Fix type-inference in sink::unfold() by specifying more of its types

[sdroege]

This is an API change, at least if

1. Any code uses different types for the existing type parameters, in which case the Sink impl was not available
2. Any code specifies the type parameters (their number changed)

Without this it's impossible to use a "unnameable" type for the state, which happened to me unfortunately.

pub fn async_write<W: AsyncWrite + Unpin + Send>(
    write: W,
) -> impl Sink<Message>, Error = std::io::Error> + Send {
    struct State<W> {
        write: W,
        buffer: Vec<u8>,
    }

    let state = State {
        write,
        buffer: Vec::with_capacity(8192),
    };

    sink::unfold(state, |mut state, item: Message| {
        async move {
            state.buffer.clear();
            item.write(&mut state.buffer).expect("can't fail");

            match state.write.write_all(&state.buffer).await {
                Ok(_) => {
                    Ok(state)
                }
                Err(err) => {
                    Err(err)
                }
            }
        }
    })
}

Without the changes from this PR this fails with

error[E0282]: type annotations needed
   --> src/utils.rs:175:35
    |
175 |     futures::sink::unfold(state, |mut state, item: Message| {
    |                                   ^^^^^^^^^ consider giving this closure parameter a type
    |
    = note: type must be known at this point

Specifying it as state: State<_> is not sufficient and the actual type can't be named in this context.

With my changes it works as expected.

[taiki-e]

Thanks! I'll prepare a new release soon.

---

EDIT(2021-01-13)

1. Any code uses different types for the existing type parameters, in which case the Sink impl was not available

Technically this is definitely a breaking change, but it was accepted because Sink is sink::unfold's only important trait implementation, and the code broken by this change is believed to have never worked before.

2. Any code specifies the type parameters (their number changed)

RFC 1105 "Policy on semver and API evolution" considers the addition of type parameters as a minor change.

[sdroege]

Thanks!

[taiki-e]

Published in 0.3.10 (and yanked 0.3.9)