make skip a double ended iterator

src/libcore/iter.rs

@@ -3851,6 +3851,17 @@ impl<I> Iterator for Skip<I> where I: Iterator {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I> ExactSizeIterator for Skip<I> where I: ExactSizeIterator {}
 
+#[stable(feature = "double_ended_skip_iterator", since = "1.8.0")]
+impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSizeIterator {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        if self.len() > 0 {
+            self.iter.next_back()
+        } else {
+            None
+        }
+    }
+}
+
 /// An iterator that only iterates over the first `n` iterations of `iter`.
 ///
 /// This `struct` is created by the [`take()`] method on [`Iterator`]. See its

src/libcoretest/iter.rs

@@ -303,6 +303,44 @@ fn test_iterator_skip() {
     assert_eq!(it.len(), 0);
 }
 
+#[test]
+fn test_iterator_skip_doubleended() {
+    let xs = [0, 1, 2, 3, 5, 13, 15, 16, 17, 19, 20, 30];
+    let mut it = xs.iter().rev().skip(5);
+    assert_eq!(it.next(), Some(&15));
+    assert_eq!(it.by_ref().rev().next(), Some(&0));
+    assert_eq!(it.next(), Some(&13));
+    assert_eq!(it.by_ref().rev().next(), Some(&1));
+    assert_eq!(it.next(), Some(&5));
+    assert_eq!(it.by_ref().rev().next(), Some(&2));
+    assert_eq!(it.next(), Some(&3));
+    assert_eq!(it.next(), None);
+    let mut it = xs.iter().rev().skip(5).rev();
+    assert_eq!(it.next(), Some(&0));
+    assert_eq!(it.rev().next(), Some(&15));
+    let mut it_base = xs.iter();
+    {
+        let mut it = it_base.by_ref().skip(5).rev();
+        assert_eq!(it.next(), Some(&30));
+        assert_eq!(it.next(), Some(&20));
+        assert_eq!(it.next(), Some(&19));
+        assert_eq!(it.next(), Some(&17));
+        assert_eq!(it.next(), Some(&16));
+        assert_eq!(it.next(), Some(&15));
+        assert_eq!(it.next(), Some(&13));
+        assert_eq!(it.next(), None);
+    }
+    // make sure the skipped parts have not been consumed
+    assert_eq!(it_base.next(), Some(&0));
+    assert_eq!(it_base.next(), Some(&1));
+    assert_eq!(it_base.next(), Some(&2));
+    assert_eq!(it_base.next(), Some(&3));
+    assert_eq!(it_base.next(), Some(&5));
+    assert_eq!(it_base.next(), None);
+    let it = xs.iter().skip(5).rev();
+    assert_eq!(it.last(), Some(&13));
+}
+
 #[test]
 fn test_iterator_skip_nth() {
     let xs = [0, 1, 2, 3, 5, 13, 15, 16, 17, 19, 20, 30];