rust - 如何等待一个盒装 future 的结果？

Question

use futures::{future, Future};

fn test() -> Box<dyn Future<Output = bool>> {
    Box::new(future::ok::<bool>(true))
}

async fn async_fn() -> bool {
    let result: bool = test().await;
    return result;
}

fn main(){
    async_fn();
    println!("Hello!");
}

Playground
错误:

error[E0277]: the trait bound `dyn core::future::future::Future<Output = bool>: std::marker::Unpin` is not satisfied
  --> src/main.rs:8:24
   |
8  |     let result: bool = test().await;
   |                        ^^^^^^^^^^^^ the trait `std::marker::Unpin` is not implemented for `dyn core::future::future::Future<Output = bool>`
   |
   = note: required because of the requirements on the impl of `core::future::future::Future` for `std::boxed::Box<dyn core::future::future::Future<Output = bool>>`

Answer 1

根据 the implementation :

impl<F> Future for Box<F>
where
    F: Unpin + Future + ?Sized, 

盒装 future 只执行 Future trait 当 future 里面 Box工具 Unpin .
由于您的函数不保证返回的 future 实现 Unpin ，您的返回值将被视为未实现 Future .您将无法 await因为你的类型基本上不是 Future .
@Stargateur 的解决方案，向签名添加显式类型边界，有效( Playground ):

fn test() -> Box<dyn Future<Output = Result<bool, ()>> + Unpin> 

如果你使用的是 futures-rs，有一个辅助类型 BoxFuture .您可以使用 BoxedFuture没有明确说明 Unpin :

use futures::future::BoxFuture;

fn test() -> BoxFuture<'static, Result<bool, ()>> {
    Box::pin(async { Ok(true) })
}

Playground