无法执行tokio :: boxed Future，因为绑定了Send的特征不满足 [英] Unable to tokio::run a boxed Future because the trait bound Send is not satisfied

问题描述

我有一个函数，根据参数的不同，应该可以选择将来运行还是不执行任何操作.我尝试在要返回的两个期货周围放置一个Box，一个立即解析为Ok(())的tokio::prelude::future::Done<Item=(), Error=()>，以及一个我正在使用and_then和map_err将两个Item和Error至().当我尝试使用tokio::run运行期货时，这似乎对我不起作用.

I have a function that should optionally run a future or do nothing, depending on parameters. I tried putting a Box around the two futures that will be returned, a tokio::prelude::future::Done<Item=(), Error=()> that immediately resolves to Ok(()), and a tokio::timer::Delay that I'm using and_then and map_err to turn both the Item and Error to (). This doesn't seem to work for me when I try to run the futures with tokio::run.

extern crate tokio;

use std::time::{Duration, Instant};
use tokio::prelude::*;
use tokio::timer;

fn main() {
    tokio::run(foo(12));
}

fn foo(x: i32) -> Box<Future<Item = (), Error = ()>> {
    if x == 0 {
        Box::new(
            timer::Delay::new(Instant::now() + Duration::from_secs(5))
                .and_then(|_| Ok(()))
                .map_err(|_| ()),
        )
    } else {
        Box::new(future::result(Ok(())))
    }
}

这无法编译并显示以下错误消息:

This fails to compile with the following error message:

error[E0277]: the trait bound `tokio::prelude::Future<Error=(), Item=()>: std::marker::Send` is not satisfied
 --> src/main.rs:8:5
  |
8 |     tokio::run(foo(12));
  |     ^^^^^^^^^^ `tokio::prelude::Future<Error=(), Item=()>` cannot be sent between threads safely
  |
  = help: the trait `std::marker::Send` is not implemented for `tokio::prelude::Future<Error=(), Item=()>`
  = note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<tokio::prelude::Future<Error=(), Item=()>>`
  = note: required because it appears within the type `std::boxed::Box<tokio::prelude::Future<Error=(), Item=()>>`
  = note: required by `tokio::run`

看来Box<Future...>没有实现Send，这对我来说没有意义.由于我要返回的Future类型都实现了Send，所以在我看来Box应该应该这样做，因为impl Send for Box<T> where T: Send是stdlib中的自动实现.我在这里想念什么?

It appears that Box<Future...> does not implement Send, which doesn't make sense to me. Since the Future types that I'm returning both implement Send, it seems to me that the Box should, since impl Send for Box<T> where T: Send is an auto implement in the stdlib. What am I missing here?

推荐答案

我意识到我需要在Foo的返回类型中指定将来是Send.编译:

I realized that I need to specify in the return type to Foo that the future is Send. This compiles:

extern crate tokio;

use std::time::{Duration, Instant};
use tokio::prelude::*;
use tokio::timer;

fn main() {
    tokio::run(foo(12));
}

fn foo(x: i32) -> Box<Future<Item = (), Error = ()> + Send> { // note the + Send at the end of this line
    if x == 0 {
        Box::new(
            timer::Delay::new(Instant::now() + Duration::from_secs(5))
                .and_then(|_| Ok(()))
                .map_err(|_| ()),
        )
    } else {
        Box::new(future::result(Ok(())))
    }
}

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