Clone a String for an async move closure in Rust

Question

Is there any way I can clone values for the async move closures for futures::stream?

I'm using future's for_each_concurrent

My code is something like this:

async fn foo() {
    use futures::stream::{self, StreamExt};

    let a: String = String::new();

    let stream = stream::once(async { 42 });

    stream
        .for_each_concurrent(None, |stream_item| async move {
            println!("{}", a);
            println!("{}", stream_item);
        })
        .await;
}

The error here:

error[E0507]: cannot move out of `a`, a captured variable in an `FnMut` closure
  --> src/main.rs:9:61
   |
4  |       let a: String = String::new();
   |           - captured outer variable
...
9  |           .for_each_concurrent(None, |stream_item| async move {
   |  _____________________________________________________________^
10 | |             println!("{}", a);
   | |                            -
   | |                            |
   | |                            move occurs because `a` has type `String`, which does not implement the `Copy` trait
   | |                            move occurs due to use in generator
11 | |             println!("{}", stream_item);
12 | |         })
   | |_________^ move out of `a` occurs here

I have to use move because of this:

error[E0373]: async block may outlive the current function, but it borrows `stream_item`, which is owned by the current function
  --> src/main.rs:9:56
   |
9  |           .for_each_concurrent(None, |stream_item| async {
   |  ________________________________________________________^
10 | |             println!("{}", a);
11 | |             println!("{}", stream_item);
   | |                            ----------- `stream_item` is borrowed here
12 | |         })
   | |_________^ may outlive borrowed value `stream_item`

If it was a loop I'd just clone as in each iteration, and move those clones inside a closure, is there a way to do something like this here?

Ibraheem Ahmed · Accepted Answer · 2021-03-01 21:28:46Z

15

You can simply clone a before the async move block:

stream
    .for_each_concurrent(None, |stream_item| {
        let a = a.clone();
        async move {
            println!("{}", a);
            println!("{}", stream_item);
        }
    })
    .await;

answered Mar 1, 2021 at 21:28

Ibraheem Ahmed

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