implement nth_back for Range

src/libcore/iter/range.rs

@@ -281,6 +281,19 @@ impl<A: Step> DoubleEndedIterator for ops::Range<A> {
             None
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<A> {
+        if let Some(minus_n) = self.end.sub_usize(n) {
+            if minus_n > self.start {
+                self.end = minus_n.sub_one();
+                return Some(self.end.clone())
+            }
+        }
+
+        self.end = self.start.clone();
+        None
+    }
 }
 
 #[stable(feature = "fused", since = "1.26.0")]

src/libcore/tests/iter.rs

@@ -1657,6 +1657,23 @@ fn test_range_nth() {
     assert_eq!(r, 20..20);
 }
 
+#[test]
+fn test_range_nth_back() {
+    assert_eq!((10..15).nth_back(0), Some(14));
+    assert_eq!((10..15).nth_back(1), Some(13));
+    assert_eq!((10..15).nth_back(4), Some(10));
+    assert_eq!((10..15).nth_back(5), None);
+    assert_eq!((-120..80_i8).nth_back(199), Some(-120));
+
+    let mut r = 10..20;
+    assert_eq!(r.nth_back(2), Some(17));
+    assert_eq!(r, 10..17);
+    assert_eq!(r.nth_back(2), Some(14));
+    assert_eq!(r, 10..14);
+    assert_eq!(r.nth_back(10), None);
+    assert_eq!(r, 10..10);
+}
+
 #[test]
 fn test_range_from_nth() {
     assert_eq!((10..).nth(0), Some(10));