rust closure to C style function

main.rs

use lazy_static::lazy_static;
use std::os::raw::{c_int};
use std::sync::Mutex;
use std::ffi::c_void;

lazy_static! {
    static ref REAL_CALLBACK: Mutex<Option<Box<FnMut(c_int, c_int) -> c_int + Send>>> = Default::default();
}

extern "C" fn callback(x: c_int, y: c_int) -> c_int {
    if let Some(ref mut real_callback) = *REAL_CALLBACK.lock().unwrap() {
        real_callback(x, y)
    } else {
        panic!("<handle error here>");
    }
}

fn main() {
    *REAL_CALLBACK.lock().unwrap() = Some(Box::new(move |x, y| {
        println!("Adding");
        x + y
    }));

    unsafe {
        call_c(callback);
    }
}

pub type Callback = extern fn(c_int, c_int) -> c_int;

fn call_c(ptr: Callback) {
    println!("ret: {}", ptr(10, 20));
}