Add TryStreamExt::{inspect_ok, inspect_err}

Author: taiki-e

futures-util/src/stream/inspect.rs

@@ -64,6 +64,13 @@ impl<St: Stream + FusedStream, F> FusedStream for Inspect<St, F> {
     }
 }
 
+// used by `TryStreamExt::{inspect_ok, inspect_err}`
+#[inline]
+pub(crate) fn inspect<T, F: FnMut(&T)>(x: T, mut f: F) -> T {
+    f(&x);
+    x
+}
+
 impl<St, F> Stream for Inspect<St, F>
     where St: Stream,
           F: FnMut(&St::Item),
@@ -74,11 +81,10 @@ impl<St, F> Stream for Inspect<St, F>
         mut self: Pin<&mut Self>,
         cx: &mut Context<'_>,
     ) -> Poll<Option<St::Item>> {
-        let item = ready!(self.as_mut().stream().poll_next(cx));
-        Poll::Ready(item.map(|e| {
-            (self.as_mut().f())(&e);
-            e
-        }))
+        self.as_mut()
+            .stream()
+            .poll_next(cx)
+            .map(|opt| opt.map(|e| inspect(e, self.as_mut().f())))
     }
 }
 

futures-util/src/stream/mod.rs

@@ -58,6 +58,7 @@ mod into_future;
 pub use self::into_future::StreamFuture;
 
 mod inspect;
+pub(crate) use self::inspect::inspect; // used by `TryStreamExt::{inspect_ok, inspect_err}`
 pub use self::inspect::Inspect;
 
 mod map;

futures-util/src/try_stream/inspect_err.rs

@@ -0,0 +1,96 @@
+use crate::stream::inspect;
+use core::pin::Pin;
+use futures_core::stream::{FusedStream, Stream, TryStream};
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_utils::{unsafe_pinned, unsafe_unpinned};
+
+/// Stream for the [`inspect_err`](super::TryStreamExt::inspect_err) method.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct InspectErr<St, F> {
+    stream: St,
+    f: F,
+}
+
+impl<St: TryStream + Unpin, F> Unpin for InspectErr<St, F> {}
+
+impl<St, F> InspectErr<St, F>
+where
+    St: TryStream,
+    F: FnMut(&St::Error),
+{
+    unsafe_pinned!(stream: St);
+    unsafe_unpinned!(f: F);
+
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, f }
+    }
+
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &St {
+        &self.stream
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut St {
+        &mut self.stream
+    }
+
+    /// Acquires a pinned mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_pin_mut<'a>(self: Pin<&'a mut Self>) -> Pin<&'a mut St> {
+        self.stream()
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> St {
+        self.stream
+    }
+}
+
+impl<St: TryStream + FusedStream, F> FusedStream for InspectErr<St, F> {
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}
+
+impl<St, F> Stream for InspectErr<St, F>
+where
+    St: TryStream,
+    F: FnMut(&St::Error),
+{
+    type Item = Result<St::Ok, St::Error>;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        self.as_mut()
+            .stream()
+            .try_poll_next(cx)
+            .map(|opt| opt.map(|res| res.map_err(|e| inspect(e, self.as_mut().f()))))
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+impl<S, F, Item> Sink<Item> for InspectErr<S, F>
+where
+    S: TryStream + Sink<Item>,
+    F: FnMut(&S::Error),
+{
+    type SinkError = S::SinkError;
+
+    delegate_sink!(stream, Item);
+}

