How can I guarantee that a type that doesn't implement Sync can actually be safely shared between threads?

Question

I have code that creates a RefCell and then wants to pass a reference to that RefCell to a single thread:

use cros         


        

Answer 1

One way would be to use a wrapper with an unsafe impl Sync:

use crossbeam; // 0.7.3
use std::cell::RefCell;

fn main() {
    struct Wrap(RefCell);
    unsafe impl Sync for Wrap {};
    let val = Wrap(RefCell::new(1));

    crossbeam::scope(|scope| {
        scope.spawn(|_| *val.0.borrow());
    })
    .unwrap();
}

As usual with unsafe, it is now up to you to guarantee that the inner RefCell is indeed never accessed from multiple threads simultaneously. As far as I understand, this should be enough for it not to cause a data race.