Add custom nth_back for Zip

src/libcore/iter/adapters/zip.rs

@@ -62,6 +62,17 @@ impl<A, B> DoubleEndedIterator for Zip<A, B> where
     fn next_back(&mut self) -> Option<(A::Item, B::Item)> {
         ZipImpl::next_back(self)
     }
+
+    #[inline]
+    fn nth_back(&mut self, mut n: usize) -> Option<(A::Item, B::Item)> {
+        while let Some(x) = ZipImpl::next_back(self) {
+            if n == 0 {
+                return Some(x)
+            }
+            n -= 1;
+        }
+        None
+    }
 }
 
 // Zip specialization trait

src/libcore/iter/traits/double_ended.rs

@@ -69,7 +69,7 @@ pub trait DoubleEndedIterator: Iterator {
     /// Returns the `n`th element from the end of the iterator.
     ///
     /// This is essentially the reversed version of [`nth`]. Although like most indexing
-    /// operations, the count starts from zero, so `nth_back(0)` returns the first value fro
+    /// operations, the count starts from zero, so `nth_back(0)` returns the first value from
     /// the end, `nth_back(1)` the second, and so on.
     ///
     /// Note that all elements between the end and the returned element will be

src/libcore/tests/iter.rs

@@ -153,6 +153,22 @@ fn test_zip_nth() {
     assert_eq!(it.nth(3), None);
 }
 
+#[test]
+fn test_zip_nth_back() {
+    let xs = [0, 1, 2, 4, 5];
+    let ys = [10, 11, 12];
+    let mut it = xs.iter().zip(&ys);
+    assert_eq!(it.nth_back(0), Some((&2, &12)));
+    assert_eq!(it.nth_back(1), Some((&0, &10)));
+    assert_eq!(it.nth_back(0), None);
+
+    let mut it = xs.iter().zip(&ys);
+    assert_eq!(it.nth_back(3), None);
+
+    let mut it = ys.iter().zip(&xs);
+    assert_eq!(it.nth_back(3), None);
+}
+
 #[test]
 fn test_zip_nth_side_effects() {
     let mut a = Vec::new();