use super::assert_sink;

use crate::never::Never;

use core::marker::PhantomData;

use core::pin::Pin;

use futures_core::task::{Context, Poll};

use futures_sink::Sink;

/// Sink for the [`drain`] function.

#[derive(Debug)]

#[must_use = "sinks do nothing unless polled"]

pub struct Drain<T> {

    marker: PhantomData<T>,

}

/// Create a sink that will just discard all items given to it.

///

/// Similar to [`io::Sink`](::std::io::Sink).

///

/// # Examples

///

/// ```

/// # futures::executor::block_on(async {

/// use futures::sink::{self, SinkExt};

///

/// let mut drain = sink::drain();

/// drain.send(5).await?;

/// # Ok::<(), futures::never::Never>(()) }).unwrap();

/// ```

pub fn drain<T>() -> Drain<T> {

    assert_sink::<T, Never, _>(Drain { marker: PhantomData })

}

impl<T> Unpin for Drain<T> {}

impl<T> Clone for Drain<T> {

    fn clone(&self) -> Self {

        drain()

    }

}

impl<T> Sink<T> for Drain<T> {

    type Error = Never;

    fn poll_ready(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {

        Poll::Ready(Ok(()))

    }

    fn start_send(self: Pin<&mut Self>, _item: T) -> Result<(), Self::Error> {

        Ok(())

    }

    fn poll_flush(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {

        Poll::Ready(Ok(()))

    }

    fn poll_close(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {

        Poll::Ready(Ok(()))

    }

}