Rollup merge of #121343 - Takashiidobe:takashi/examples-for-slice, r=Mark-Simulacrum

Add examples for some methods on slices

Adds some examples to some methods on slice.

`is_empty` didn't have an example for an empty slice, even though `str` and the collections all have one, so I added that in.

`first_mut` and `last_mut` didn't have an example for what happens when the slice is empty, whereas `first` and `last` do, so I added that too.

library/core/src/slice/mod.rs

@@ -146,6 +146,9 @@ impl<T> [T] {
     /// ```
     /// let a = [1, 2, 3];
     /// assert!(!a.is_empty());
+    ///
+    /// let b: &[i32] = &[];
+    /// assert!(b.is_empty());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_stable(feature = "const_slice_is_empty", since = "1.39.0")]
@@ -185,6 +188,9 @@ impl<T> [T] {
     ///     *first = 5;
     /// }
     /// assert_eq!(x, &[5, 1, 2]);
+    ///
+    /// let y: &mut [i32] = &mut [];
+    /// assert_eq!(None, y.first_mut());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]
@@ -297,7 +303,7 @@ impl<T> [T] {
         if let [.., last] = self { Some(last) } else { None }
     }
 
-    /// Returns a mutable reference to the last item in the slice.
+    /// Returns a mutable reference to the last item in the slice, or `None` if it is empty.
     ///
     /// # Examples
     ///
@@ -308,6 +314,9 @@ impl<T> [T] {
     ///     *last = 10;
     /// }
     /// assert_eq!(x, &[0, 1, 10]);
+    ///
+    /// let y: &mut [i32] = &mut [];
+    /// assert_eq!(None, y.last_mut());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     #[rustc_const_unstable(feature = "const_slice_first_last", issue = "83570")]