[Fix #461] Add new LoadDefaultsMismatch cop

changelog/new_add_rails_load_defaults_mismatch_cop.md

@@ -0,0 +1 @@
+* [#461](https://github.com/rubocop/rubocop-rails/issues/461): Add new `Rails/LoadDefaultsMismatch` cop that flags when the rails version in `config.load_defaults` version is outdated. ([@nzlaura][])

config/default.yml

@@ -714,6 +714,15 @@ Rails/LinkToBlank:
   Enabled: true
   VersionAdded: '0.62'
 
+Rails/LoadDefaultsMismatch:
+  Description: 'Checks that `config.load_defaults` version has the same major version as the target Rails version.'
+  StyleGuide: 'https://rails.rubystyle.guide/#config-defaults'
+  Enabled: pending
+  Safe: false
+  VersionAdded: '<<next>>'
+  Include:
+    - '**/config/application.rb'
+
 Rails/MailerName:
   Description: 'Mailer should end with `Mailer` suffix.'
   StyleGuide: 'https://rails.rubystyle.guide/#mailer-name'

lib/rubocop/cop/rails/load_defaults_mismatch.rb

@@ -0,0 +1,56 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks that `config.load_defaults` is called with a version that
+      # matches the target Rails version.
+      #
+      # When upgrading Rails, `rails app:update` sets `load_defaults` to the
+      # previous Rails version so you can incrementally adopt new defaults.
+      # Once the upgrade is complete, `load_defaults` should be updated to
+      # match the current Rails version. Not updating the version can mean your app
+      # drifts from the newer default configuration, making future upgrades
+      # harder.
+      #
+      # @safety
+      #   This cop's offense may be a false positive if you are still
+      #   mid-upgrade and have not yet finished adopting the new defaults.
+      #
+      # @example
+      #   # Assuming the project is using Rails 8.1
+      #
+      #   # bad
+      #   config.load_defaults 6.1
+      #
+      #   # good
+      #   config.load_defaults 8.1
+      #
+      class LoadDefaultsMismatch < Base
+        MSG = '`load_defaults` is set to `%<load_version>s` but the target Rails version is `%<rails_version>s`. ' \
+              'Update `load_defaults` to match the Rails version after completing the upgrade.'
+
+        RESTRICT_ON_SEND = %i[load_defaults].freeze
+
+        # @!method load_defaults_version(node)
+        def_node_matcher :load_defaults_version, <<~PATTERN
+          (send _ :load_defaults ${float_type? int_type?})
+        PATTERN
+
+        def on_send(node)
+          load_defaults_version(node) do |version_node|
+            load_version = version_node.value.to_f
+            rails_version = target_rails_version
+
+            return unless rails_version
+
+            return if load_version == rails_version
+
+            message = format(MSG, load_version: load_version, rails_version: rails_version)
+            add_offense(version_node, message: message)
+          end
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -79,6 +79,7 @@
 require_relative 'rails/inverse_of'
 require_relative 'rails/lexically_scoped_action_filter'
 require_relative 'rails/link_to_blank'
+require_relative 'rails/load_defaults_mismatch'
 require_relative 'rails/mailer_name'
 require_relative 'rails/match_route'
 require_relative 'rails/migration_class_name'

spec/rubocop/cop/rails/load_defaults_mismatch_spec.rb

@@ -0,0 +1,62 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::LoadDefaultsMismatch, :config do
+  context 'when targeting Rails 8.1', :rails81 do
+    it 'does not register an offense when load_defaults matches the target version' do
+      expect_no_offenses(<<~RUBY)
+        config.load_defaults 8.1
+      RUBY
+    end
+
+    it 'registers an offense when load_defaults has a different minor version' do
+      expect_offense(<<~RUBY)
+        config.load_defaults 7.0
+                             ^^^ `load_defaults` is set to `7.0` but the target Rails version is `8.1`. Update `load_defaults` to match the Rails version after completing the upgrade.
+      RUBY
+    end
+
+    it 'registers an offense when load_defaults has a different major version' do
+      expect_offense(<<~RUBY)
+        config.load_defaults 6.1
+                             ^^^ `load_defaults` is set to `6.1` but the target Rails version is `8.1`. Update `load_defaults` to match the Rails version after completing the upgrade.
+      RUBY
+    end
+
+    it 'registers an offense when load_defaults major version is ahead' do
+      expect_offense(<<~RUBY)
+        config.load_defaults 8.0
+                             ^^^ `load_defaults` is set to `8.0` but the target Rails version is `8.1`. Update `load_defaults` to match the Rails version after completing the upgrade.
+      RUBY
+    end
+  end
+
+  context 'when targeting Rails 8.0', :rails80 do
+    it 'does not register an offense when load_defaults matches' do
+      expect_no_offenses(<<~RUBY)
+        config.load_defaults 8.0
+      RUBY
+    end
+
+    it 'registers an offense when load_defaults is 7.0' do
+      expect_offense(<<~RUBY)
+        config.load_defaults 7.0
+                             ^^^ `load_defaults` is set to `7.0` but the target Rails version is `8.0`. Update `load_defaults` to match the Rails version after completing the upgrade.
+      RUBY
+    end
+  end
+
+  context 'when load_defaults receives an integer', :rails80 do
+    it 'does not register an offense when it matches' do
+      expect_no_offenses(<<~RUBY)
+        config.load_defaults 8
+      RUBY
+    end
+
+    it 'registers an offense when it does not match' do
+      expect_offense(<<~RUBY)
+        config.load_defaults 6
+                             ^ `load_defaults` is set to `6.0` but the target Rails version is `8.0`. Update `load_defaults` to match the Rails version after completing the upgrade.
+      RUBY
+    end
+  end
+end