Arc<Mutex<T>> inside closure: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`

[guerlando]

I'm trying to share an Arc<Mutex<T>> with a closure. I get the error that closure is FnOncebecause it moves the variableshared_wake_deque_ out of its environment:

use core::task::{Context, Poll, Waker};
use hyper::client::connect::{Connection, Connected};
use std::sync::{Arc, Mutex};
use std::collections::VecDeque;

pub type OnPollRead = Arc<dyn Fn(&mut Context<'_>, &mut tokio::io::ReadBuf<'_>) -> Poll<std::io::Result<()>> + Send + Sync>;
pub type OnPollWrite = Arc<dyn Fn(&mut Context<'_>, &[u8]) -> Poll<core::result::Result<usize, std::io::Error>> + Send + Sync>;

#[derive(Clone)]
pub struct CustomTransporter {
    on_poll_read: Option<OnPollRead>,
    on_poll_write: Option<OnPollWrite>,
}

pub struct OVPNClient {
}

impl OVPNClient {
    pub fn send(&self, buffer: &[u8]) {
        
    }
}

unsafe impl Send for OVPNClient {}


unsafe impl Send for CustomTransporter {}

impl CustomTransporter {
    pub fn new(on_poll_read: Option<OnPollRead>, on_poll_write: Option<OnPollWrite>) -> CustomTransporter {
        CustomTransporter{
            on_poll_read: on_poll_read,
            on_poll_write: on_poll_write
        }
    }
}

fn main () {
    let openvpn_client = Arc::new(Mutex::new(OVPNClient{}));

    let shared_wake_deque = Arc::new(Mutex::new(VecDeque::<Waker>::new()));
    
    let shared_wake_deque_ = shared_wake_deque.clone();
    let on_poll_read = Arc::new(|context: &mut Context, buffer: &mut tokio::io::ReadBuf| -> Poll<std::io::Result<()>> {
        let shared_wake_deque__ = shared_wake_deque_;
        Poll::Ready(Ok(()))
    });

    let on_poll_write = Arc::new(|context: &mut Context, buffer: &[u8]| -> Poll<core::result::Result<usize, std::io::Error>>{
        openvpn_client.lock().unwrap().send(buffer);
        Poll::Ready(Ok(0))
    });

    let connector = CustomTransporter::new(Some(on_poll_read), Some(on_poll_write));
}

Error:

error[E0525]: expected a closure that implements the `Fn` trait, but this closure only implements `FnOnce`
  --> src/main.rs:44:33
   |
44 |     let on_poll_read = Arc::new(|context: &mut Context, buffer: &mut tokio::io::ReadBuf| -> Poll<std::io::Result<()>> {
   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ this closure implements `FnOnce`, not `Fn`
45 |         let shared_wake_deque__ = shared_wake_deque_;
   |                                   ------------------ closure is `FnOnce` because it moves the variable `shared_wake_deque_` out of its environment
...
54 |     let connector = CustomTransporter::new(Some(on_poll_read), Some(on_poll_write));
   |                                                 ------------ the requirement to implement `Fn` derives from here

I get that moving can only occur one time. I didn't even made the closure move. As you see, I'm trying to use a clone of the Arc<Mutex<T>> so I'm not exactly moving it. You can see that I tried cloning so I move a variable that is used just on this closure and never anymore, but it didn't work.

Rust Playground

[mbrubeck]

You can move the Arc into the closure, and then clone it again inside the closure if you need a new Arc each time the closure is called:

    let on_poll_read = Arc::new(move |context: &mut Context, buffer: &mut tokio::io::ReadBuf| -> Poll<std::io::Result<()>> {
        let shared_wake_deque__ = shared_wake_deque_.clone();
        Poll::Ready(Ok(()))
    });

Playground