Rollup merge of rust-lang#60454 - acrrd:issues/54054_skip, r=scottmcm

Add custom nth_back to Skip

Implementation of nth_back for Skip.
Part of rust-lang#54054

src/libcore/iter/adapters/mod.rs

@@ -1509,6 +1509,20 @@ impl<I> DoubleEndedIterator for Skip<I> where I: DoubleEndedIterator + ExactSize
         }
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<I::Item> {
+        let len = self.len();
+        if n < len {
+            self.iter.nth_back(n)
+        } else {
+            if len > 0 {
+                // consume the original iterator
+                self.iter.nth_back(len-1);
+            }
+            None
+        }
+    }
+
     fn try_rfold<Acc, Fold, R>(&mut self, init: Acc, mut fold: Fold) -> R where
         Self: Sized, Fold: FnMut(Acc, Self::Item) -> R, R: Try<Ok=Acc>
     {

src/libcore/tests/iter.rs

@@ -2333,6 +2333,40 @@ fn test_skip_try_folds() {
     assert_eq!(iter.next_back(), Some(24));
 }
 
+#[test]
+fn test_skip_nth_back() {
+    let xs = [0, 1, 2, 3, 4, 5];
+    let mut it = xs.iter().skip(2);
+    assert_eq!(it.nth_back(0), Some(&5));
+    assert_eq!(it.nth_back(1), Some(&3));
+    assert_eq!(it.nth_back(0), Some(&2));
+    assert_eq!(it.nth_back(0), None);
+
+    let ys = [2, 3, 4, 5];
+    let mut ity = ys.iter();
+    let mut it = xs.iter().skip(2);
+    assert_eq!(it.nth_back(1), ity.nth_back(1));
+    assert_eq!(it.clone().nth(0), ity.clone().nth(0));
+    assert_eq!(it.nth_back(0), ity.nth_back(0));
+    assert_eq!(it.clone().nth(0), ity.clone().nth(0));
+    assert_eq!(it.nth_back(0), ity.nth_back(0));
+    assert_eq!(it.clone().nth(0), ity.clone().nth(0));
+    assert_eq!(it.nth_back(0), ity.nth_back(0));
+    assert_eq!(it.clone().nth(0), ity.clone().nth(0));
+
+    let mut it = xs.iter().skip(2);
+    assert_eq!(it.nth_back(4), None);
+    assert_eq!(it.nth_back(0), None);
+
+    let mut it = xs.iter();
+    it.by_ref().skip(2).nth_back(3);
+    assert_eq!(it.next_back(), Some(&1));
+
+    let mut it = xs.iter();
+    it.by_ref().skip(2).nth_back(10);
+    assert_eq!(it.next_back(), Some(&1));
+}
+
 #[test]
 fn test_take_try_folds() {
     let f = &|acc, x| i32::checked_add(2*acc, x);