implement nth_back for Box

src/liballoc/boxed.rs

@@ -662,6 +662,9 @@ impl<I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for Box<I> {
     fn next_back(&mut self) -> Option<I::Item> {
         (**self).next_back()
     }
+    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
+        (**self).nth_back(n)
+    }
 }
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for Box<I> {

src/liballoc/lib.rs

@@ -115,6 +115,7 @@
 #![feature(maybe_uninit)]
 #![feature(alloc_layout_extra)]
 #![feature(try_trait)]
+#![feature(iter_nth_back)]
 
 // Allow testing this library