Override nth for VecDeque Iter and IterMut

src/liballoc/collections/vec_deque.rs

@@ -2286,6 +2286,16 @@ fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
         final_res
     }
 
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        if n >= count(self.tail, self.head, self.ring.len()) {
+            self.tail = self.head;
+            None
+        } else {
+            self.tail = wrap_index(self.tail.wrapping_add(n), self.ring.len());
+            self.next()
+        }
+    }
+
     #[inline]
     fn last(mut self) -> Option<&'a T> {
         self.next_back()
@@ -2404,6 +2414,16 @@ fn fold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
         back.iter_mut().fold(accum, &mut f)
     }
 
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        if n >= count(self.tail, self.head, self.ring.len()) {
+            self.tail = self.head;
+            None
+        } else {
+            self.tail = wrap_index(self.tail.wrapping_add(n), self.ring.len());
+            self.next()
+        }
+    }
+
     #[inline]
     fn last(mut self) -> Option<&'a mut T> {
         self.next_back()