alloc: Implement rfold for VecDeque iterators

src/liballoc/lib.rs

@@ -98,6 +98,7 @@
 #![feature(generic_param_attrs)]
 #![feature(i128_type)]
 #![feature(inclusive_range)]
+#![feature(iter_rfold)]
 #![feature(lang_items)]
 #![feature(needs_allocator)]
 #![feature(nonzero)]

src/liballoc/vec_deque.rs

@@ -1973,6 +1973,14 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
         self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
         unsafe { Some(self.ring.get_unchecked(self.head)) }
     }
+
+    fn rfold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
+        where F: FnMut(Acc, Self::Item) -> Acc
+    {
+        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
+        accum = back.iter().rfold(accum, &mut f);
+        front.iter().rfold(accum, &mut f)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -2058,6 +2066,14 @@ impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
             Some(&mut *(elem as *mut _))
         }
     }
+
+    fn rfold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
+        where F: FnMut(Acc, Self::Item) -> Acc
+    {
+        let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
+        accum = back.iter_mut().rfold(accum, &mut f);
+        front.iter_mut().rfold(accum, &mut f)
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]