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

fn main() {
    let val = RefCell::new(1);

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

In the complete code, I'm using a type that has a RefCell embedded in it (a typed_arena::Arena). I'm using crossbeam to ensure that the thread does not outlive the reference it takes.

This produces the error:

error[E0277]: `std::cell::RefCell<i32>` cannot be shared between threads safely
 --> src/main.rs:8:15
  |
8 |         scope.spawn(|_| *val.borrow());
  |               ^^^^^ `std::cell::RefCell<i32>` cannot be shared between threads safely
  |
  = help: the trait `std::marker::Sync` is not implemented for `std::cell::RefCell<i32>`
  = note: required because of the requirements on the impl of `std::marker::Send` for `&std::cell::RefCell<i32>`
  = note: required because it appears within the type `[closure@src/main.rs:8:21: 8:38 val:&std::cell::RefCell<i32>]`

I believe I understand why this error happens: RefCell is not designed to be called concurrently from multiple threads, and since it uses internal mutability, the normal mechanism of requiring a single mutable borrow won't prevent multiple concurrent actions. This is even documented on Sync:

Types that are not Sync are those that have "interior mutability" in a non-thread-safe form, such as cell::Cell and cell::RefCell.

This is all well and good, but in this case, I know that only one thread is able to access the RefCell. How can I affirm to the compiler that I understand what I am doing and I ensure this is the case? Of course, if my reasoning that this is actually safe is incorrect, I'd be more than happy to be told why.

Answer 1

Another solution for this case is to move a mutable reference to the item into the thread, even though mutability isn't required. Since there can be only one mutable reference, the compiler knows that it's safe to be used in another thread.

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

fn main() {
    let mut val = RefCell::new(1);
    let val2 = &mut val;

    crossbeam::scope(|scope| {
        scope.spawn(move |_| *val2.borrow());
    })
    .unwrap();
}

As bluss points out:

This is allowed because RefCell<i32> implements Send.

Answer 2

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<i32>);
    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.