let test_cb = move |n: i32| {
    n*2
};
let mut _test_worker = new(test_cb);

pub struct Worker {
    ptr: WorkerPtr,        // Pointer to the C data structure
    cb: fn(i32) -> i32,    // Callback function
}

struct WorkerPtr(NonNull<worker>);
unsafe impl marker::Send for WorkerPtr {}
struct worker_s {
    void* rust_object;  // Pointer to the Rust data structure
};

#[no_mangle]
pub extern fn rust_callback(w: *mut Worker, raw_n: c_int) -> c_int {
    let n = raw_n as i32;
    println!("rust_callback: {}", n);
    unsafe {
        let out = (*w).trigger_callback(n);
        out as c_int
    }
}

worker_new: worker = 0x7f9834d000c0
worker_set_rust_object: worker = 0x7f9834d000c0 rust_object = 0x10ca21000
trigger_callback: worker = 0x7f9834d000c0 rust_object = 0x10ca21000
rust_callback: 100
trigger_callback: 100
got: 200