implement nth_back for Fuse

src/libcore/iter/adapters/mod.rs

@@ -1789,6 +1789,17 @@ impl<I> DoubleEndedIterator for Fuse<I> where I: DoubleEndedIterator {
         }
     }
 
+    #[inline]
+    default fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
+        if self.done {
+            None
+        } else {
+            let nth = self.iter.nth_back(n);
+            self.done = nth.is_none();
+            nth
+        }
+    }
+
     #[inline]
     default fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
@@ -1877,6 +1888,11 @@ fn next_back(&mut self) -> Option<<I as Iterator>::Item> {
         self.iter.next_back()
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<<I as Iterator>::Item> {
+        self.iter.nth_back(n)
+    }
+
     #[inline]
     fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>