We'll take a look at a sample program where five threads push data to a shared Vec. The following program tries to do the same:

// thread_mut.rs

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

fn main() {
    let mut nums = Arc::new(vec![]);
    for n in 0..5 {
        let mut ns = nums.clone();
        thread::spawn(move || {
            nums.push(n);
        });
    }
}

We have the same nums wrapped with Arc. But we cannot mutate it, as the compiler gives the following error:

This doesn't work as cloning Arc hands out immutable reference to the inner value. To mutate data from multiple threads, we need to use a type that provides shared mutability just like RefCell. But similar to Rc, RefCell cannot be used across multiple threads. Instead, we need to use their thread-safe variants such as the Mutex or RwLock wrapper types. Let's explore them next.