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

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")]

src/libcore/tests/slice.rs

@@ -374,6 +374,25 @@ fn test_chunks_exact_mut_nth() {
     assert_eq!(c2.next(), None);
 }
 
+#[test]
+fn test_chunks_exact_mut_nth_back() {
+    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
+    let mut c = v.chunks_exact_mut(2);
+    assert_eq!(c.nth_back(1).unwrap(), &[2, 3]);
+    assert_eq!(c.next().unwrap(), &[0, 1]);
+    assert_eq!(c.next(), None);
+
+    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c2 = v2.chunks_exact_mut(3);
+    assert_eq!(c2.nth_back(0).unwrap(), &[0, 1, 2]);
+    assert_eq!(c2.next(), None);
+    assert_eq!(c2.next_back(), None);
+
+    let v3: &mut [i32] = &mut [0, 1, 2, 3, 4];
+    let mut c3 = v3.chunks_exact_mut(10);
+    assert_eq!(c3.nth_back(0), None);
+}
+
 #[test]
 fn test_chunks_exact_mut_last() {
     let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];