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

implement nth_back for Range(Inclusive)

This is part of  rust-lang#54054.

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")]
@@ -438,6 +451,34 @@ impl<A: Step> DoubleEndedIterator for ops::RangeInclusive<A> {
         })
     }
 
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<A> {
+        self.compute_is_empty();
+        if self.is_empty.unwrap_or_default() {
+            return None;
+        }
+
+        if let Some(minus_n) = self.end.sub_usize(n) {
+            use crate::cmp::Ordering::*;
+
+            match minus_n.partial_cmp(&self.start) {
+                Some(Greater) => {
+                    self.is_empty = Some(false);
+                    self.end = minus_n.sub_one();
+                    return Some(minus_n);
+                }
+                Some(Equal) => {
+                    self.is_empty = Some(true);
+                    return Some(minus_n);
+                }
+                _ => {}
+            }
+        }
+
+        self.is_empty = Some(true);
+        None
+    }
+
     #[inline]
     fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R where
         Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>

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));
@@ -1714,6 +1731,26 @@ fn test_range_inclusive_nth() {
     assert_eq!(ExactSizeIterator::is_empty(&r), true);
 }
 
+#[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)]