you make a unit struct MyCFuncs or whatever and then you only call the C functions from methods of that unit struct. Then you can only access those methods once you lock the Mutex and get the instance of the unit struct. It feel elegant to me.