Add StreamReader to io module

[Darksonn]

Allow conversion from a stream of chunks of bytes to an AsyncRead.

Motivation

Http crates such as hyper and reqwest provide their response as a stream of chunks of bytes, which is not easily convertible into a type that implements AsyncRead.

This type was originally motivated when helping someone on the official Rust discord.

Solution

This PR provides a new type called IntoAsyncRead that buffers the last chunk received from the stream, and implements both AsyncRead and AsyncBufRead on this new type.

The futures crate has a very similar type, however that type operates on the AsyncRead trait from the futures crate as opposed to the tokio version of the trait.

[carllerche]

At this point, I’m a bit hesitant to add more traits right now related to async-io given the current uncertainty re: future.

How would this trait be used in practice?

[Darksonn]

That's fair enough. The main use case (and what originally prompted this) is when reading a response provided by e.g. hyper.

[carllerche]

For some reason, I thought this was a trait... i should have paid more attention.

I think an adapter is good. As I mentioned on discord, some thoughts:

• place it in io as stream is more abstract than "io".
• The constructor should be a free fn to match stream::iter.
• Generic over T: AsRef<[u8]>
• I would avoid AsyncRead in the name.

Maybe something like:

fn byte_stream<T: ...>(stream: T) -> ByteStream<T> { ... }

I don't think ByteStream is great so I would love to hear other naming suggestions.

[Darksonn]

Regarding being generic over T: AsRef<[u8]>, notice that this actually somewhat complicates the implementation, as you need both an option around the chunk, and an index to track how much of the chunk you've read.

A few naming suggestions: ByteStreamReader, ChunkStream, ChunkStreamReader, ChunkedStream, ChunkReader.

[carllerche]

tokio::io::StreamReader is probably best. Assuming it is generic over T: Buf, i think that will be the only adapter in tokio::io from T: Stream.

[LucioFranco]

@carllerche ping

[carllerche]

Looks great