implement nth_back for Enumerate

src/libcore/iter/adapters/mod.rs

@@ -980,6 +980,16 @@ 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>

src/libcore/tests/iter.rs

@@ -389,6 +389,24 @@ fn test_iterator_enumerate_nth() {
     assert_eq!(i, 3);
 }
 
+#[test]
+fn test_iterator_enumerate_nth_back() {
+    let xs = [0, 1, 2, 3, 4, 5];
+    let mut it = xs.iter().enumerate();
+    while let Some((i, &x)) = it.nth_back(0) {
+        assert_eq!(i, x);
+    }
+
+    let mut it = xs.iter().enumerate();
+    while let Some((i, &x)) = it.nth_back(1) {
+        assert_eq!(i, x);
+    }
+
+    let (i, &x) = xs.iter().enumerate().nth_back(3).unwrap();
+    assert_eq!(i, x);
+    assert_eq!(i, 2);
+}
+
 #[test]
 fn test_iterator_enumerate_count() {
     let xs = [0, 1, 2, 3, 4, 5];