std: optimize the example of as_slices and as_mut_slices

Signed-off-by: xizheyin <xizheyin@smail.nju.edu.cn>

Author: xizheyin

library/alloc/src/collections/vec_deque/mod.rs

@@ -1328,14 +1328,18 @@ impl<T, A: Allocator> VecDeque<T, A> {
     /// deque.push_back(1);
     /// deque.push_back(2);
     ///
-    /// let slices = deque.as_slices();
-    /// assert!(slices.0.iter().chain(slices.1.iter()).eq([0, 1, 2].iter()));
+    /// let expected = [0, 1, 2];
+    /// let (front, back) = deque.as_slices();
+    /// assert_eq!(&expected[..front.len()], front);
+    /// assert_eq!(&expected[front.len()..], back);
     ///
     /// deque.push_front(10);
     /// deque.push_front(9);
     ///
-    /// let slices = deque.as_slices();
-    /// assert!(slices.0.iter().chain(slices.1.iter()).eq([9, 10, 0, 1, 2].iter()));
+    /// let expected = [9, 10, 0, 1, 2];
+    /// let (front, back) = deque.as_slices();
+    /// assert_eq!(&expected[..front.len()], front);
+    /// assert_eq!(&expected[front.len()..], back);
     /// ```
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
@@ -1363,20 +1367,28 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// let mut deque = VecDeque::new();
     ///
+    /// deque.push_back(0);
     /// deque.push_back(1);
-    /// deque.push_back(2);
-    /// deque.push_back(3);
-    /// deque.push_back(4);
     ///
-    /// let (front, back) = deque.as_mut_slices();
+    /// deque.push_front(10);
+    /// deque.push_front(9);
     ///
-    /// if !front.is_empty() {
-    ///     front[0] = 5;
-    ///     assert_eq!(deque[0], 5);
-    /// } else {
-    ///     back[back.len() - 1] = 5;
-    ///     assert_eq!(deque[deque.len() - 1], 5);
-    /// }
+    /// // Since the split point is not guaranteed, we may need to update
+    /// // either slice.
+    /// let mut update_nth = |index: usize, val: u32| {
+    ///     let (front, back) = deque.as_mut_slices();
+    ///     if index > front.len() - 1 {
+    ///         back[index - front.len()] = val;
+    ///     } else {
+    ///         front[index] = val;
+    ///     }
+    /// };
+    ///
+    /// update_nth(0, 42);
+    /// update_nth(2, 24);
+    ///
+    /// let v: Vec<_> = deque.into();
+    /// assert_eq!(v, [42, 10, 24, 1]);
     /// ```
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]