我正在尝试学习异步编程，但这个非常基本的示例不起作用:
I'm trying to learn async programming, but this very basic example doesn't work:
use std::future::Future; fn main() { let t = async { println!("Hello, world!"); }; t.poll(); }我从规范中读到的所有内容都说这应该可行，但是货物抱怨在impl std::future::Future"中找不到方法poll".我做错了什么?
Everything I've read from the specs says this should work, but cargo complains that method "poll" can't be found in "impl std::future::Future". What am I doing wrong?
推荐答案poll 有这个签名:
poll has this signature:
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output>;以您的方式调用它有两个问题:
There are two problems with calling this in the way you do:
poll 不是在未来的 Fut 上实现的，而是在 Pin 上实现的，所以你需要首先获得固定参考.pin_mut! 通常很有用，并且如果未来实现 Unpin，你可以使用Pin::new 也是.
poll is not implement on a future Fut, but on Pin<&mut Fut>, so you need to get a pinned reference first. pin_mut! is often useful, and if the future implements Unpin, you can use Pin::new as well.
然而，更大的问题是 poll 需要一个 &mut Context 参数.上下文由异步运行时创建并传递给最外层 future 的 poll 函数.这意味着您不能像那样轮询未来，您需要在异步运行时中才能做到这一点.
The bigger problem however is that poll takes a &mut Context<'_> argument. The context is created by the asynchronous runtime and passed in to the poll function of the outermost future. This means that you can't just poll a future like that, you need to be in an asynchronous runtime to do it.
相反，您可以使用像 tokio 这样的板条箱或 async-std 以运行未来同步上下文:
Instead, you can use a crate like tokio or async-std to run a future in a synchronous context:
// tokio use tokio::runtime::Runtime; let runtime = Runtime::new().unwrap(); let result = runtime.block_on(async { // ... }); // async-std let result = async_std::task::block_on(async { // ... })或者更好的是，您可以使用 #[tokio::main] 或 #[async_std::main] 将您的 main 函数转换为异步函数:
Or even better, you can use #[tokio::main] or #[async_std::main] to convert your main function into an asynchronous function:
// tokio #[tokio::main] async fn main() { // ... } // async-std #[async_std::main] async fn main() { // ... }更多推荐
Rust:方法“轮询"在 `impl std::future::Future` 中找不到
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