Rollup merge of #102507 - scottmcm:more-binary-search-docs, r=m-ou-se

More slice::partition_point examples

After seeing the discussion of `binary_search` vs `partition_point` in #101999, I thought some more example code could be helpful.

library/core/src/slice/mod.rs

@@ -2359,6 +2359,28 @@ impl<T> [T] {
     /// assert!(match r { Ok(1..=4) => true, _ => false, });
     /// ```
     ///
+    /// If you want to find that whole *range* of matching items, rather than
+    /// an arbitrary matching one, that can be done using [`partition_point`]:
+    /// ```
+    /// let s = [0, 1, 1, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55];
+    ///
+    /// let low = s.partition_point(|x| x < &1);
+    /// assert_eq!(low, 1);
+    /// let high = s.partition_point(|x| x <= &1);
+    /// assert_eq!(high, 5);
+    /// let r = s.binary_search(&1);
+    /// assert!((low..high).contains(&r.unwrap()));
+    ///
+    /// assert!(s[..low].iter().all(|&x| x < 1));
+    /// assert!(s[low..high].iter().all(|&x| x == 1));
+    /// assert!(s[high..].iter().all(|&x| x > 1));
+    ///
+    /// // For something not found, the "range" of equal items is empty
+    /// assert_eq!(s.partition_point(|x| x < &11), 9);
+    /// assert_eq!(s.partition_point(|x| x <= &11), 9);
+    /// assert_eq!(s.binary_search(&11), Err(9));
+    /// ```
+    ///
     /// If you want to insert an item to a sorted vector, while maintaining
     /// sort order, consider using [`partition_point`]:
     ///
@@ -3787,6 +3809,16 @@ impl<T> [T] {
     /// assert!(v[i..].iter().all(|&x| !(x < 5)));
     /// ```
     ///
+    /// If all elements of the slice match the predicate, including if the slice
+    /// is empty, then the length of the slice will be returned:
+    ///
+    /// ```
+    /// let a = [2, 4, 8];
+    /// assert_eq!(a.partition_point(|x| x < &100), a.len());
+    /// let a: [i32; 0] = [];
+    /// assert_eq!(a.partition_point(|x| x < &100), 0);
+    /// ```
+    ///
     /// If you want to insert an item to a sorted vector, while maintaining
     /// sort order:
     ///