How do I solve "cannot return value referencing local data" when using threads and async/await?

Question

I am learning Rust especially multithreading and async requests in parallel.

I read the documentation and still I do not understand where I made a mistake. I assume I know where, but do not see how to resolve it.

main.rs

use std::thread;

struct Request {
    url: String,
}

impl Request {
    fn new(name: &str) -> Request {
        Request {
            url: name.to_string(),
        }
    }

    async fn call(&self, x: &str) -> Result<(), Box<dyn std::error::Error>> {
        let resp = reqwest::get(x).await;
        Ok(())
    }
}

#[tokio::main]
async fn main() {
    let requests = vec![
        Request::new("https://www.google.com/"),
        Request::new("https://www.google.com/"),
    ];
    let handles: Vec<_> = requests
        .into_iter()
        .map(|request| {
            thread::spawn(move || async {
                request.call(&request.url).await;
            })
        })
        .collect();

    for y in handles {
        println!("{:?}", y);
    }
}

error[E0515]: cannot return value referencing local data `request`
  --> src/main.rs:29:35
   |
29 |               thread::spawn(move || async {
   |  ___________________________________^
30 | |                 request.call(&request.url).await;
   | |                 ------- `request` is borrowed here
31 | |             })
   | |_____________^ returns a value referencing data owned by the current function

Cargo.toml

[dependencies]
reqwest = "0.10.4"
tokio = { version = "0.2", features = ["full"] }

Shepmaster · Accepted Answer · 2020-03-30T19:57:06.287

Like closures, async blocks capture their variables as weakly as possible. In order of preference:

immutable reference
mutable reference
by value

This is determined by how the variable is used in the closure / async block. In your example, request is only used by reference, so it is only captured by reference:

async {
    request.call(&request.url).await;
}

However, you need to transfer ownership of the variable to the async block so that the variable is still alive when the future is eventually executed. Like closures, this is done via the move keyword:

thread::spawn(move || async move {
    request.call(&request.url).await;
})

How do I solve "cannot return value referencing local data" when using threads and async/await?

1 Answers1