在异步 Function 参数中声明相关的特征类型 Object

Question

I'd like a function which asynchronously processes a variable amount of ( Sink , Stream ) tuples.

use futures::channel::mpsc;
use futures::{Sink, Stream, SinkExt, StreamExt};

async fn foo(v: Vec<(Box<dyn Sink<Error = std::io::Error>>, Box<dyn Stream<Item = u8>>)>) {
    for (mut tx, mut rx) in v {
        let _ = tx.send(0);
        let _ = rx.next().await;
    }
}

#[tokio::main]
pub async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let (tx, mut rx) = mpsc::channel(32);
    foo(vec![(Box::new(tx), Box::new(rx))]).await;
    
    Ok(())
}

But I get this compilation error:

 src/main.rs:4:30
  
 async fn foo(v: Vec<(Box<dyn Sink<Error = std::io::Error>>, Box<dyn Stream<Item = u8>>)>) {
                                

I was prompted to declare the associated type for the trait object that way by the compiler itself. I'm unsure why it does not accept it.

Answer 1

The compiler wants you to specify the "type argument" of the Sink . This is not the error type, but the type of the item being sent down the sink, as in Sink<Foo> . You specify u8 as the type of the stream, and are sending the value unchanged between one and the other, so you probably want a Sink<u8> .

Once you do that, the compiler will next complain that you need to specify the Error associated type (this time for real). However if you specify std::io::Error , the call to foo() from main() won't compile because the implementation of Sink for mpsc::Sender specifies its own mpsc::SendError as the error type.

Finally, both the sink and the stream need to be pinned so they can live across await points. This is done by using Pin<Box<...>> instead of Box<...> and Box::pin(...) instead of Box::new(...) .

With the above changes, a version that compiles looks like this:

use futures::channel::mpsc;
use futures::{Sink, SinkExt, Stream, StreamExt};
use std::pin::Pin;

async fn foo(
    v: Vec<(
        Pin<Box<dyn Sink<u8, Error = mpsc::SendError>>>,
        Pin<Box<dyn Stream<Item = u8>>>,
    )>,
) {
    for (mut tx, mut rx) in v {
        let _ = tx.send(0);
        let _ = rx.next().await;
    }
}

#[tokio::main]
pub async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let (tx, rx) = mpsc::channel(32);
    foo(vec![(Box::pin(tx), Box::pin(rx))]).await;

    Ok(())
}