How do I share a mutable object between threads using Arc?

Arc's documentation says:

Shared references in Rust disallow mutation by default, and Arc is no exception: you cannot generally obtain a mutable reference to something inside an Arc. If you need to mutate through an Arc, use Mutex, RwLock, or one of the Atomic types.

You will likely want a Mutex combined with an Arc:

use std::{    sync::{Arc, Mutex},    thread,};struct Stats;impl Stats {    fn add_stats(&mut self, _other: &Stats) {}}fn main() {    let shared_stats = Arc::new(Mutex::new(Stats));    let threads = 5;    for _ in 0..threads {        let my_stats = shared_stats.clone();        thread::spawn(move || {            let mut shared = my_stats.lock().unwrap();            shared.add_stats(&Stats);        });        // Note: Immediately joining, no multithreading happening!        // THIS WAS A LIE, see below    }}

This is largely cribbed from the Mutex documentation.

How can I use shared_stats after the for? (I'm talking about the Stats object). It seems that the shared_stats cannot be easily converted to Stats.

As of Rust 1.15, it's possible to get the value back. See my additional answer for another solution as well.

[A comment in the example] says that there is no multithreading. Why?

Because I got confused! :-)

In the example code, the result of thread::spawn (a JoinHandle) is immediately dropped because it's not stored anywhere. When the handle is dropped, the thread is detached and may or may not ever finish. I was confusing it with JoinGuard, a old, removed API that joined when it is dropped. Sorry for the confusion!

For a bit of editorial, I suggest avoiding mutability completely:

use std::{ops::Add, thread};#[derive(Debug)]struct Stats(u64);// Implement addition on our typeimpl Add for Stats {    type Output = Stats;    fn add(self, other: Stats) -> Stats {        Stats(self.0 + other.0)    }}fn main() {    let threads = 5;    // Start threads to do computation    let threads: Vec<_> = (0..threads).map(|_| thread::spawn(|| Stats(4))).collect();    // Join all the threads, fail if any of them failed    let result: Result<Vec<_>, _> = threads.into_iter().map(|t| t.join()).collect();    let result = result.unwrap();    // Add up all the results    let sum = result.into_iter().fold(Stats(0), |i, sum| sum + i);    println!("{:?}", sum);}

Here, we keep a reference to the JoinHandle and then wait for all the threads to finish. We then collect the results and add them all up. This is the common map-reduce pattern. Note that no thread needs any mutability, it all happens in the master thread.