Rollup merge of #61671 - koalatux:nth-back-range, r=KodrAus

implement nth_back for Range(Inclusive)

This is part of  #54054.

1 files changed, 37 insertions, 0 deletions

diff --git a/src/libcore/tests/iter.rs b/src/libcore/tests/iter.rs
index bedb9e75612..020618ae7ae 100644
--- a/src/libcore/tests/iter.rs
+++ b/src/libcore/tests/iter.rs
@@ -1658,6 +1658,23 @@ fn test_range_nth() {
 }
 
 #[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));
     assert_eq!((10..).nth(1), Some(11));
@@ -1715,6 +1732,26 @@ fn test_range_inclusive_nth() {
 }
 
 #[test]
+fn test_range_inclusive_nth_back() {
+    assert_eq!((10..=15).nth_back(0), Some(15));
+    assert_eq!((10..=15).nth_back(1), Some(14));
+    assert_eq!((10..=15).nth_back(5), Some(10));
+    assert_eq!((10..=15).nth_back(6), None);
+    assert_eq!((-120..=80_i8).nth_back(200), Some(-120));
+
+    let mut r = 10_u8..=20;
+    assert_eq!(r.nth_back(2), Some(18));
+    assert_eq!(r, 10..=17);
+    assert_eq!(r.nth_back(2), Some(15));
+    assert_eq!(r, 10..=14);
+    assert_eq!(r.is_empty(), false);
+    assert_eq!(ExactSizeIterator::is_empty(&r), false);
+    assert_eq!(r.nth_back(10), None);
+    assert_eq!(r.is_empty(), true);
+    assert_eq!(ExactSizeIterator::is_empty(&r), true);
+}
+
+#[test]
 fn test_range_step() {
     #![allow(deprecated)]