为什么在使用 TryFuture 而不是等效的 Future 时会收到有关类型不匹配的错误？

Question

我有以下使用 Future.

的工作代码

use futures::{future, Future};

fn fut_res() -> impl Future<Output = Result<(), failure::Error>> {
    future::ok::<(), failure::Error>(())
}

#[tokio::main]
async fn main() -> Result<(), failure::Error> {
    if let Err(e) = fut_res().await {
        println!("{:?}", e);
    }

    Ok(())
}

根据我在文档中阅读的内容，我应该能够更改代码以使用 TryFuture，如下所示：

use futures::{future, TryFuture};

fn try_fut() -> impl TryFuture<Ok = (), Error = failure::Error> {
    future::ok::<(), failure::Error>(())
}

#[tokio::main]
async fn main() -> Result<(), failure::Error> {
    if let Err(e) = try_fut().await {
        println!("{:?}", e);
    }

    Ok(())
}

编译器抱怨 try_fut 必须 return 关联类型 Output，但根据定义，此类型是 Result<(), failure::Error> :

error[E0308]: mismatched types
 --> src/lib.rs:9:12
  |
9 |     if let Err(e) = try_fut().await {
  |            ^^^^^^ expected associated type, found enum `std::result::Result`
  |
  = note: expected type `<impl futures_core::future::TryFuture as core::future::future::Future>::Output`
             found type `std::result::Result<_, _>`
  = note: consider constraining the associated type `<impl futures_core::future::TryFuture as core::future::future::Future>::Output` to `std::result::Result<_, _>` or calling a method that returns `<impl futures_core::future::TryFuture as core::future::future::Future>::Output`
  = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html

如何向编译器解释这一点？

Playground 不起作用，因为 tokio 仍处于 alpha 阶段，但也有一个合适的 Cargo.toml.

Answer 1

您的语法目前无效。 TryFuture 的文档显示您必须暂时将 Result 包装在 Poll 中（重点是我的）：

fn try_poll(
    self: PinMut<Self>, 
    cx: &mut Context ) -> Poll<Result<Self::Ok, Self::Error>> [−]
Poll this TryFuture as if it were a Future.

This method is a stopgap for a compiler limitation that prevents us from directly inheriting from the Future trait; in the future it won't be needed.

playground

解决此编译器限制后，将不再需要此方法，您将能够做您所做的事情（或类似的事情）

为什么在使用 TryFuture 而不是等效的 Future 时会收到有关类型不匹配的错误？

Why do I get an error about mismatched types when using TryFuture instead of the equivalent Future?

future

rust

参考资料