如何指定绑定在作为结果的关联类型上的特征？

Question

I'm trying to write a function that can take a Stream that returns Result s. The trouble is in specifying boundaries for the success values of the Result .

This is an example that illustrates the problem (yes, it won't run without an async runtime, but that's not really the issue at hand):

use std::{pin::Pin, task::{Context, Poll}};
use futures::{Stream, StreamExt};

struct Data;

impl Stream for Data {
    type Item = Result<String, std::io::Error>;

    #[inline]
    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        Poll::Ready(Some(Ok("Hello".to_owned())))
    }
}

async fn handle_stream<S>(mut stream: S) where S: Stream + Unpin, <S as Stream>::Item: AsRef<str> {
    while let Some(item) = stream.next().await {
        // TODO
    }
}

fn main() {
    let mut data = Data{};
    async move {
    handle_stream(data).await;
    };
}

The compiler error below makes sense, since Result does not implement AsRef :

error[E0277]: the trait bound `Result<String, std::io::Error>: AsRef<str>` is not satisfied
  --> src/main.rs:24:5
   |
24 |     handle_stream(data).await;
   |     ^^^^^^^^^^^^^ the trait `AsRef<str>` is not implemented for `Result<String, std::io::Error>`
   |
note: required by a bound in `handle_stream`
  --> src/main.rs:15:88
   |
15 | async fn handle_stream<S>(mut stream: S) where S: Stream + Unpin, <S as Stream>::Item: AsRef<str> {
   |                                                                                        ^^^^^^^^^^ required by this bound in `handle_stream`

For more information about this error, try `rustc --explain E0277`.

But my attempts to specify that the associated Item for S should actually be a Result fail, since Result is not a trait:

error[E0404]: expected trait, found enum `Result`
  --> src/main.rs:15:88
   |
15 | async fn handle_stream<S>(mut stream: S) where S: Stream + Unpin, <S as Stream>::Item: Result<AsRef<str>> {
   |                                                                                        ^^^^^^^^^^^^^^^^^^ not a trait

For more information about this error, try `rustc --explain E0404`.

How can I specify that I want Stream s that return Result s that have AsRef<str> values? I realize I will likely need to specify something for the Err type as well; assuming that's a similar syntax.

Answer 1

The syntax for equating associated types is Trait<Assoc = Type> .

So:

async fn handle_stream<S, I, E>(mut stream: S)
where
    S: Stream<Item = Result<I, E>> + Unpin,
    I: AsRef<str>,
{
    // ...
}