Introduce futures_util::stream::select_bias.

The current select function only supports round robin. To change that
would be a breaking change, so for now I propose a `select_bias` which
takes a closure that defines the selection strategy.

- When a breaking change is planned, we could re-consider this, but it
  does have 4 generic parameters of which one is a closure, compared to
  the 2 streams in `select`.
- note the name select_bias. It's not consistent with `select_biased` used
  elsewhere in the lib. This can obviously be changed, but I feel the "ed"
  at the end does not add any semantic value but makes function names
  longer.
- There isn't really integration tests included right now, but the examples
  in the docs get tested by doc test.

futures-util/src/stream/mod.rs

@@ -92,6 +92,9 @@ pub use self::poll_fn::{poll_fn, PollFn};
 mod select;
 pub use self::select::{select, Select};
 
+mod select_bias;
+pub use self::select_bias::{select_bias, PollNext, SelectBias};
+
 mod unfold;
 pub use self::unfold::{unfold, Unfold};
 

futures-util/src/stream/select_bias.rs

@@ -0,0 +1,220 @@
+use super::assert_stream;
+use crate::stream::{Fuse, StreamExt};
+use core::pin::Pin;
+use futures_core::stream::{FusedStream, Stream};
+use futures_core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+/// Type to tell [`SelectBias`] which stream to poll next.
+#[derive(Debug, PartialEq, Eq, Copy, Clone, Hash)]
+pub enum PollNext {
+    /// Poll the first stream.
+    Left,
+    /// Poll the second stream.
+    Right,
+}
+
+impl PollNext {
+    /// Toggle the value and return the old one.
+    pub fn toggle(&mut self) -> Self {
+        let old = *self;
+
+        match self {
+            PollNext::Left => *self = PollNext::Right,
+            PollNext::Right => *self = PollNext::Left,
+        }
+
+        old
+    }
+}
+
+impl Default for PollNext {
+    fn default() -> Self {
+        PollNext::Left
+    }
+}
+
+pin_project! {
+    /// Stream for the [`select_bias()`] function. See function docs for details.
+    #[derive(Debug)]
+    #[must_use = "streams do nothing unless polled"]
+    pub struct SelectBias<St1, St2, Clos, State> {
+        #[pin]
+        stream1: Fuse<St1>,
+        #[pin]
+        stream2: Fuse<St2>,
+        state: State,
+        clos: Clos,
+    }
+}
+
+/// This function will attempt to pull items from both streams. You provide a
+/// closure to tell the [`SelectBias`] which stream to poll. The closure can
+/// store state on `SelectBias` to which it will receive a `&mut` on every
+/// invocation. This allows basing the strategy on prior choices.
+///
+/// After one of the two input stream completes, the remaining one will be
+/// polled exclusively. The returned stream completes when both input
+/// streams have completed.
+///
+/// Note that this function consumes both streams and returns a wrapped
+/// version of them.
+///
+/// ## Examples
+///
+/// ### Priority
+/// This example shows how to always prioritize the left stream.
+///
+/// ```rust
+///
+/// # futures::executor::block_on(async {
+/// use futures_util::stream::{ select_bias, PollNext, StreamExt };
+/// use futures::stream::repeat;
+///
+/// let left = repeat(1);
+/// let right = repeat(2);
+///
+/// // We don't need any state, so lets make it an empty tuple.
+/// // We must provide some type here, as there is no way for the compiler
+/// // to infer it.
+/// let prio_left = |_: &mut ()| PollNext::Left;
+///
+/// let mut out = select_bias(left, right, prio_left);
+///
+/// for _ in 0..100 {
+///     // Whenever we poll out, we will alwasy get `1`.
+///     assert_eq!(1, out.select_next_some().await);
+/// }
+///
+/// });
+/// ```
+///
+/// ### Round Robin
+/// This example shows how to select from both streams round robin.
+///
+/// ```rust
+///
+/// # futures::executor::block_on(async {
+/// use futures_util::stream::{ select_bias, PollNext, StreamExt };
+/// use futures::stream::repeat;
+///
+/// let left = repeat(1);
+/// let right = repeat(2);
+///
+/// // We don't need any state, so lets make it an empty tuple.
+/// let rrobin = |last: &mut PollNext| last.toggle();
+///
+/// let mut out = select_bias(left, right, rrobin);
+///
+/// for _ in 0..100 {
+///     // We should be alternating now.
+///     assert_eq!(1, out.select_next_some().await);
+///     assert_eq!(2, out.select_next_some().await);
+/// }
+///
+/// });
+/// ```
+pub fn select_bias<St1, St2, Clos, State>(
+    stream1: St1,
+    stream2: St2,
+    which: Clos,
+) -> SelectBias<St1, St2, Clos, State>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+    Clos: FnMut(&mut State) -> PollNext,
+    State: Default,
+{
+    assert_stream::<St1::Item, _>(SelectBias {
+        stream1: stream1.fuse(),
+        stream2: stream2.fuse(),
+        state: Default::default(),
+        clos: which,
+    })
+}
+
+impl<St1, St2, Clos, State> SelectBias<St1, St2, Clos, State> {
+    /// Acquires a reference to the underlying streams that this combinator is
+    /// pulling from.
+    pub fn get_ref(&self) -> (&St1, &St2) {
+        (self.stream1.get_ref(), self.stream2.get_ref())
+    }
+
+    /// Acquires a mutable reference to the underlying streams 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 St1, &mut St2) {
+        (self.stream1.get_mut(), self.stream2.get_mut())
+    }
+
+    /// Acquires a pinned mutable reference to the underlying streams 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(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) {
+        let this = self.project();
+        (this.stream1.get_pin_mut(), this.stream2.get_pin_mut())
+    }
+
+    /// Consumes this combinator, returning the underlying streams.
+    ///
+    /// 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) -> (St1, St2) {
+        (self.stream1.into_inner(), self.stream2.into_inner())
+    }
+}
+
+impl<St1, St2, Clos, State> FusedStream for SelectBias<St1, St2, Clos, State>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+    Clos: FnMut(&mut State) -> PollNext,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream1.is_terminated() && self.stream2.is_terminated()
+    }
+}
+
+impl<St1, St2, Clos, State> Stream for SelectBias<St1, St2, Clos, State>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+    Clos: FnMut(&mut State) -> PollNext,
+{
+    type Item = St1::Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<St1::Item>> {
+        let this = self.project();
+
+        match (this.clos)(this.state) {
+            PollNext::Left => poll_inner(this.stream1, this.stream2, cx),
+            PollNext::Right => poll_inner(this.stream2, this.stream1, cx),
+        }
+    }
+}
+
+fn poll_inner<St1, St2>(
+    a: Pin<&mut St1>,
+    b: Pin<&mut St2>,
+    cx: &mut Context<'_>,
+) -> Poll<Option<St1::Item>>
+where
+    St1: Stream,
+    St2: Stream<Item = St1::Item>,
+{
+    let a_done = match a.poll_next(cx) {
+        Poll::Ready(Some(item)) => return Poll::Ready(Some(item)),
+        Poll::Ready(None) => true,
+        Poll::Pending => false,
+    };
+
+    match b.poll_next(cx) {
+        Poll::Ready(Some(item)) => Poll::Ready(Some(item)),
+        Poll::Ready(None) if a_done => Poll::Ready(None),
+        Poll::Ready(None) | Poll::Pending => Poll::Pending,
+    }
+}