1 files changed, 99 insertions, 0 deletions
diff --git a/third_party/rust/itertools/src/rciter_impl.rs b/third_party/rust/itertools/src/rciter_impl.rs
new file mode 100644
index 0000000000..7298350a88
--- /dev/null
+++ b/third_party/rust/itertools/src/rciter_impl.rs
@@ -0,0 +1,99 @@
+
+use std::iter::{FusedIterator, IntoIterator};
+use alloc::rc::Rc;
+use std::cell::RefCell;
+
+/// A wrapper for `Rc<RefCell<I>>`, that implements the `Iterator` trait.
+#[derive(Debug)]
+pub struct RcIter<I> {
+    /// The boxed iterator.
+    pub rciter: Rc<RefCell<I>>,
+}
+
+/// Return an iterator inside a `Rc<RefCell<_>>` wrapper.
+///
+/// The returned `RcIter` can be cloned, and each clone will refer back to the
+/// same original iterator.
+///
+/// `RcIter` allows doing interesting things like using `.zip()` on an iterator with
+/// itself, at the cost of runtime borrow checking which may have a performance
+/// penalty.
+///
+/// Iterator element type is `Self::Item`.
+///
+/// ```
+/// use itertools::rciter;
+/// use itertools::zip;
+///
+/// // In this example a range iterator is created and we iterate it using
+/// // three separate handles (two of them given to zip).
+/// // We also use the IntoIterator implementation for `&RcIter`.
+///
+/// let mut iter = rciter(0..9);
+/// let mut z = zip(&iter, &iter);
+///
+/// assert_eq!(z.next(), Some((0, 1)));
+/// assert_eq!(z.next(), Some((2, 3)));
+/// assert_eq!(z.next(), Some((4, 5)));
+/// assert_eq!(iter.next(), Some(6));
+/// assert_eq!(z.next(), Some((7, 8)));
+/// assert_eq!(z.next(), None);
+/// ```
+///
+/// **Panics** in iterator methods if a borrow error is encountered in the
+/// iterator methods. It can only happen if the `RcIter` is reentered in
+/// `.next()`, i.e. if it somehow participates in an “iterator knot”
+/// where it is an adaptor of itself.
+pub fn rciter<I>(iterable: I) -> RcIter<I::IntoIter>
+    where I: IntoIterator
+{
+    RcIter { rciter: Rc::new(RefCell::new(iterable.into_iter())) }
+}
+
+impl<I> Clone for RcIter<I> {
+    clone_fields!(rciter);
+}
+
+impl<A, I> Iterator for RcIter<I>
+    where I: Iterator<Item = A>
+{
+    type Item = A;
+    #[inline]
+    fn next(&mut self) -> Option<Self::Item> {
+        self.rciter.borrow_mut().next()
+    }
+
+    #[inline]
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        // To work sanely with other API that assume they own an iterator,
+        // so it can't change in other places, we can't guarantee as much
+        // in our size_hint. Other clones may drain values under our feet.
+        (0, self.rciter.borrow().size_hint().1)
+    }
+}
+
+impl<I> DoubleEndedIterator for RcIter<I>
+    where I: DoubleEndedIterator
+{
+    #[inline]
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.rciter.borrow_mut().next_back()
+    }
+}
+
+/// Return an iterator from `&RcIter<I>` (by simply cloning it).
+impl<'a, I> IntoIterator for &'a RcIter<I>
+    where I: Iterator
+{
+    type Item = I::Item;
+    type IntoIter = RcIter<I>;
+
+    fn into_iter(self) -> RcIter<I> {
+        self.clone()
+    }
+}
+
+
+impl<A, I> FusedIterator for RcIter<I>
+    where I: FusedIterator<Item = A>
+{}