futures-util/src/try_stream/inspect_ok.rs

@@ -0,0 +1,96 @@
+use crate::stream::inspect;
+use core::pin::Pin;
+use futures_core::stream::{FusedStream, Stream, TryStream};
+use futures_core::task::{Context, Poll};
+use futures_sink::Sink;
+use pin_utils::{unsafe_pinned, unsafe_unpinned};
+
+/// Stream for the [`inspect_ok`](super::TryStreamExt::inspect_ok) method.
+#[derive(Debug)]
+#[must_use = "streams do nothing unless polled"]
+pub struct InspectOk<St, F> {
+    stream: St,
+    f: F,
+}
+
+impl<St: TryStream + Unpin, F> Unpin for InspectOk<St, F> {}
+
+impl<St, F> InspectOk<St, F>
+where
+    St: TryStream,
+    F: FnMut(&St::Ok),
+{
+    unsafe_pinned!(stream: St);
+    unsafe_unpinned!(f: F);
+
+    pub(super) fn new(stream: St, f: F) -> Self {
+        Self { stream, f }
+    }
+
+    /// Acquires a reference to the underlying stream that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> &St {
+        &self.stream
+    }
+
+    /// Acquires a mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_mut(&mut self) -> &mut St {
+        &mut self.stream
+    }
+
+    /// Acquires a pinned mutable reference to the underlying stream that this
+    /// combinator is pulling from.
+    ///
+    /// Note that care must be taken to avoid tampering with the state of the
+    /// stream which may otherwise confuse this combinator.
+    pub fn get_pin_mut<'a>(self: Pin<&'a mut Self>) -> Pin<&'a mut St> {
+        self.stream()
+    }
+
+    /// Consumes this combinator, returning the underlying stream.
+    ///
+    /// Note that this may discard intermediate state of this combinator, so
+    /// care should be taken to avoid losing resources when this is called.
+    pub fn into_inner(self) -> St {
+        self.stream
+    }
+}
+
+impl<St: TryStream + FusedStream, F> FusedStream for InspectOk<St, F> {
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}
+
+impl<St, F> Stream for InspectOk<St, F>
+where
+    St: TryStream,
+    F: FnMut(&St::Ok),
+{
+    type Item = Result<St::Ok, St::Error>;
+
+    fn poll_next(
+        mut self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+    ) -> Poll<Option<Self::Item>> {
+        self.as_mut()
+            .stream()
+            .try_poll_next(cx)
+            .map(|opt| opt.map(|res| res.map(|e| inspect(e, self.as_mut().f()))))
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+impl<S, F, Item> Sink<Item> for InspectOk<S, F>
+where
+    S: TryStream + Sink<Item>,
+    F: FnMut(&S::Ok),
+{
+    type SinkError = S::SinkError;
+
+    delegate_sink!(stream, Item);
+}

futures-util/src/try_stream/mod.rs

@@ -17,6 +17,12 @@ pub use self::and_then::AndThen;
 mod err_into;
 pub use self::err_into::ErrInto;
 
+mod inspect_ok;
+pub use self::inspect_ok::InspectOk;
+
+mod inspect_err;
+pub use self::inspect_err::InspectErr;
+
 mod into_stream;
 pub use self::into_stream::IntoStream;
 
@@ -223,6 +229,31 @@ pub trait TryStreamExt: TryStream {
         OrElse::new(self, f)
     }
 
+    /// Do something with the success value of this stream, afterwards passing
+    /// it on.
+    ///
+    /// This is similar to the `StreamExt::inspect` method where it allows
+    /// easily inspecting the success value as it passes through the stream, for
+    /// example to debug what's going on.
+    fn inspect_ok<F>(self, f: F) -> InspectOk<Self, F>
+        where F: FnMut(&Self::Ok),
+              Self: Sized,
+    {
+        InspectOk::new(self, f)
+    }
+
+    /// Do something with the error value of this stream, afterwards passing it on.
+    ///
+    /// This is similar to the `StreamExt::inspect` method where it allows
+    /// easily inspecting the error value as it passes through the stream, for
+    /// example to debug what's going on.
+    fn inspect_err<F>(self, f: F) -> InspectErr<Self, F>
+        where F: FnMut(&Self::Error),
+              Self: Sized,
+    {
+        InspectErr::new(self, f)
+    }
+
     /// Wraps a [`TryStream`] into a type that implements
     /// [`Stream`](futures_core::Stream)
     ///

futures/src/lib.rs

@@ -404,10 +404,10 @@ pub mod stream {
     pub use futures_util::try_stream::{
         TryStreamExt,
         AndThen, ErrInto, MapOk, MapErr, OrElse,
+        InspectOk, InspectErr,
         TryNext, TryForEach, TryFilterMap,
         TryCollect, TryFold, TrySkipWhile,
         IntoStream,
-        // ToDo: InspectErr
     };
 
     #[cfg_attr(