Adding upstream version 115.7.0esr.upstream/115.7.0esrupstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 119 insertions, 0 deletions

diff --git a/third_party/rust/futures-util/src/stream/unfold.rs b/third_party/rust/futures-util/src/stream/unfold.rs
new file mode 100644
index 0000000000..7d8ef6babc
--- /dev/null
+++ b/third_party/rust/futures-util/src/stream/unfold.rs
@@ -0,0 +1,119 @@
+use super::assert_stream;
+use crate::unfold_state::UnfoldState;
+use core::fmt;
+use core::pin::Pin;
+use futures_core::future::Future;
+use futures_core::ready;
+use futures_core::stream::{FusedStream, Stream};
+use futures_core::task::{Context, Poll};
+use pin_project_lite::pin_project;
+
+/// Creates a `Stream` from a seed and a closure returning a `Future`.
+///
+/// This function is the dual for the `Stream::fold()` adapter: while
+/// `Stream::fold()` reduces a `Stream` to one single value, `unfold()` creates a
+/// `Stream` from a seed value.
+///
+/// `unfold()` will call the provided closure with the provided seed, then wait
+/// for the returned `Future` to complete with `(a, b)`. It will then yield the
+/// value `a`, and use `b` as the next internal state.
+///
+/// If the closure returns `None` instead of `Some(Future)`, then the `unfold()`
+/// will stop producing items and return `Poll::Ready(None)` in future
+/// calls to `poll()`.
+///
+/// This function can typically be used when wanting to go from the "world of
+/// futures" to the "world of streams": the provided closure can build a
+/// `Future` using other library functions working on futures, and `unfold()`
+/// will turn it into a `Stream` by repeating the operation.
+///
+/// # Example
+///
+/// ```
+/// # futures::executor::block_on(async {
+/// use futures::stream::{self, StreamExt};
+///
+/// let stream = stream::unfold(0, |state| async move {
+///     if state <= 2 {
+///         let next_state = state + 1;
+///         let yielded = state  * 2;
+///         Some((yielded, next_state))
+///     } else {
+///         None
+///     }
+/// });
+///
+/// let result = stream.collect::<Vec<i32>>().await;
+/// assert_eq!(result, vec![0, 2, 4]);
+/// # });
+/// ```
+pub fn unfold<T, F, Fut, Item>(init: T, f: F) -> Unfold<T, F, Fut>
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = Option<(Item, T)>>,
+{
+    assert_stream::<Item, _>(Unfold { f, state: UnfoldState::Value { value: init } })
+}
+
+pin_project! {
+    /// Stream for the [`unfold`] function.
+    #[must_use = "streams do nothing unless polled"]
+    pub struct Unfold<T, F, Fut> {
+        f: F,
+        #[pin]
+        state: UnfoldState<T, Fut>,
+    }
+}
+
+impl<T, F, Fut> fmt::Debug for Unfold<T, F, Fut>
+where
+    T: fmt::Debug,
+    Fut: fmt::Debug,
+{
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Unfold").field("state", &self.state).finish()
+    }
+}
+
+impl<T, F, Fut, Item> FusedStream for Unfold<T, F, Fut>
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = Option<(Item, T)>>,
+{
+    fn is_terminated(&self) -> bool {
+        if let UnfoldState::Empty = self.state {
+            true
+        } else {
+            false
+        }
+    }
+}
+
+impl<T, F, Fut, Item> Stream for Unfold<T, F, Fut>
+where
+    F: FnMut(T) -> Fut,
+    Fut: Future<Output = Option<(Item, T)>>,
+{
+    type Item = Item;
+
+    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
+        let mut this = self.project();
+
+        if let Some(state) = this.state.as_mut().take_value() {
+            this.state.set(UnfoldState::Future { future: (this.f)(state) });
+        }
+
+        let step = match this.state.as_mut().project_future() {
+            Some(fut) => ready!(fut.poll(cx)),
+            None => panic!("Unfold must not be polled after it returned `Poll::Ready(None)`"),
+        };
+
+        if let Some((item, next_state)) = step {
+            this.state.set(UnfoldState::Value { value: next_state });
+            Poll::Ready(Some(item))
+        } else {
+            this.state.set(UnfoldState::Empty);
+            Poll::Ready(None)
+        }
+    }
+}