Rollup merge of #63265 - JohnTitor:implement-nth-back-for-chunksexactmut, r=scottmcm

Implement `nth_back` for ChunksExactMut

This is a part of #54054.

r? @scottmcm

1 files changed, 16 insertions, 0 deletions

diff --git a/src/libcore/slice/mod.rs b/src/libcore/slice/mod.rs
index ce5af13d4ca..bfbbb15c8d4 100644
--- a/src/libcore/slice/mod.rs
+++ b/src/libcore/slice/mod.rs
@@ -4637,6 +4637,22 @@ impl<'a, T> DoubleEndedIterator for ChunksExactMut<'a, T> {
             Some(tail)
         }
     }
+
+    #[inline]
+    fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
+        let len = self.len();
+        if n >= len {
+            self.v = &mut [];
+            None
+        } else {
+            let start = (len - 1 - n) * self.chunk_size;
+            let end = start + self.chunk_size;
+            let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);
+            let (head, nth_back) = temp.split_at_mut(start);
+            self.v = head;
+            Some(nth_back)
+        }
+    }
 }
 
 #[stable(feature = "chunks_exact", since = "1.31.0")]