tokio-stream: implement FusedStream for various stream adaptors (#8096)

`MapWhile` is intentionally excluded because its current implementation
does not track when the closure returns `None` early, making a correct
`is_terminated()` impossible without a separate semantic change.

tokio-stream/src/stream_ext/chain.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 
 use core::pin::Pin;
 use core::task::{ready, Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -48,3 +49,13 @@ where
         super::merge_size_hints(self.a.size_hint(), self.b.size_hint())
     }
 }
+
+impl<T, U> FusedStream for Chain<T, U>
+where
+    T: Stream,
+    U: FusedStream<Item = T::Item>,
+{
+    fn is_terminated(&self) -> bool {
+        self.a.is_terminated() && self.b.is_terminated()
+    }
+}

tokio-stream/src/stream_ext/filter.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{ready, Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -56,3 +57,13 @@ where
         (0, self.stream.size_hint().1) // can't know a lower bound, due to the predicate
     }
 }
+
+impl<St, F> FusedStream for Filter<St, F>
+where
+    St: FusedStream,
+    F: FnMut(&St::Item) -> bool,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}

tokio-stream/src/stream_ext/filter_map.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{ready, Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -56,3 +57,13 @@ where
         (0, self.stream.size_hint().1) // can't know a lower bound, due to the predicate
     }
 }
+
+impl<St, F, T> FusedStream for FilterMap<St, F>
+where
+    St: FusedStream,
+    F: FnMut(St::Item) -> Option<T>,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}

tokio-stream/src/stream_ext/map.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -49,3 +50,13 @@ where
         self.stream.size_hint()
     }
 }
+
+impl<St, F, T> FusedStream for Map<St, F>
+where
+    St: FusedStream,
+    F: FnMut(St::Item) -> T,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}

tokio-stream/src/stream_ext/merge.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 
 use core::pin::Pin;
 use core::task::{Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -57,6 +58,16 @@ where
     }
 }
 
+impl<T, U> FusedStream for Merge<T, U>
+where
+    T: Stream,
+    U: Stream<Item = T::Item>,
+{
+    fn is_terminated(&self) -> bool {
+        self.a.is_terminated() && self.b.is_terminated()
+    }
+}
+
 fn poll_next<T, U>(
     first: Pin<&mut T>,
     second: Pin<&mut U>,

tokio-stream/src/stream_ext/skip.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{ready, Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -61,3 +62,12 @@ where
         (lower, upper)
     }
 }
+
+impl<St> FusedStream for Skip<St>
+where
+    St: FusedStream,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}

tokio-stream/src/stream_ext/skip_while.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{ready, Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -71,3 +72,13 @@ where
         (lower, upper)
     }
 }
+
+impl<St, F> FusedStream for SkipWhile<St, F>
+where
+    St: FusedStream,
+    F: FnMut(&St::Item) -> bool,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}

tokio-stream/src/stream_ext/take.rs

@@ -4,6 +4,7 @@ use core::cmp;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -74,3 +75,12 @@ where
         (lower, upper)
     }
 }
+
+impl<St> FusedStream for Take<St>
+where
+    St: Stream,
+{
+    fn is_terminated(&self) -> bool {
+        self.remaining == 0
+    }
+}

tokio-stream/src/stream_ext/take_while.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -77,3 +78,13 @@ where
         (0, upper)
     }
 }
+
+impl<St, F> FusedStream for TakeWhile<St, F>
+where
+    St: Stream,
+    F: FnMut(&St::Item) -> bool,
+{
+    fn is_terminated(&self) -> bool {
+        self.done
+    }
+}

tokio-stream/src/stream_ext/then.rs

@@ -4,6 +4,7 @@ use core::fmt;
 use core::future::Future;
 use core::pin::Pin;
 use core::task::{Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -81,3 +82,14 @@ where
         (lower, upper)
     }
 }
+
+impl<St, F, Fut> FusedStream for Then<St, Fut, F>
+where
+    St: FusedStream,
+    Fut: Future,
+    F: FnMut(St::Item) -> Fut,
+{
+    fn is_terminated(&self) -> bool {
+        self.future.is_none() && self.stream.is_terminated()
+    }
+}