Arc<Mutex<T>> — Shared Mutable State

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

fn main() {
    let counter = Arc::new(Mutex::new(0u64));

    let mut handles = vec![];
    for _ in 0..10 {
        let counter = Arc::clone(&counter);
        handles.push(thread::spawn(move || {
            for _ in 0..1_000 {
                let mut n = counter.lock().unwrap();
                *n += 1;
                // lock is released when `n` (the guard) goes out of scope
            }
        }));
    }
    for h in handles { h.join().unwrap(); }

    println!("final: {}", *counter.lock().unwrap());     // 10000

    // For simple counters, prefer AtomicU64 — lock-free and faster.
}