Rollup merge of #106985 - jofas:106746-fix, r=ChrisDenton

Enhanced doucmentation of binary search methods for `slice` and `VecDeque` for unsorted instances

Fixes #106746. Issue #106746 raises the concern that the binary search methods for slices and deques aren't explicit enough about the fact that they are only applicable to sorted slices/deques. I changed the explanation for these methods. I took the relatively harsh description of the behaviour of binary search on unsorted collections ("unspecified and meaningless") from the description of the [`partition_point`](https://doc.rust-lang.org/std/primitive.slice.html#method.partition_point) method:

> If this slice is not partitioned, the returned result is unspecified and meaningless, as this method performs a kind of binary search.

Author: JohnTitor

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

@@ -2394,7 +2394,8 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Binary searches this `VecDeque` for a given element.
-    /// This behaves similarly to [`contains`] if this `VecDeque` is sorted.
+    /// If the `VecDeque` is not sorted, the returned result is unspecified and
+    /// meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2404,7 +2405,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// See also [`binary_search_by`], [`binary_search_by_key`], and [`partition_point`].
     ///
-    /// [`contains`]: VecDeque::contains
     /// [`binary_search_by`]: VecDeque::binary_search_by
     /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
     /// [`partition_point`]: VecDeque::partition_point
@@ -2450,12 +2450,13 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Binary searches this `VecDeque` with a comparator function.
-    /// This behaves similarly to [`contains`] if this `VecDeque` is sorted.
     ///
-    /// The comparator function should implement an order consistent
-    /// with the sort order of the deque, returning an order code that
-    /// indicates whether its argument is `Less`, `Equal` or `Greater`
-    /// than the desired target.
+    /// The comparator function should return an order code that indicates
+    /// whether its argument is `Less`, `Equal` or `Greater` the desired
+    /// target.
+    /// If the `VecDeque` is not sorted or if the comparator function does not
+    /// implement an order consistent with the sort order of the underlying
+    /// `VecDeque`, the returned result is unspecified and meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2465,7 +2466,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// See also [`binary_search`], [`binary_search_by_key`], and [`partition_point`].
     ///
-    /// [`contains`]: VecDeque::contains
     /// [`binary_search`]: VecDeque::binary_search
     /// [`binary_search_by_key`]: VecDeque::binary_search_by_key
     /// [`partition_point`]: VecDeque::partition_point
@@ -2505,10 +2505,11 @@ impl<T, A: Allocator> VecDeque<T, A> {
     }
 
     /// Binary searches this `VecDeque` with a key extraction function.
-    /// This behaves similarly to [`contains`] if this `VecDeque` is sorted.
     ///
     /// Assumes that the deque is sorted by the key, for instance with
     /// [`make_contiguous().sort_by_key()`] using the same key extraction function.
+    /// If the deque is not sorted by the key, the returned result is
+    /// unspecified and meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2518,7 +2519,6 @@ impl<T, A: Allocator> VecDeque<T, A> {
     ///
     /// See also [`binary_search`], [`binary_search_by`], and [`partition_point`].
     ///
-    /// [`contains`]: VecDeque::contains
     /// [`make_contiguous().sort_by_key()`]: VecDeque::make_contiguous
     /// [`binary_search`]: VecDeque::binary_search
     /// [`binary_search_by`]: VecDeque::binary_search_by

library/core/src/slice/mod.rs

@@ -2387,7 +2387,8 @@ impl<T> [T] {
     }
 
     /// Binary searches this slice for a given element.
-    /// This behaves similarly to [`contains`] if this slice is sorted.
+    /// If the slice is not sorted, the returned result is unspecified and
+    /// meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2399,7 +2400,6 @@ impl<T> [T] {
     ///
     /// See also [`binary_search_by`], [`binary_search_by_key`], and [`partition_point`].
     ///
-    /// [`contains`]: slice::contains
     /// [`binary_search_by`]: slice::binary_search_by
     /// [`binary_search_by_key`]: slice::binary_search_by_key
     /// [`partition_point`]: slice::partition_point
@@ -2462,12 +2462,13 @@ impl<T> [T] {
     }
 
     /// Binary searches this slice with a comparator function.
-    /// This behaves similarly to [`contains`] if this slice is sorted.
     ///
-    /// The comparator function should implement an order consistent
-    /// with the sort order of the underlying slice, returning an
-    /// order code that indicates whether its argument is `Less`,
-    /// `Equal` or `Greater` the desired target.
+    /// The comparator function should return an order code that indicates
+    /// whether its argument is `Less`, `Equal` or `Greater` the desired
+    /// target.
+    /// If the slice is not sorted or if the comparator function does not
+    /// implement an order consistent with the sort order of the underlying
+    /// slice, the returned result is unspecified and meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2479,7 +2480,6 @@ impl<T> [T] {
     ///
     /// See also [`binary_search`], [`binary_search_by_key`], and [`partition_point`].
     ///
-    /// [`contains`]: slice::contains
     /// [`binary_search`]: slice::binary_search
     /// [`binary_search_by_key`]: slice::binary_search_by_key
     /// [`partition_point`]: slice::partition_point
@@ -2548,10 +2548,11 @@ impl<T> [T] {
     }
 
     /// Binary searches this slice with a key extraction function.
-    /// This behaves similarly to [`contains`] if this slice is sorted.
     ///
     /// Assumes that the slice is sorted by the key, for instance with
     /// [`sort_by_key`] using the same key extraction function.
+    /// If the slice is not sorted by the key, the returned result is
+    /// unspecified and meaningless.
     ///
     /// If the value is found then [`Result::Ok`] is returned, containing the
     /// index of the matching element. If there are multiple matches, then any
@@ -2563,7 +2564,6 @@ impl<T> [T] {
     ///
     /// See also [`binary_search`], [`binary_search_by`], and [`partition_point`].
     ///
-    /// [`contains`]: slice::contains
     /// [`sort_by_key`]: slice::sort_by_key
     /// [`binary_search`]: slice::binary_search
     /// [`binary_search_by`]: slice::binary_search_by