Add a poll_read_buf shim to tokio-util to mimic now deprecated AsyncRead API

[udoprog]

While working to migrate a project to 0.3.0 I found uses of the now deprecated poll_read_buf API that needed to be migrated. This turned out to be nontrivial for types implementing BufMut which does not immediately provide an API to access an underlying contiguous buffer like Chain.

This adds a shim for it which provides the old API to ease migrations to 0.3.0. The naming and shape of the API is preliminary and I'll happily move and rename it to whatever is appropriate if it warrants inclusion. At the very least I'm putting it here in the hope so that people who are currently migrating might find it. It might also be a good idea to provide it as a library function since the generic implementation involves a sprinkle of unsafe it would be a shame for people to have to replicate.

[Darksonn]

Thanks! I would like to see a function with this signature too:

pub async fn read_buf<R, B>(
    read: &mut R,
    buf: &mut B,
) -> io::Result<usize>
where
    R: AsyncRead + Unpin,
    B: BufMut,
{

It can just internally call poll_read_buf.

EDIT: Added async to signature.

[udoprog]

Thanks! I would like to see a function with this signature too:

pub fn read_buf<R, B>(
    read: &mut R,
    buf: &mut B,
) -> io::Result<usize>
where
    R: AsyncRead + Unpin,
    B: BufMut,
{

It can just internally call poll_read_buf.

So I'm guessing this should either take a Context argument or be async?

[udoprog]

Note: For whatever reason cargo fmt is not working on Windows, so I have to run rustfmt over all modified source files like the CI test does it.

[Darksonn]

Yeah, it should be async. The cargo fmt issue is well known and described in CONTRIBUTING.md, not specific to windows, and I think it has to do with the feature flags.

[udoprog]

Added read_buf helper fn per suggestion.

Couldn't find any non-dev dependency poll_fn, so implemented the future by hand which was pretty straight forward since it's Unpin.

[Darksonn]

I think this is more or less ready.