预期的“async”块，发现了不同的“async”块

Question

我正在尝试使用

futures::future::select_ok

（游乐场）：

use std::time::Duration;
use tokio; // 1.16.1
use futures; // 0.3.19

#[tokio::main]
async fn main() {
    let first_future = async {
        tokio::time::sleep(Duration::from_secs(1)).await;
        Ok(3)
    };
    let second_future = async {
        tokio::time::sleep(Duration::from_millis(100)).await;
        Err(())
    };
    let third_future = async {
        tokio::time::sleep(Duration::from_secs(300)).await;
        Ok(3)
    };
    futures::future::select_ok(&[first_future,second_future.await,third_future]).await;
}

并遇到错误：

error[E0308]: mismatched types
  --> src/main.rs:19:47
   |
7  |       let first_future = async {
   |  ______________________________-
8  | |         tokio::time::sleep(Duration::from_secs(1)).await;
9  | |         Ok(3)
10 | |     };
   | |_____- the expected `async` block
...
19 |       futures::future::select_ok(&[first_future,second_future.await,third_future]).await;
   |                                                 ^^^^^^^^^^^^^^^^^^^ expected opaque type, found enum `Result`
   |
   = note: expected opaque type `impl futures::Future<Output = [async output]>`
                     found enum `Result<_, ()>`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

我不知道我在这里缺少什么，非常感谢任何帮助。

Answer 1

每个异步块都被视为不同的类型，就像闭包一样（IIRC）。它们都实现了

Future

，因此您可能希望实际动态调度它们。为此，您需要将它们包裹在

Pin<Box>

:

use futures;
use std::future::Future;
use std::pin::Pin;
use std::time::Duration;
use tokio; // 1.16.1 // 0.3.19

#[tokio::main]
async fn main() {
    let first_future = async {
        tokio::time::sleep(Duration::from_secs(1)).await;
        Ok(3)
    };
    let second_future = async {
        tokio::time::sleep(Duration::from_millis(100)).await;
        Err(())
    };
    let third_future = async {
        tokio::time::sleep(Duration::from_secs(300)).await;
        Ok(3)
    };
    let futures: [Pin<Box<dyn Future<Output = Result<usize, ()>>>>; 3] = [
        Box::pin(first_future),
        Box::pin(second_future),
        Box::pin(third_future),
    ];
    futures::future::select_ok(futures).await;
}

游乐场

Answer 2

这是对 @Netwave 的出色回答的补充：

如果有人希望避免

Pin<Box<dyn ..>>

所需的分配，请查看 enum_dispatch 或 smallbox 板条箱，它们支持堆栈分配不同大小的对象。