Add RuboCop cop enforcing Data.define over Struct.new

.rubocop.yml

@@ -4,6 +4,9 @@ inherit_mode:
   merge:
     - Exclude
 
+require:
+  - ./lib/rubocop/cop/style/prefer_data_define
+
 # Find out more about the rubocop cops  https://github.com/bbatsov/rubocop/blob/master/config/enabled.yml
 
 AllCops:

config/application.rb

@@ -19,7 +19,7 @@ class Application < Rails::Application
     # Please, add to the `ignore` list any other `lib` subdirectories that do
     # not contain `.rb` files, or that should not be reloaded or eager loaded.
     # Common ones are `templates`, `generators`, or `middleware`, for example.
-    config.autoload_lib(ignore: %w[assets tasks omniauth])  
+    config.autoload_lib(ignore: %w[assets tasks omniauth rubocop])
 
     # Configuration for the application, engines, and railties goes here.
     #

lib/rubocop/cop/style/prefer_data_define.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Style
+      # Prefer Data.define over Struct.new for immutable value objects.
+      #
+      # Data defines immutable value objects with keyword-aware equality;
+      # Struct instances are mutable, so the rewrite is not safely mechanical.
+      class PreferDataDefine < Base
+        MSG = 'Prefer `Data.define` over `Struct.new` for immutable value objects.'
+
+        def on_send(node)
+          return unless node.receiver&.const_type? && node.receiver.const_name == 'Struct'
+          return unless node.method?(:new)
+
+          add_offense(node)
+        end
+      end
+    end
+  end
+end

spec/helpers/email_header_helper_spec.rb

@@ -6,7 +6,7 @@
   end
 
   describe '#mail_to_member' do
-    let(:member) { Struct.new(:id, :email).new(1, 'test@example.com') }
+    let(:member) { instance_double(Member, id: 1, email: 'test@example.com') }
 
     it 'calls mail with correct arguments for valid email' do
       allow(helper).to receive(:mail).with(
@@ -38,37 +38,37 @@
     end
 
     it 'returns SkippedEmail for nil email' do
-      member = Struct.new(:id, :email).new(1, nil)
+      member = instance_double(Member, id: 1, email: nil)
       result = helper.mail_to_member(member, 'Test Subject')
       expect(result).to be_a(EmailHeaderHelper::SkippedEmail)
     end
 
     it 'returns SkippedEmail for blank email' do
-      member = Struct.new(:id, :email).new(1, '')
+      member = instance_double(Member, id: 1, email: '')
       result = helper.mail_to_member(member, 'Test Subject')
       expect(result).to be_a(EmailHeaderHelper::SkippedEmail)
     end
 
     it 'returns SkippedEmail for invalid email format' do
-      member = Struct.new(:id, :email).new(1, 'invalid-email')
+      member = instance_double(Member, id: 1, email: 'invalid-email')
       result = helper.mail_to_member(member, 'Test Subject')
       expect(result).to be_a(EmailHeaderHelper::SkippedEmail)
     end
 
     it 'returns SkippedEmail for email missing @ symbol' do
-      member = Struct.new(:id, :email).new(1, 'invalidexample.com')
+      member = instance_double(Member, id: 1, email: 'invalidexample.com')
       result = helper.mail_to_member(member, 'Test Subject')
       expect(result).to be_a(EmailHeaderHelper::SkippedEmail)
     end
 
     it 'returns SkippedEmail for email missing TLD' do
-      member = Struct.new(:id, :email).new(1, 'invalid@example')
+      member = instance_double(Member, id: 1, email: 'invalid@example')
       result = helper.mail_to_member(member, 'Test Subject')
       expect(result).to be_a(EmailHeaderHelper::SkippedEmail)
     end
 
     it 'logs the skip' do
-      member = Struct.new(:id, :email).new(1, 'bad-email')
+      member = instance_double(Member, id: 1, email: 'bad-email')
       allow(Rails.logger).to receive(:info).with(/Skipped email to member 1/)
       helper.mail_to_member(member, 'Test Subject')
 

spec/rubocop/cop/style/prefer_data_define_spec.rb

@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+require 'rubocop'
+require 'rubocop/rspec/expect_offense'
+require 'rubocop/rspec/support'
+require_relative '../../../../lib/rubocop/cop/style/prefer_data_define'
+
+RSpec.describe RuboCop::Cop::Style::PreferDataDefine, :config, :ruby40 do
+  include RuboCop::RSpec::ExpectOffense
+
+  it 'registers an offense for Struct.new' do
+    expect_offense(<<~RUBY)
+      Struct.new(:id, :email)
+      ^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Data.define` over `Struct.new` for immutable value objects.
+    RUBY
+  end
+
+  it 'registers an offense for ::Struct.new' do
+    expect_offense(<<~RUBY)
+      ::Struct.new(:id, :email)
+      ^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer `Data.define` over `Struct.new` for immutable value objects.
+    RUBY
+  end
+
+  it 'does not register an offense for Data.define' do
+    expect_no_offenses(<<~RUBY)
+      Data.define(:id, :email)
+    RUBY
+  end
+
+  it 'does not register an offense for namespaced receivers' do
+    expect_no_offenses(<<~RUBY)
+      Other::Struct.new(:id, :email)
+    RUBY
+  end
+
+  it 'does not register an offense for other Struct methods' do
+    expect_no_offenses(<<~RUBY)
+      Struct.members
+    RUBY
+  end
+
+  it 'does not register an offense for OtherClass.new' do
+    expect_no_offenses(<<~RUBY)
+      Member.new(:id, :email)
+    RUBY
+  end
+end