use super::assert_stream;

use core::marker::PhantomData;

use core::pin::Pin;

use futures_core::stream::{FusedStream, Stream};

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

/// Stream for the [`empty`] function.

#[derive(Debug)]

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

pub struct Empty<T> {

    _phantom: PhantomData<T>,

}

/// Creates a stream which contains no elements.

///

/// The returned stream will always return `Ready(None)` when polled.

pub fn empty<T>() -> Empty<T> {

    assert_stream::<T, _>(Empty { _phantom: PhantomData })

}

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

impl<T> FusedStream for Empty<T> {

    fn is_terminated(&self) -> bool {

        true

    }

}

impl<T> Stream for Empty<T> {

    type Item = T;

    fn poll_next(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Option<Self::Item>> {

        Poll::Ready(None)

    }

    fn size_hint(&self) -> (usize, Option<usize>) {

        (0, Some(0))

    }

}

impl<T> Clone for Empty<T> {

    fn clone(&self) -> Self {

        empty()

    }

}