rust - 使用线程和async/await时如何解决 "cannot return value referencing local data"？

Question

我正在学习 Rust，尤其是多线程和并行异步请求。

我阅读了文档，但仍然不明白我在哪里犯了错误。我假设我知道在哪里，但不知道如何解决它。

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"] }

Answer 1

像闭包一样，async block 尽可能弱地捕获它们的变量。按优先顺序:

1. 不可变引用
2. 可变引用
3. 按值(value)

这取决于变量在闭包/异步 block 中的使用方式。在您的示例中，request 仅通过引用使用，因此它仅通过引用捕获:

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

但是，您需要将变量的所有权转移到异步 block ，以便在最终执行 future 时变量仍然存在。与闭包一样，这是通过 move 关键字完成的:

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

另见:

• What is the difference between `|_| async move {}` and `async move |_| {}`
• Is there a way to have a Rust closure that moves only some variables into it?

---

在您的理解中，极不可能此时您想混合使用线程和异步。一种本质上是阻塞的，另一种期望代码不会阻塞。您应该遵循 How can I perform parallel asynchronous HTTP GET requests with reqwest? 中概述的示例相反。

另见:

• What is the best approach to encapsulate blocking I/O in future-rs?