Implement nth_back

src/iter.rs

@@ -217,6 +217,18 @@ impl<T, N: ArrayLength> DoubleEndedIterator for GenericArrayIter<T, N> {
 
         ret
     }
+
+    fn nth_back(&mut self, n: usize) -> Option<T> {
+        let next_back = self.index_back - cmp::min(n, self.len());
+
+        unsafe {
+            ptr::drop_in_place(self.array.get_unchecked_mut(next_back..self.index_back));
+        }
+
+        self.index_back = next_back;
+
+        self.next_back()
+    }
 }
 
 impl<T, N: ArrayLength> ExactSizeIterator for GenericArrayIter<T, N> {

tests/iter.rs

@@ -90,6 +90,20 @@ fn test_into_iter_nth() {
     assert_eq!(iter.nth(1).unwrap(), v[4]);
 }
 
+#[test]
+fn test_into_iter_nth_back() {
+    let v = arr![0, 1, 2, 3, 4];
+
+    for i in 0..v.len() {
+        assert_eq!(v.into_iter().nth_back(i).unwrap(), v[v.len() - i - 1]);
+    }
+    assert_eq!(v.into_iter().nth_back(v.len()), None);
+
+    let mut iter = v.into_iter();
+    assert_eq!(iter.nth_back(2).unwrap(), v[2]);
+    assert_eq!(iter.nth_back(1).unwrap(), v[0]);
+}
+
 #[test]
 fn test_into_iter_last() {
     let v = arr![0, 1, 2, 3, 4];
@@ -173,6 +187,16 @@ fn test_into_iter_drops() {
     }
     assert_eq!(i.get(), 5);
 
+    let i = Cell::new(0);
+    {
+        let mut iter = v(&i).into_iter();
+        let _x = iter.nth_back(2);
+        assert_eq!(i.get(), 2);
+        let _y = iter.last();
+        assert_eq!(i.get(), 3);
+    }
+    assert_eq!(i.get(), 5);
+
     let i = Cell::new(0);
     for (index, _x) in v(&i).into_iter().enumerate() {
         assert_eq!(i.get(), index);