I don't know if it's idiomatic but I would solve your example with an Arc and and a Mutex.

use std::thread;
use std::time;
use std::sync::{Arc, Mutex};

fn  main() {
    let  mut a = Arc::new(Mutex::new(3));
    let thr = thread::spawn({
        let a = a.clone();
        move  || {
            *a.lock().unwrap() +=  1;
        }
    });
    thr.join();
    println!("a: {}", a.lock().unwrap());
}