Revert FusedFuture implementations for Feed and FeedAll

The provided implementations did not account for the underlying
stream or sink returning an Err. Accounting for the error cases
requires extra logic or flags, which may not be worth it.

Author: mzabaluev

futures-util/src/sink/feed.rs

@@ -1,5 +1,5 @@
 use core::pin::Pin;
-use futures_core::future::{Future, FusedFuture};
+use futures_core::future::Future;
 use futures_core::task::{Context, Poll};
 use futures_sink::Sink;
 
@@ -25,6 +25,10 @@ impl<'a, Si: Sink<Item> + Unpin + ?Sized, Item> Feed<'a, Si, Item> {
     pub(super) fn sink_pin_mut(&mut self) -> Pin<&mut Si> {
         Pin::new(self.sink)
     }
+
+    pub(super) fn is_item_pending(&self) -> bool {
+        self.item.is_some()
+    }
 }
 
 impl<Si: Sink<Item> + Unpin + ?Sized, Item> Future for Feed<'_, Si, Item> {
@@ -42,9 +46,3 @@ impl<Si: Sink<Item> + Unpin + ?Sized, Item> Future for Feed<'_, Si, Item> {
         Poll::Ready(Ok(()))
     }
 }
-
-impl<Si: Sink<Item> + Unpin + ?Sized, Item> FusedFuture for Feed<'_, Si, Item> {
-    fn is_terminated(&self) -> bool {
-        self.item.is_none()
-    }
-}

futures-util/src/sink/feed_all.rs

@@ -1,8 +1,8 @@
 use crate::stream::TryStreamExt;
 use core::fmt;
 use core::pin::Pin;
-use futures_core::future::{Future, FusedFuture};
-use futures_core::stream::{TryStream, Stream, FusedStream};
+use futures_core::future::Future;
+use futures_core::stream::{TryStream, Stream};
 use futures_core::task::{Context, Poll};
 use futures_sink::Sink;
 
@@ -113,13 +113,3 @@ where
         }
     }
 }
-
-impl<Si, St, Ok, Error> FusedFuture for FeedAll<'_, Si, St>
-where
-    Si: Sink<Ok, Error = Error> + Unpin + ?Sized,
-    St: Stream<Item = Result<Ok, Error>> + FusedStream + Unpin + ?Sized,
-{
-    fn is_terminated(&self) -> bool {
-        self.buffered.is_none() && self.stream.is_terminated()
-    }
-}

futures-util/src/sink/send.rs

@@ -1,6 +1,6 @@
 use super::Feed;
 use core::pin::Pin;
-use futures_core::future::{Future, FusedFuture};
+use futures_core::future::Future;
 use futures_core::task::{Context, Poll};
 use futures_sink::Sink;
 
@@ -30,8 +30,10 @@ impl<Si: Sink<Item> + Unpin + ?Sized, Item> Future for Send<'_, Si, Item> {
         cx: &mut Context<'_>,
     ) -> Poll<Self::Output> {
         let this = &mut *self;
-        if !this.feed.is_terminated() {
+
+        if this.feed.is_item_pending() {
             ready!(Pin::new(&mut this.feed).poll(cx))?;
+            debug_assert!(!this.feed.is_item_pending());
         }
 
         // we're done sending the item, but want to block on flushing the