Thread-unsafety in Rust

gistfile1.rs

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

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

    for _ in 1..100 {
        let x = p.clone();
        thread::spawn(move || {
            let n;
            {
                let mut x_ = x.lock().unwrap();
                *x_ += 1;
                n = *x_;
            }
            thread::sleep_ms(50);
            {
                // because Rust guarantees thread-safety, we can safely assume x hasn't been changed
                let x_ = x.lock().unwrap();
                if *x_ != n {
                    panic!();
                }
            }
        });
    }

    thread::sleep_ms(2000);
}