Add option for class_attribute default (#29270)

* Allow a default value to be declared for class_attribute

* Convert to using class_attribute default rather than explicit setter

* Removed instance_accessor option by mistake

* False is a valid default value

* Documentation

3 files changed, 4 insertions, 7 deletions

diff --git a/activemodel/lib/active_model/attribute_methods.rb b/activemodel/lib/active_model/attribute_methods.rb
index b5c0b43b61..b3b39bf7ae 100644
--- a/activemodel/lib/active_model/attribute_methods.rb
+++ b/activemodel/lib/active_model/attribute_methods.rb
@@ -68,9 +68,8 @@ module ActiveModel
     CALL_COMPILABLE_REGEXP = /\A[a-zA-Z_]\w*[!?]?\z/
 
     included do
-      class_attribute :attribute_aliases, :attribute_method_matchers, instance_writer: false
-      self.attribute_aliases = {}
-      self.attribute_method_matchers = [ClassMethods::AttributeMethodMatcher.new]
+      class_attribute :attribute_aliases, instance_writer: false, default: {}
+      class_attribute :attribute_method_matchers, instance_writer: false, default: [ ClassMethods::AttributeMethodMatcher.new ]
     end
 
     module ClassMethods
diff --git a/activemodel/lib/active_model/serializers/json.rb b/activemodel/lib/active_model/serializers/json.rb
index a9d92eb92a..205b84ddb4 100644
--- a/activemodel/lib/active_model/serializers/json.rb
+++ b/activemodel/lib/active_model/serializers/json.rb
@@ -10,8 +10,7 @@ module ActiveModel
       included do
         extend ActiveModel::Naming
 
-        class_attribute :include_root_in_json, instance_writer: false
-        self.include_root_in_json = false
+        class_attribute :include_root_in_json, instance_writer: false, default: false
       end
 
       # Returns a hash representing the model. Some configuration can be
diff --git a/activemodel/lib/active_model/validations.rb b/activemodel/lib/active_model/validations.rb
index 1f14a068d1..ae1d69f685 100644
--- a/activemodel/lib/active_model/validations.rb
+++ b/activemodel/lib/active_model/validations.rb
@@ -49,8 +49,7 @@ module ActiveModel
       private :validation_context=
       define_callbacks :validate, scope: :name
 
-      class_attribute :_validators, instance_writer: false
-      self._validators = Hash.new { |h, k| h[k] = [] }
+      class_attribute :_validators, instance_writer: false, default: Hash.new { |h, k| h[k] = [] }
     end
 
     module ClassMethods