Rollup merge of #61048 - wizAmit:feature/nth_back_chunks, r=scottmcm

Feature/nth back chunks

A succinct implementation for nth_back on chunks. Thank you @timvermeulen for the guidance.

r? @timvermeulen

1 files changed, 18 insertions, 0 deletions

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index 1c67e750e40..0e782bef39d 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -4158,6 +4158,24 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
             Some(snd)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &[];
+            None
+        } else {
+            let start = (len - 1 - n) * self.chunk_size;
+            let end = match start.checked_add(self.chunk_size) {
+                Some(res) => cmp::min(res, self.v.len()),
+                None => self.v.len(),
+            };
+            let nth_back = &self.v[start..end];
+            self.v = &self.v[..start];
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]