1 files changed, 110 insertions, 0 deletions
diff --git a/third_party/rust/itertools/src/exactly_one_err.rs b/third_party/rust/itertools/src/exactly_one_err.rs
new file mode 100644
index 0000000000..c54ae77ca9
--- /dev/null
+++ b/third_party/rust/itertools/src/exactly_one_err.rs
@@ -0,0 +1,110 @@
+#[cfg(feature = "use_std")]
+use std::error::Error;
+use std::fmt::{Debug, Display, Formatter, Result as FmtResult};
+
+use std::iter::ExactSizeIterator;
+
+use either::Either;
+
+use crate::size_hint;
+
+/// Iterator returned for the error case of `IterTools::exactly_one()`
+/// This iterator yields exactly the same elements as the input iterator.
+///
+/// During the execution of `exactly_one` the iterator must be mutated.  This wrapper
+/// effectively "restores" the state of the input iterator when it's handed back.
+///
+/// This is very similar to `PutBackN` except this iterator only supports 0-2 elements and does not
+/// use a `Vec`.
+#[derive(Clone)]
+pub struct ExactlyOneError<I>
+where
+    I: Iterator,
+{
+    first_two: Option<Either<[I::Item; 2], I::Item>>,
+    inner: I,
+}
+
+impl<I> ExactlyOneError<I>
+where
+    I: Iterator,
+{
+    /// Creates a new `ExactlyOneErr` iterator.
+    pub(crate) fn new(first_two: Option<Either<[I::Item; 2], I::Item>>, inner: I) -> Self {
+        Self { first_two, inner }
+    }
+
+    fn additional_len(&self) -> usize {
+        match self.first_two {
+            Some(Either::Left(_)) => 2,
+            Some(Either::Right(_)) => 1,
+            None => 0,
+        }
+    }
+}
+
+impl<I> Iterator for ExactlyOneError<I>
+where
+    I: Iterator,
+{
+    type Item = I::Item;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        match self.first_two.take() {
+            Some(Either::Left([first, second])) => {
+                self.first_two = Some(Either::Right(second));
+                Some(first)
+            },
+            Some(Either::Right(second)) => {
+                Some(second)
+            }
+            None => {
+                self.inner.next()
+            }
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        size_hint::add_scalar(self.inner.size_hint(), self.additional_len())
+    }
+}
+
+
+impl<I> ExactSizeIterator for ExactlyOneError<I> where I: ExactSizeIterator {}
+
+impl<I> Display for ExactlyOneError<I> 
+    where I: Iterator,
+{
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        let additional = self.additional_len();
+        if additional > 0 {
+            write!(f, "got at least 2 elements when exactly one was expected")
+        } else {
+            write!(f, "got zero elements when exactly one was expected")
+        }
+    }
+}
+
+impl<I> Debug for ExactlyOneError<I> 
+    where I: Iterator + Debug,
+          I::Item: Debug,
+{
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        match &self.first_two {
+            Some(Either::Left([first, second])) => {
+                write!(f, "ExactlyOneError[First: {:?}, Second: {:?}, RemainingIter: {:?}]", first, second, self.inner)
+            },
+            Some(Either::Right(second)) => {
+                write!(f, "ExactlyOneError[Second: {:?}, RemainingIter: {:?}]", second, self.inner)
+            }
+            None => {
+                write!(f, "ExactlyOneError[RemainingIter: {:?}]", self.inner)
+            }
+        }
+    }
+}
+
+#[cfg(feature = "use_std")]
+impl<I> Error for ExactlyOneError<I>  where I: Iterator + Debug, I::Item: Debug, {}
+
+