1 files changed, 100 insertions, 0 deletions
diff --git a/third_party/rust/itertools/src/with_position.rs b/third_party/rust/itertools/src/with_position.rs
new file mode 100644
index 0000000000..1388503d1c
--- /dev/null
+++ b/third_party/rust/itertools/src/with_position.rs
@@ -0,0 +1,100 @@
+use std::iter::{Fuse,Peekable, FusedIterator};
+
+/// An iterator adaptor that wraps each element in an [`Position`].
+///
+/// Iterator element type is `Position<I::Item>`.
+///
+/// See [`.with_position()`](crate::Itertools::with_position) for more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct WithPosition<I>
+    where I: Iterator,
+{
+    handled_first: bool,
+    peekable: Peekable<Fuse<I>>,
+}
+
+impl<I> Clone for WithPosition<I>
+    where I: Clone + Iterator,
+          I::Item: Clone,
+{
+    clone_fields!(handled_first, peekable);
+}
+
+/// Create a new `WithPosition` iterator.
+pub fn with_position<I>(iter: I) -> WithPosition<I>
+    where I: Iterator,
+{
+    WithPosition {
+        handled_first: false,
+        peekable: iter.fuse().peekable(),
+    }
+}
+
+/// A value yielded by `WithPosition`.
+/// Indicates the position of this element in the iterator results.
+///
+/// See [`.with_position()`](crate::Itertools::with_position) for more information.
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Position<T> {
+    /// This is the first element.
+    First(T),
+    /// This is neither the first nor the last element.
+    Middle(T),
+    /// This is the last element.
+    Last(T),
+    /// This is the only element.
+    Only(T),
+}
+
+impl<T> Position<T> {
+    /// Return the inner value.
+    pub fn into_inner(self) -> T {
+        match self {
+            Position::First(x) |
+            Position::Middle(x) |
+            Position::Last(x) |
+            Position::Only(x) => x,
+        }
+    }
+}
+
+impl<I: Iterator> Iterator for WithPosition<I> {
+    type Item = Position<I::Item>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.peekable.next() {
+            Some(item) => {
+                if !self.handled_first {
+                    // Haven't seen the first item yet, and there is one to give.
+                    self.handled_first = true;
+                    // Peek to see if this is also the last item,
+                    // in which case tag it as `Only`.
+                    match self.peekable.peek() {
+                        Some(_) => Some(Position::First(item)),
+                        None => Some(Position::Only(item)),
+                    }
+                } else {
+                    // Have seen the first item, and there's something left.
+                    // Peek to see if this is the last item.
+                    match self.peekable.peek() {
+                        Some(_) => Some(Position::Middle(item)),
+                        None => Some(Position::Last(item)),
+                    }
+                }
+            }
+            // Iterator is finished.
+            None => None,
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        self.peekable.size_hint()
+    }
+}
+
+impl<I> ExactSizeIterator for WithPosition<I>
+    where I: ExactSizeIterator,
+{ }
+
+impl<I: Iterator> FusedIterator for WithPosition<I> 
+{}