如何在 Tokio 中使用 async/await 语法？

Question

我正在尝试将 async/await 与 Rust 中的进程一起使用。我正在使用tokio 和tokio-process：

#![feature(await_macro, async_await, futures_api)]

extern crate tokio;
extern crate tokio_process;

use std::process::Command;
use tokio_process::CommandExt;

fn main() {
    tokio::run_async(main_async());
}

async fn main_async() {
    let out = Command::new("echo")
        .arg("hello")
        .arg("world")
        .output_async();
    let s = await!(out);
}

这是我得到的错误：

error[E0277]: the trait bound `tokio_process::OutputAsync: std::future::Future` is not satisfied
  --> src/main.rs:21:13
   |
21 |     let s = await!(out);
   |             ^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `tokio_process::OutputAsync`
   |
   = note: required by `std::future::poll_with_tls_waker`
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

error[E0277]: the trait bound `tokio_process::OutputAsync: std::future::Future` is not satisfied
  --> src/main.rs:21:13
   |
21 |     let s = await!(out);
   |             ^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `tokio_process::OutputAsync`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

我怎样才能做到这一点？

Answer 1

TL;DR：使用 Tokio 0.2 或更高版本，它应该可以正常工作。

Tokio 0.1 和相关的 crate 是使用 futures 0.1 crate 实现的。此 crate 中的 Future 特征在概念上类似于标准库中的 Future 特征版本，但在细节上有很大不同。 async / await 语法是围绕标准库中的 trait 版本构建的。

Tokio 0.2 和相关 crates 使用标准库 Future 实现，并经过重新设计以更好地支持 async / await 语法。

东京 0.2

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

use tokio::process::Command;

#[tokio::main]
async fn main() -> std::io::Result<()> {
    let out = Command::new("echo")
        .arg("hello")
        .arg("world")
        .output()
        .await?;

    let s = String::from_utf8_lossy(&out.stdout);

    println!("{}", s);
    Ok(())
}

$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.05s
     Running `target/debug/tokio2`
hello world

测试：

• Rustc 1.39.0

东京 0.1

你必须translate between a Future implemented against the trait from the futures 0.1 crate and from the standard library。此外，您需要启动 Tokio 运行时服务，理想情况下适用于 Tokio 0.1 和 Tokio 0.3。这就是tokio-compat 的用武之地：

[dependencies]
futures = { version = "0.3", features = ["compat"] }
tokio-compat = "0.1"
tokio-process = "0.2"

use futures::compat::Future01CompatExt;
use std::process::Command;
use tokio_process::CommandExt;

fn main() {
    tokio_compat::run_std(main_async());
}

async fn main_async() {
    let out = Command::new("echo")
        .arg("hello")
        .arg("world")
        .output_async();

    // Convert future from 0.1 to 0.3
    let s = out.compat().await;

    println!("{:?}", s);
}

% cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/tt`
Ok(Output { status: ExitStatus(ExitStatus(0)), stdout: "hello world\n", stderr: "" })

用 Rust 1.43.0 测试

另见：

• How do I convert an async / standard library future to futures 0.1?
• Is there a way that we can convert from futures 0.1 to the standard library futures?
• Why is a trait not implemented for a type that clearly has it implemented?