为什么我的匹配块中需要 Send 特性？

Question

我认为我的问题与 Rust Issue 57017 有关。

以下 code 无法编译并生成 error: future cannot be sent between threads safely，因为 future created by async block is not 'Send' 源自 async fn execute_fail。另一方面，async fn execute_work 编译并具有与 execute_fail 相同的功能和逻辑，但方式不太优雅。

async fn stream() -> Result<(), Box<dyn std::error::Error>> {
    Ok(())
}

async fn process() {}

async fn execute_fail() -> Result<(), Box<dyn std::error::Error>> {
    match stream().await {
        Err(err) => {
            return Err(err);
        }
        Ok(()) => {
            process().await;
        }
    }
    Ok(())
}

async fn execute_work() -> Result<(), Box<dyn std::error::Error>> {
    let mut is_success = false;
    match stream().await {
        Err(err) => {
            return Err(err);
        }
        Ok(()) => {
            is_success = true;
        }
    }

    if is_success {
        process().await;
    }
    Ok(())
}

async fn my_fn() {
    tokio::spawn(async {
        if let Err(err) = execute_fail().await {
            panic!("error here");
        }
    });

    tokio::spawn(async {
        if let Err(err) = execute_work().await {
            panic!("error here");
        }
    });
}

给出：

error: future cannot be sent between threads safely
   --> src/lib.rs:39:5
    |
39  |     tokio::spawn(async {
    |     ^^^^^^^^^^^^ future created by async block is not `Send`
    | 
   ::: /playground/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.11.0/src/task/spawn.rs:127:21
    |
127 |         T: Future + Send + 'static,
    |                     ---- required by this bound in `tokio::spawn`
    |
    = help: the trait `Send` is not implemented for `dyn std::error::Error`
note: future is not `Send` as this value is used across an await
   --> src/lib.rs:15:13
    |
10  |     match stream().await {
    |           -------------- has type `Result<(), Box<dyn std::error::Error>>` which is not `Send`
...
15  |             process().await;
    |             ^^^^^^^^^^^^^^^ await occurs here, with `stream().await` maybe used later
16  |         }
17  |     }
    |     - `stream().await` is later dropped here

我想解释为什么 execute_fail 不像 execute_work 那样编译。编译器注释 note: future is not Send as this value is used across an await 对我来说没有意义，因为我看不到任何值是如何在等待中使用的。

Answer 1

I would like an explanation why execute_fail does not compile where as execute_work does.

当您 match stream().await 时，它基本上是：

let v = stream().await;
match v {
   ...
}

因此 v 在整个匹配过程中都处于活动状态，这意味着就编译器而言，它的“生命”与 process().await 调用重叠。值必须是 Send 才能通过 await 调用，因为协程可能会在与暂停不同的线程上恢复。

注意：这里的代码可以大大简化：

async fn execute_fail() -> Result<(), Box<dyn std::error::Error>> {
    if let Err(err) = stream().await {
        return Err(err);
    }
    process().await;
    Ok(())
}

甚至

async fn execute_fail() -> Result<(), Box<dyn std::error::Error>> {
    stream().await?;
    process().await;
    Ok(())
}

为什么我的匹配块中需要 Send 特性？

Why is the Send trait required inside my match block?

lifetime

rust

rust-tokio

rust-async-std