Sending to async method seems to lead to an impossible move

1

I have the following simplified sample producing a problem with moving a clone of a string to a closure. Somehow my usual cloning, Arc/Mutex use did not help here either. So I am a bit puzzled what could be a possible resolution here. Any pointers on further reading etc?

use futures::Future;

struct A {
    v:String,
}

impl A {
    pub async fn m(&self) {
    let cl=self.v.clone();
    b(|s| async move {
        let cl2=cl.clone();
        println!("Hello{}{}",cl2,s);
    });
}
}

#[tokio::main]
async fn main() {
    let a=A { v: String::from(" ") };
    a.m().await;
    println!("Later use: HELLO{}WORLD!",a.v);
}

async fn b<F,G>(f:F) 
where F: Fn(String) -> G + Send + 'static + Sync + Clone,
      G: Future<Output=()> + Send + 'static,
{
    f(String::from("World!")).await
}

The error message:

   Compiling playground v0.0.1 (/playground)
error[E0507]: cannot move out of `cl`, a captured variable in an `Fn` closure
  --> src/main.rs:11:22
   |
10 |       let cl=self.v.clone();
   |           -- captured outer variable
11 |       b(|s| async move {
   |  _______---____________^
   | |       |
   | |       captured by this `Fn` closure
12 | |         let cl2=cl.clone();
   | |                 --
   | |                 |
   | |                 variable moved due to use in generator
   | |                 move occurs because `cl` has type `String`, which does not implement the `Copy` trait
13 | |         println!("Hello{}{}",cl2,s);
14 | |     });
   | |_____^ move out of `cl` occurs here

For more information about this error, try `rustc --explain E0507`.
error: could not compile `playground` due to previous error
2

It seems that you want FnOnce(), not Fn() for the trait bound. (You're also missing an .await, but that's irrelevant from the perspective of this error.) Playground.

3

The trait bound comes from a lib unfortunately, so I cannot change that. The lib requires an Fn as it plans to call my method multiple times in a background thread.

4

In this case, you have to move the original string into the closure, clone it outside the async block, then move the clone into the block:

impl A {
    pub async fn m(&self) {
        let cl = self.v.clone();
        b(move |s| {
            let cl2 = cl.clone();
            async move {
                println!("Hello{}{}", cl2, s);
            }
        }).await;
    }
}

Playground

2 Likes
5

Thank you! This is definitely working now I am just puzzled why... I would not even have guessed that this would be ok as an async function.

6

Which part did you think was unsuitable for an async function?

7

I thought that you have had to do async next to the || to make the closure async.

8

No, it's not "making the closure async". That's merely an async block inside a perfectly regular closure. (The type annotations of a closure are only of the form |args: Types| -> RetType; everything else beyond that is part of the body/return value.)

"Async" from the point of the type system isn't really special, it just means that the expression has type impl Future<Output = …>, and closures have no async declaration. (A proper async |…| closure form is being developed on nightly, if one can believe the compiler.)

1 Like
9

Thanks for the explanation, it definitely helps. Not to mention the github issue linked by rustc when compiling your code in playground.

10

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