The way to do this in idiomatic Rust is to make a wrapper type that implements drop:

    struct MyForeignPtr(*mut c_void);

    impl Drop for MyForeignPtr {
        fn drop(&mut self) {
            unsafe { my_free_func(self.0); }
        }
    }

Then wrap the foreign pointer with MyForeignPtr as soon as it crosses the FFI boundary into your Rust code, and only ever access the raw pointer via this wrapper object. Don't pass the raw pointer around.