try_flatten_stream() in futures-preview 0.3.0

[ebkalderon]

With futures 0.1, it was possible to have a Future that resolves to a Stream and use flatten_stream() to get access to the underlying stream, but if the Future resulted in an Error, the resulting stream would short-circuit and resolve to an Error as well.

When working with futures-preview 0.3.0, handling this situation is pretty cumbersome:

let future = async {
    let response = await!(client.get(uri))?;
    Ok(response.into_body())
};

// Worse
let stream = future
    .then(|result| match result {
        Ok(stream) => future::ready(Box::pin(stream) as Pin<Box<dyn Stream<Item = _> + Send>>),
        Err(err) => future::ready(Box::pin(stream::once(future::err(err))) as Pin<Box<dyn Stream<Item = _> + Send>>),
    }).flatten_stream();

// Slightly better
let stream = future
    .map_ok(|stream| Box::pin(stream) as Pin<Box<dyn Stream<Item = _> + Send>>)
    .unwrap_or_else(|err| Box::pin(stream::once(future::err(err))))
    .flatten_stream();

It would be nice if there was a TryFutureExt::try_flatten_stream() method which could simplify the code above like so:

let future = async {
    let response = await!(client.get(uri))?;
    Ok(response.into_body())
};

let stream = future.try_flatten_stream();