使用TcpConnectionNew时，不满足特征绑定`():futures :: Future` [英] The trait bound `(): futures::Future` is not satisfied when using TcpConnectionNew

问题描述

我正在尝试使用 Tokio 板条箱在Rust中编写一个简单的TCP客户端.我的代码非常接近此示例，减去了TLS:

I am trying to write a simple TCP client in Rust using Tokio crate. My code is pretty close to this example minus the TLS:

extern crate futures;
extern crate tokio_core;
extern crate tokio_io;

use futures::Future;
use tokio_core::net::TcpStream;
use tokio_core::reactor::Core;
use tokio_io::io;

fn main() {
    let mut core = Core::new().unwrap();
    let handle = core.handle();

    let connection = TcpStream::connect(&"127.0.0.1:8080".parse().unwrap(), &handle);

    let server = connection.and_then(|stream| {
        io::write_all(stream, b"hello");
    });

    core.run(server).unwrap();
}

但是，编译失败并显示以下错误:

However, compilation fails with the error:

error[E0277]: the trait bound `(): futures::Future` is not satisfied
  --> src/main.rs:16:29
   |
16 |     let server = connection.and_then(|stream| {
   |                             ^^^^^^^^ the trait `futures::Future` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `futures::IntoFuture` for `()`

error[E0277]: the trait bound `(): futures::Future` is not satisfied
  --> src/main.rs:20:10
   |
20 |     core.run(server).unwrap();
   |          ^^^ the trait `futures::Future` is not implemented for `()`
   |
   = note: required because of the requirements on the impl of `futures::IntoFuture` for `()`

我觉得很奇怪，因为根据文档应该实现.

I find it strange because according to the documentation it should be implemented.

我正在使用

• 铁锈1.19.0
• 未来0.1.16
• tokio-core 0.1.10
• tokio-io 0.1.3

我想念什么?

推荐答案

TL; DR:在io::write_all之后删除分号.

TL;DR: remove the semicolon after io::write_all.

查看 and_then :

fn and_then<F, B>(self, f: F) -> AndThen<Self, B, F> 
where
    F: FnOnce(Self::Item) -> B,
    B: IntoFuture<Error = Self::Error>,
    Self: Sized, 

闭包(F)必须返回某种类型(B)，该类型可以转换为将来的类型(B: IntoFuture)，其错误类型与起始闭包(Error = Self::Error)相匹配.

The closure (F) has to return some type (B) that can be converted into a future (B: IntoFuture) with an error type that matches the starting closure (Error = Self::Error).

您的闭包返回什么? ().这是为什么?因为您已经在行的末尾放置了分号(;). ()不能实现特征IntoFuture ，由错误消息部分在()的futures::IntoFuture的impl上"指示:

What does your closure return? (). Why is that? Because you've placed a semicolon (;) at the end of your line. () does not implement the trait IntoFuture, which is indicated by the error message part "on the impl of futures::IntoFuture for ()":

impl<F: Future> IntoFuture for F {
    type Future = F;
    type Item = F::Item;
    type Error = F::Error;
}

删除分号会使io::write_all返回的Future返回到and_then，并且程序将编译.

Removing the semicolon will cause the Future returned by io::write_all to be returned back to and_then and the program will compile.

通常，期货通过将本身就是期货的较小部分组合在一起而起作用.所有这些共同作用，将建立一个单一的大型未来，这实质上是一个状态机.请牢记这一点，因为使用此类组合器时几乎总是需要返回将来.

In general, futures work by combining together smaller components which are themselves futures. All of this works together to build a single large future which is essentially a state machine. It's good to keep this in mind, as you will almost always need to return a future when using such combinators.

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