Use linear versions of descendants in GroupDescendantsFinder

app/finders/group_descendants_finder.rb

@@ -87,9 +87,13 @@ class GroupDescendantsFinder
       visible_to_user = visible_to_user.or(authorized_to_user)
     end
 
-    hierarchy_for_parent
-      .descendants
-      .where(visible_to_user)
+    group_to_query = if Feature.enabled?(:linear_group_descendants_finder, current_user, default_enabled: :yaml)
+                       parent_group
+                     else
+                       hierarchy_for_parent
+                     end
+
+    group_to_query.descendants.where(visible_to_user)
     # rubocop: enable CodeReuse/Finder
   end
   # rubocop: enable CodeReuse/ActiveRecord
@@ -155,7 +159,13 @@ class GroupDescendantsFinder
   # rubocop: disable CodeReuse/ActiveRecord
   def projects_matching_filter
     # rubocop: disable CodeReuse/Finder
-    projects_nested_in_group = Project.where(namespace_id: hierarchy_for_parent.base_and_descendants.select(:id))
+    objects_in_hierarchy = if Feature.enabled?(:linear_group_descendants_finder, current_user, default_enabled: :yaml)
+                             parent_group.self_and_descendants.as_ids
+                           else
+                             hierarchy_for_parent.base_and_descendants.select(:id)
+                           end
+
+    projects_nested_in_group = Project.where(namespace_id: objects_in_hierarchy)
     params_with_search = params.merge(search: params[:filter])
 
     ProjectsFinder.new(params: params_with_search,

config/feature_flags/development/linear_group_descendants_finder.yml

+---
+name: linear_group_descendants_finder
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68954
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/339440
+milestone: '14.6'
+type: development
+group: group::access
+default_enabled: false