calculate the ivars to remove in advance as a set and cache them in a

constant.

`view_assigns` can use the precalculated sets and remove instance
variables without allocating any extra arrays

actionmailer/lib/action_mailer/base.rb

@@ -373,7 +373,11 @@ class Base < AbstractController::Base
     include AbstractController::AssetPaths
     include AbstractController::Callbacks
 
-    self.protected_instance_variables = [:@_action_has_layout]
+    PROTECTED_IVARS = AbstractController::Rendering::DEFAULT_PROTECTED_INSTANCE_VARIABLES + [:@_action_has_layout]
+
+    def _protected_ivars # :nodoc:
+      PROTECTED_IVARS
+    end
 
     helper ActionMailer::MailHelper
 

actionpack/lib/abstract_controller/rendering.rb

 require 'active_support/concern'
 require 'active_support/core_ext/class/attribute'
+require 'set'
 
 module AbstractController
   class DoubleRenderError < Error
@@ -13,11 +14,6 @@ def initialize(message = nil)
   module Rendering
     extend ActiveSupport::Concern
 
-    included do
-      class_attribute :protected_instance_variables
-      self.protected_instance_variables = []
-    end
-
     # Normalize arguments, options and then delegates render_to_body and
     # sticks the result in self.response_body.
     # :api: public
@@ -55,8 +51,6 @@ def rendered_format
       Mime::TEXT
     end
 
-    require 'set'
-
     DEFAULT_PROTECTED_INSTANCE_VARIABLES = Set.new %w(
       @_action_name @_response_body @_formats @_prefixes @_config
       @_view_context_class @_view_renderer @_lookup_context
@@ -113,7 +107,7 @@ def _normalize_render(*args, &block)
     end
 
     def _protected_ivars # :nodoc:
-      DEFAULT_PROTECTED_INSTANCE_VARIABLES + protected_instance_variables
+      DEFAULT_PROTECTED_INSTANCE_VARIABLES
     end
   end
 end

actionpack/lib/action_controller/base.rb

@@ -261,10 +261,17 @@ def self.without_modules(*modules)
     end
 
     # Define some internal variables that should not be propagated to the view.
-    self.protected_instance_variables = [
+    PROTECTED_IVARS = AbstractController::Rendering::DEFAULT_PROTECTED_INSTANCE_VARIABLES + [
       :@_status, :@_headers, :@_params, :@_env, :@_response, :@_request,
-      :@_view_runtime, :@_stream, :@_url_options, :@_action_has_layout
-    ]
+      :@_view_runtime, :@_stream, :@_url_options, :@_action_has_layout ]
+
+    def _protected_ivars # :nodoc:
+      PROTECTED_IVARS
+    end
+
+    def self.protected_instance_variables
+      PROTECTED_IVARS
+    end
 
     ActiveSupport.run_load_hooks(:action_controller, self)
   end