--- /dev/null
+use std::fmt;
+use std::iter::ExactSizeIterator;
+use std::iter::FusedIterator;
+use std::iter::Iterator;
+
+/// Iterator which may contain instance of
+/// one of two specific implementations.
+///
+/// Note: For most methods providing custom
+/// implementation may margianlly
+/// improve performance by avoiding
+/// doing Left/Right match on every step
+/// and doing it only once instead.
+#[derive(Clone)]
+pub enum EitherIter<L, R> {
+ Left(L),
+ Right(R),
+}
+
+impl<L, R> Iterator for EitherIter<L, R>
+where
+ L: Iterator,
+ R: Iterator<Item = L::Item>,
+{
+ type Item = L::Item;
+
+ fn next(&mut self) -> Option<Self::Item> {
+ match self {
+ EitherIter::Left(l) => l.next(),
+ EitherIter::Right(r) => r.next(),
+ }
+ }
+
+ fn size_hint(&self) -> (usize, Option<usize>) {
+ match self {
+ EitherIter::Left(l) => l.size_hint(),
+ EitherIter::Right(r) => r.size_hint(),
+ }
+ }
+}
+
+impl<L, R> ExactSizeIterator for EitherIter<L, R>
+where
+ L: ExactSizeIterator,
+ R: ExactSizeIterator,
+ EitherIter<L, R>: Iterator,
+{
+ fn len(&self) -> usize {
+ match self {
+ EitherIter::Left(l) => l.len(),
+ EitherIter::Right(r) => r.len(),
+ }
+ }
+}
+
+impl<L, R> FusedIterator for EitherIter<L, R>
+where
+ L: FusedIterator,
+ R: FusedIterator,
+ EitherIter<L, R>: Iterator,
+{
+}
+
+impl<L, R> fmt::Debug for EitherIter<L, R>
+where
+ L: fmt::Debug,
+ R: fmt::Debug,
+{
+ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+ match self {
+ EitherIter::Left(l) => l.fmt(f),
+ EitherIter::Right(r) => r.fmt(f),
+ }
+ }
+}