Lazy-allocate @api_class and @point_in_time_copies on InheritableSetting

CHANGELOG.md

@@ -43,6 +43,7 @@
 * [#2726](https://github.com/ruby-grape/grape/pull/2726): Reuse one `AttributesIterator` per validator and drop the unused `Enumerable` mixin - [@ericproulx](https://github.com/ericproulx).
 * [#2728](https://github.com/ruby-grape/grape/pull/2728): Deprecate passing a positional options Hash to `auth`/`http_basic`/`http_digest`; pass keyword arguments instead - [@ericproulx](https://github.com/ericproulx).
 * [#2733](https://github.com/ruby-grape/grape/pull/2733): Drop the dead `active_support/core_ext/hash/reverse_merge` require; call `ActiveSupport::HashWithIndifferentAccess.new(...)` directly at call sites - [@ericproulx](https://github.com/ericproulx).
+* [#2740](https://github.com/ruby-grape/grape/pull/2740): Lazy-allocate `@api_class` and `@point_in_time_copies` on `Grape::Util::InheritableSetting` so unused settings layers don't carry an empty Hash and empty Array each - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 #### Fixes

lib/grape/util/inheritable_setting.rb

@@ -5,7 +5,17 @@ module Util
     # A branchable, inheritable settings object which can store both stackable
     # and inheritable values (see InheritableValues and StackableValues).
     class InheritableSetting
-      attr_reader :route, :api_class, :namespace, :namespace_inheritable, :namespace_stackable, :namespace_reverse_stackable, :parent, :point_in_time_copies
+      attr_reader :route, :namespace, :namespace_inheritable, :namespace_stackable, :namespace_reverse_stackable, :parent
+
+      # Lazy-allocated; +api_class+ and +point_in_time_copies+ are rarely
+      # written on most settings layers, so don't pay for a Hash/Array each.
+      def api_class
+        @api_class ||= {}
+      end
+
+      def point_in_time_copies
+        @point_in_time_copies ||= []
+      end
 
       # Retrieve global settings.
       def self.global
@@ -24,14 +34,13 @@ def self.reset_global!
       # #inherit_from).
       def initialize
         @route = {}
-        @api_class = {}
         @namespace = InheritableValues.new # only inheritable from a parent when
         # used with a mount, or should every API::Class be a separate namespace by default?
         @namespace_inheritable = InheritableValues.new
         @namespace_stackable = StackableValues.new
         @namespace_reverse_stackable = ReverseStackableValues.new
-        @point_in_time_copies = []
         @parent = nil
+        # @api_class and @point_in_time_copies stay nil until first access.
       end
 
       # Return the class-level global properties.
@@ -53,7 +62,7 @@ def inherit_from(parent)
         namespace_reverse_stackable.inherited_values = parent.namespace_reverse_stackable
         @route = parent.route.merge(route)
 
-        point_in_time_copies.each { |cloned_one| cloned_one.inherit_from parent }
+        @point_in_time_copies&.each { |cloned_one| cloned_one.inherit_from parent }
       end
 
       # Create a point-in-time copy of this settings instance, with clones of