Rust closure as mutable callback.

mutable_closure.rs

use std::thread;

fn do_something<'a>(mut callback: Box<FnMut() + 'a>) {
    // check http://stackoverflow.com/questions/35651279/error-closure-may-outlive-the-current-function-but-it-will-not-outlive-it
    // about why adding 'a will work
    callback();
}

fn main() {
    let mut ret = 0;
    do_something(Box::new(|| {ret += 1;}));
    println!("ret = {}", ret); // <- 1
}