use std::sync::Arc;
use std::sync::Mutex;
use std::thread;

fn main() {
    let unimportant_variable = 3;

    let my_shared_variable: Arc<Mutex<Option<Box<[i32]>>>> = Arc::new(Mutex::new(None));
    let my_shared_variable_tmp = Arc::clone(&my_shared_variable);

    let thread_handle_1 = thread::spawn(move || {
        let mut var = my_shared_variable_tmp.lock().unwrap();
        *var = Some(function_returning_box(unimportant_variable));
    });

    thread_handle_1.join().expect("joining thread 1");

    println!(
        "{:?}",
        my_shared_variable.lock().expect("Error locking mutex")
    )
}

fn function_returning_box(v: i32) -> Box<[i32]> {
    (0..v).collect()
}