futures-util 0.3.5

use core::fmt;
use core::marker::PhantomData;
use core::pin::Pin;
use futures_core::future::Future;
use futures_core::stream::Stream;
use futures_core::task::{Context, Poll};
use futures_sink::Sink;
use pin_project::{pin_project, project};

/// Sink for the [`with`](super::SinkExt::with) method.
#[pin_project]
#[must_use = "sinks do nothing unless polled"]
pub struct With<Si, Item, U, Fut, F> {
    #[pin]
    sink: Si,
    f: F,
    #[pin]
    state: Option<Fut>,
    _phantom: PhantomData<fn(U) -> Item>,
}

impl<Si, Item, U, Fut, F> fmt::Debug for With<Si, Item, U, Fut, F>
where
    Si: fmt::Debug,
    Fut: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("With")
            .field("sink", &self.sink)
            .field("state", &self.state)
            .finish()
    }
}

impl<Si, Item, U, Fut, F> With<Si, Item, U, Fut, F>
where Si: Sink<Item>,
      F: FnMut(U) -> Fut,
      Fut: Future,
{
    pub(super) fn new<E>(sink: Si, f: F) -> Self
        where
            Fut: Future<Output = Result<Item, E>>,
            E: From<Si::Error>,
    {
        With {
            state: None,
            sink,
            f,
            _phantom: PhantomData,
        }
    }
}

// Forwarding impl of Stream from the underlying sink
impl<S, Item, U, Fut, F> Stream for With<S, Item, U, Fut, F>
    where S: Stream + Sink<Item>,
          F: FnMut(U) -> Fut,
          Fut: Future
{
    type Item = S::Item;

    delegate_stream!(sink);
}

impl<Si, Item, U, Fut, F, E> With<Si, Item, U, Fut, F>
    where Si: Sink<Item>,
          F: FnMut(U) -> Fut,
          Fut: Future<Output = Result<Item, E>>,
          E: From<Si::Error>,
{
    delegate_access_inner!(sink, Si, ());

    /// Completes the processing of previous item if any.
    #[project]
    fn poll(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), E>> {
        #[project]
        let With { mut state, sink, .. } = self.project();

        let item = match state.as_mut().as_pin_mut() {
            None => return Poll::Ready(Ok(())),
            Some(fut) => ready!(fut.poll(cx))?,
        };
        state.set(None);
        sink.start_send(item)?;
        Poll::Ready(Ok(()))
    }
}

impl<Si, Item, U, Fut, F, E> Sink<U> for With<Si, Item, U, Fut, F>
    where Si: Sink<Item>,
          F: FnMut(U) -> Fut,
          Fut: Future<Output = Result<Item, E>>,
          E: From<Si::Error>,
{
    type Error = E;

    fn poll_ready(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>> {
        ready!(self.as_mut().poll(cx))?;
        ready!(self.project().sink.poll_ready(cx)?);
        Poll::Ready(Ok(()))
    }

    #[project]
    fn start_send(
        self: Pin<&mut Self>,
        item: U,
    ) -> Result<(), Self::Error> {
        #[project]
        let With { mut state, f, .. } = self.project();

        assert!(state.is_none());
        state.set(Some(f(item)));
        Ok(())
    }

    fn poll_flush(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>> {
        ready!(self.as_mut().poll(cx))?;
        ready!(self.project().sink.poll_flush(cx)?);
        Poll::Ready(Ok(()))
    }

    fn poll_close(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>> {
        ready!(self.as_mut().poll(cx))?;
        ready!(self.project().sink.poll_close(cx)?);
        Poll::Ready(Ok(()))
    }
}