Implement try_take_while.

futures-util/src/stream/mod.rs

@@ -45,7 +45,7 @@ mod try_stream;
 pub use self::try_stream::{
     try_unfold, AndThen, ErrInto, InspectErr, InspectOk, IntoStream, MapErr, MapOk, OrElse,
     TryCollect, TryConcat, TryFilter, TryFilterMap, TryFlatten, TryFold, TryForEach, TryNext,
-    TrySkipWhile, TryStreamExt, TryUnfold,
+    TrySkipWhile, TryStreamExt, TryTakeWhile, TryUnfold,
 };
 
 #[cfg(feature = "io")]

futures-util/src/stream/try_stream/mod.rs

@@ -103,6 +103,10 @@ mod try_skip_while;
 #[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
 pub use self::try_skip_while::TrySkipWhile;
 
+mod try_take_while;
+#[allow(unreachable_pub)] // https://github.com/rust-lang/rust/issues/57411
+pub use self::try_take_while::TryTakeWhile;
+
 cfg_target_has_atomic! {
     #[cfg(feature = "alloc")]
     mod try_buffer_unordered;
@@ -432,6 +436,36 @@ pub trait TryStreamExt: TryStream {
         TrySkipWhile::new(self, f)
     }
 
+    /// Take elements on this stream while the provided asynchronous predicate
+    /// resolves to `true`.
+    ///
+    /// This function is similar to
+    /// [`StreamExt::take_while`](crate::stream::StreamExt::take_while) but exits
+    /// early if an error occurs.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # futures::executor::block_on(async {
+    /// use futures::future;
+    /// use futures::stream::{self, TryStreamExt};
+    ///
+    /// let stream = stream::iter(vec![Ok::<i32, i32>(1), Ok(2), Ok(3), Ok(2)]);
+    /// let stream = stream.try_take_while(|x| future::ready(Ok(*x < 3)));
+    ///
+    /// let output: Result<Vec<i32>, i32> = stream.try_collect().await;
+    /// assert_eq!(output, Ok(vec![1, 2]));
+    /// # })
+    /// ```
+    fn try_take_while<Fut, F>(self, f: F) -> TryTakeWhile<Self, Fut, F>
+    where
+        F: FnMut(&Self::Ok) -> Fut,
+        Fut: TryFuture<Ok = bool, Error = Self::Error>,
+        Self: Sized,
+    {
+        TryTakeWhile::new(self, f)
+    }
+
     /// Attempts to run this stream to completion, executing the provided asynchronous
     /// closure for each element on the stream concurrently as elements become
     /// available, exiting as soon as an error occurs.

futures-util/src/stream/try_stream/try_take_while.rs

@@ -0,0 +1,132 @@
+use core::fmt;
+use core::pin::Pin;
+use futures_core::future::TryFuture;
+use futures_core::stream::{FusedStream, Stream, TryStream};
+use futures_core::task::{Context, Poll};
+#[cfg(feature = "sink")]
+use futures_sink::Sink;
+use pin_project::pin_project;
+
+/// Stream for the [`try_take_while`](super::TryStreamExt::try_take_while)
+/// method.
+#[pin_project]
+#[must_use = "streams do nothing unless polled"]
+pub struct TryTakeWhile<St, Fut, F>
+where
+    St: TryStream,
+{
+    #[pin]
+    stream: St,
+    f: F,
+    #[pin]
+    pending_fut: Option<Fut>,
+    pending_item: Option<St::Ok>,
+    done_taking: bool,
+}
+
+impl<St, Fut, F> fmt::Debug for TryTakeWhile<St, Fut, F>
+where
+    St: TryStream + fmt::Debug,
+    St::Ok: fmt::Debug,
+    Fut: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("TryTakeWhile")
+            .field("stream", &self.stream)
+            .field("pending_fut", &self.pending_fut)
+            .field("pending_item", &self.pending_item)
+            .field("done_taking", &self.done_taking)
+            .finish()
+    }
+}
+
+impl<St, Fut, F> TryTakeWhile<St, Fut, F>
+where
+    St: TryStream,
+    F: FnMut(&St::Ok) -> Fut,
+    Fut: TryFuture<Ok = bool, Error = St::Error>,
+{
+    pub(super) fn new(stream: St, f: F) -> TryTakeWhile<St, Fut, F> {
+        TryTakeWhile {
+            stream,
+            f,
+            pending_fut: None,
+            pending_item: None,
+            done_taking: false,
+        }
+    }
+
+    delegate_access_inner!(stream, St, ());
+}
+
+impl<St, Fut, F> Stream for TryTakeWhile<St, Fut, F>
+where
+    St: TryStream,
+    F: FnMut(&St::Ok) -> Fut,
+    Fut: TryFuture<Ok = bool, Error = St::Error>,
+{
+    type Item = Result<St::Ok, St::Error>;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+
+        if *this.done_taking {
+            return Poll::Ready(None);
+        }
+
+        Poll::Ready(loop {
+            if let Some(fut) = this.pending_fut.as_mut().as_pin_mut() {
+                let take = ready!(fut.try_poll(cx)?);
+                let item = this.pending_item.take();
+                this.pending_fut.set(None);
+                if take {
+                    break item.map(Ok);
+                } else {
+                    *this.done_taking = true;
+                    break None;
+                }
+            } else if let Some(item) = ready!(this.stream.as_mut().try_poll_next(cx)?) {
+                this.pending_fut.set(Some((this.f)(&item)));
+                *this.pending_item = Some(item);
+            } else {
+                break None;
+            }
+        })
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        if self.done_taking {
+            return (0, Some(0));
+        }
+
+        let pending_len = if self.pending_item.is_some() { 1 } else { 0 };
+        let (_, upper) = self.stream.size_hint();
+        let upper = match upper {
+            Some(x) => x.checked_add(pending_len),
+            None => None,
+        };
+        (0, upper) // can't know a lower bound, due to the predicate
+    }
+}
+
+impl<St, Fut, F> FusedStream for TryTakeWhile<St, Fut, F>
+where
+    St: TryStream + FusedStream,
+    F: FnMut(&St::Ok) -> Fut,
+    Fut: TryFuture<Ok = bool, Error = St::Error>,
+{
+    fn is_terminated(&self) -> bool {
+        self.done_taking || self.pending_item.is_none() && self.stream.is_terminated()
+    }
+}
+
+// Forwarding impl of Sink from the underlying stream
+#[cfg(feature = "sink")]
+impl<S, Fut, F, Item, E> Sink<Item> for TryTakeWhile<S, Fut, F>
+where
+    S: TryStream + Sink<Item, Error = E>,
+{
+    type Error = E;
+
+    delegate_sink!(stream, Item);
+}

futures/src/lib.rs

@@ -465,7 +465,7 @@ pub mod stream {
         AndThen, ErrInto, MapOk, MapErr, OrElse,
         InspectOk, InspectErr,
         TryNext, TryForEach, TryFilter, TryFilterMap, TryFlatten,
-        TryCollect, TryConcat, TryFold, TrySkipWhile,
+        TryCollect, TryConcat, TryFold, TrySkipWhile, TryTakeWhile,
         IntoStream,
     };