BTree: clarify order sanity enforced by range searches

library/alloc/src/collections/btree/map.rs

@@ -1018,6 +1018,9 @@ impl<K, V> BTreeMap<K, V> {
     ///
     /// Panics if range `start > end`.
     /// Panics if range `start == end` and both bounds are `Excluded`.
+    /// May panic if the [`Ord`] implementation of type `T` is ill-defined,
+    /// either because it does not form a total order or because it does not
+    /// correspond to the [`Ord`] implementation of type `K`.
     ///
     /// # Examples
     ///
@@ -1061,6 +1064,9 @@ impl<K, V> BTreeMap<K, V> {
     ///
     /// Panics if range `start > end`.
     /// Panics if range `start == end` and both bounds are `Excluded`.
+    /// May panic if the [`Ord`] implementation of type `T` is ill-defined,
+    /// either because it does not form a total order or because it does not
+    /// correspond to the [`Ord`] implementation of type `K`.
     ///
     /// # Examples
     ///

library/alloc/src/collections/btree/navigate.rs

@@ -29,11 +29,18 @@ impl<BorrowType, K, V> LeafRange<BorrowType, K, V> {
 
 impl<BorrowType: marker::BorrowType, K, V> NodeRef<BorrowType, K, V, marker::LeafOrInternal> {
     /// Finds the distinct leaf edges delimiting a specified range in a tree.
-    /// Returns either a pair of different handles into the same tree or a pair
-    /// of empty options.
+    ///
+    /// If such distinct edges exist, returns them in ascending order, meaning
+    /// that a non-zero number of calls to `next_unchecked` on the `front` of
+    /// the result and/or calls to `next_back_unchecked` on the `back` of the
+    /// result will eventually reach the same edge.
+    ///
+    /// If there are no such edges, i.e., if the tree contains no key within
+    /// the range, returns a pair of empty options.
+    ///
     /// # Safety
-    /// Unless `BorrowType` is `Immut`, do not use the duplicate handles to
-    /// visit the same KV twice.
+    /// Unless `BorrowType` is `Immut`, do not use the handles to visit the same
+    /// KV twice.
     unsafe fn find_leaf_edges_spanning_range<Q: ?Sized, R>(
         self,
         range: R,

library/alloc/src/collections/btree/search.rs

@@ -68,13 +68,18 @@ impl<BorrowType: marker::BorrowType, K, V> NodeRef<BorrowType, K, V, marker::Lea
     /// of the range is different from the edge matching the upper bound, i.e.,
     /// the nearest node that has at least one key contained in the range.
     ///
-    /// If found, returns an `Ok` with that node, the pair of edge indices in it
-    /// delimiting the range, and the corresponding pair of bounds for
-    /// continuing the search in the child nodes, in case the node is internal.
+    /// If found, returns an `Ok` with that node, the strictly ascending pair of
+    /// edge indices in the node delimiting the range, and the corresponding
+    /// pair of bounds for continuing the search in the child nodes, in case
+    /// the node is internal.
     ///
     /// If not found, returns an `Err` with the leaf edge matching the entire
     /// range.
     ///
+    /// As a diagnostic service, panics if the range specifies impossible bounds
+    /// or if it witnesses that the `Ord` implementation of `Q` violates total
+    /// order or is inconsistent with the `Ord` implementation of `K`.
+    ///
     /// The result is meaningful only if the tree is ordered by key.
     pub fn search_tree_for_bifurcation<'r, Q: ?Sized, R>(
         mut self,
@@ -115,6 +120,10 @@ impl<BorrowType: marker::BorrowType, K, V> NodeRef<BorrowType, K, V, marker::Lea
             let (lower_edge_idx, lower_child_bound) = self.find_lower_bound_index(lower_bound);
             let (upper_edge_idx, upper_child_bound) = self.find_upper_bound_index(upper_bound);
             if lower_edge_idx > upper_edge_idx {
+                // Since we already checked the range bounds, this can only
+                // happen if `Q: Ord` does not implement a total order or does
+                // not correspond to the `K: Ord` implementation that is used
+                // while populating the tree.
                 panic!("Ord is ill-defined in BTreeMap range")
             }
             if lower_edge_idx < upper_edge_idx {