implement nth_back for Enumerate

1 files changed, 10 insertions, 0 deletions

diff --git a/src/libcore/iter/adapters/mod.rs b/src/libcore/iter/adapters/mod.rs
index f08f2a5ec75..9f9146a1523 100644
--- a/src/libcore/iter/adapters/mod.rs
+++ b/src/libcore/iter/adapters/mod.rs
@@ -981,6 +981,16 @@ impl<I> DoubleEndedIterator for Enumerate<I> where
     }
 
     #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<(usize, <I as Iterator>::Item)> {
+        self.iter.nth_back(n).map(|a| {
+            let len = self.iter.len();
+            // Can safely add, `ExactSizeIterator` promises that the number of
+            // elements fits into a `usize`.
+            (self.count + len, a)
+        })
+    }
+
+    #[inline]
     fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
     {