我有一个使用组合器的流，我需要运行它直到完成。我可以使用while循环或for_each组合器。它们都可以工作，但我认为一定有更好的方法。
Sink看起来像我正在寻找的，尤其是sink::drain()，但我还不知道如何使用它。
使用while循环

use futures::{StreamExt, TryStreamExt}; // 0.3.6
use tokio; // 0.3.0

#[tokio::main]
async fn main() {
    let mut stream = Box::pin(
        futures::stream::iter(0..20)
            .map(foo)
            .map_ok(|x| x * 10)
            .and_then(bar)
            .filter(|x| futures::future::ready(x.is_ok())),
    );

    while let Some(_) = stream.next().await {
        // Nothing to do here. I just need to run stream.
    }
}

fn foo(x: i32) -> Result<i32, String> {
    if x != 10 {
        Ok(x)
    } else {
        Err("eeer".to_string())
    }
}

async fn bar(x: i32) -> Result<(), String> {
    async {
        if x == 13 {
            Err("errr".to_string())
        } else {
            Ok(())
        }
    }
    .await
}

使用for_each：

use futures::{StreamExt, TryStreamExt}; // 0.3.6
use tokio; // 0.3.0

#[tokio::main]
async fn main() {
    futures::stream::iter(0..20)
        .map(foo)
        .map_ok(|x| x * 10)
        .and_then(bar)
        .filter(|x| futures::future::ready(x.is_ok()))
        .for_each(|_| futures::future::ready(())) // Nothing to do here, just to run stream
        .await;
}

fn foo(x: i32) -> Result<i32, String> {
    if x != 10 {
        Ok(x)
    } else {
        Err("eeer".to_string())
    }
}

async fn bar(x: i32) -> Result<(), String> {
    async {
        if x == 13 {
            Err("errr".to_string())
        } else {
            Ok(())
        }
    }
    .await
}

我希望有如下的代码，不需要使用drain组合子，只需要使用一些组合子来运行流：

use futures::{StreamExt, TryStreamExt}; // 0.3.6
use tokio; // 0.3.0

#[tokio::main]
async fn main() {
    futures::stream::iter(0..20)
        .map(foo)
        .map_ok(|x| x * 10)
        .and_then(bar)
        .filter(|x| futures::future::ready(x.is_ok()))
        .forward(futures::sink::drain())
        .await;
}

fn foo(x: i32) -> Result<i32, String> {
    if x != 10 {
        Ok(x)
    } else {
        Err("eeer".to_string())
    }
}

async fn bar(x: i32) -> Result<(), String> {
    async {
        if x == 13 {
            Err("errr".to_string())
        } else {
            Ok(())
        }
    }
    .await
}

这不起作用，可能是因为drain在Error类型上设置了一些界限：

error[E0271]: type mismatch resolving `<futures::sink::Drain<()> as futures::Sink<()>>::Error == std::string::String`
  --> src/main.rs:11:10
   |
11 |         .forward(futures::sink::drain())
   |          ^^^^^^^ expected enum `std::convert::Infallible`, found struct `std::string::String`

error[E0271]: type mismatch resolving `<futures::stream::Filter<futures::stream::AndThen<futures::stream::MapOk<futures::stream::Map<futures::stream::Iter<std::ops::Range<i32>>, fn(i32) -> std::result::Result<i32, std::string::String> {foo}>, [closure@src/main.rs:8:17: 8:27]>, impl futures::Future, fn(i32) -> impl futures::Future {bar}>, futures::future::Ready<bool>, [closure@src/main.rs:10:17: 10:54]> as futures::Stream>::Item == std::result::Result<(), std::convert::Infallible>`
  --> src/main.rs:6:5
   |
6  | /     futures::stream::iter(0..20)
7  | |         .map(foo)
8  | |         .map_ok(|x| x * 10)
9  | |         .and_then(bar)
10 | |         .filter(|x| futures::future::ready(x.is_ok()))
11 | |         .forward(futures::sink::drain())
12 | |         .await;
   | |______________^ expected struct `std::string::String`, found enum `std::convert::Infallible`
   |
   = note: expected enum `std::result::Result<_, std::string::String>`
              found enum `std::result::Result<_, std::convert::Infallible>`
   = note: required because of the requirements on the impl of `futures::Future` for `futures_util::stream::stream::forward::Forward<futures::stream::Filter<futures::stream::AndThen<futures::stream::MapOk<futures::stream::Map<futures::stream::Iter<std::ops::Range<i32>>, fn(i32) -> std::result::Result<i32, std::string::String> {foo}>, [closure@src/main.rs:8:17: 8:27]>, impl futures::Future, fn(i32) -> impl futures::Future {bar}>, futures::future::Ready<bool>, [closure@src/main.rs:10:17: 10:54]>, futures::sink::Drain<()>, ()>`