Type not Send/Sync even though wrapped in an Arc<Mutex<...>>. Why?

[gbutler69]

I have a type defined as follows:

#[derive(Clone)]
pub struct Filter(pub Arc<Mutex<dyn FnMut(&Path) -> bool>>);

When I attempt to use this in a threaded context the compiler complains that it is not "Send". Why would this not be "Send" and "Sync"?

[quinedot]

Your dyn FnMut could be a closure that holds state which is not Send.

    let rc = Rc::new(0);
    let closure = move |_: &Path| { println!("{}", rc); true };
    let filter = Filter(Arc::new(Mutex::new(closure)));
    // The `Rc` is part of your compiler-derived closure struct
    // and thus the struct (closure) is not Send

You can prevent this by putting a bound on it:

pub struct Filter(pub Arc<Mutex<dyn FnMut(&Path) -> bool + Send>>);

[gbutler69]

Thanks. I just figured it out before you responded, but thanks for the solution anyway.

[cole-miller]

More broadly, wrapping in Arc can never make anything Send or Sync that isn't already. Wrapping in Mutex or a similar type can add Sync, but only if Send is already present.