Add ConstantInRoutes Cop

changelog/new_add_constant_in_routes_cop.md

@@ -0,0 +1 @@
+* [#1492](https://github.com/rubocop/rubocop-rails/pull/1492) Add new `ConstantInRoutes` Cop. ([@skipkayhil][])

config/default.yml

@@ -298,6 +298,12 @@ Rails/CompactBlank:
   Safe: false
   VersionAdded: '2.13'
 
+Rails/ConstantInRoutes:
+  Description: 'Checks for constants defined in config/routes.rb files.'
+  Enabled: pending
+  Safe: false
+  VersionAdded: <<next>>
+
 Rails/ContentTag:
   Description: 'Use `tag.something` instead of `tag(:something)`.'
   Reference:

lib/rubocop/cop/rails/constant_in_routes.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Prevent defining constants in config/routes.rb files.
+      #
+      # Defining constants in a routes.rb file can lead to constant redefinition
+      # warnings because routes.rb is a reloadable file.
+      #
+      # @example
+      #   # bad
+      #   Rails.application.routes.draw do
+      #     ADMIN_CONSTRAINT = ->(req) { req.subdomain == 'admin' }
+      #
+      #     get "/users", constraints: ADMIN_CONSTRAINT
+      #   end
+      #
+      #   # good
+      #   Rails.application.routes.draw do
+      #     admin_constraint = ->(req) { req.subdomain == 'admin' }
+      #
+      #     get "/users", constraints: admin_constraint
+      #   end
+      #
+      class ConstantInRoutes < Base
+        MSG = 'Do not define constants in config/routes.rb. Use a local variable or move this constant somewhere else.'
+
+        def on_casgn(node)
+          return unless in_routes_file?
+
+          add_offense(node)
+        end
+
+        private
+
+        def in_routes_file?
+          processed_source.file_path&.end_with?('config/routes.rb')
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -33,6 +33,7 @@
 require_relative 'rails/blank'
 require_relative 'rails/bulk_change_table'
 require_relative 'rails/compact_blank'
+require_relative 'rails/constant_in_routes'
 require_relative 'rails/content_tag'
 require_relative 'rails/create_table_with_timestamps'
 require_relative 'rails/dangerous_column_names'

spec/rubocop/cop/rails/constant_in_routes_spec.rb

@@ -0,0 +1,41 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::ConstantInRoutes, :config do
+  context 'when in config/routes.rb file' do
+    let(:source) { 'config/routes.rb' }
+
+    it 'registers an offense when defining constants in routes' do
+      expect_offense(<<~RUBY, source)
+        API_VERSION = 'v1'
+        ^^^^^^^^^^^^^^^^^^ Do not define constants in config/routes.rb. Use a local variable or move this constant somewhere else.
+        Rails.application.routes.draw do
+          BASE_PATH = '/api'
+          ^^^^^^^^^^^^^^^^^^ Do not define constants in config/routes.rb. Use a local variable or move this constant somewhere else.
+          get "\#{BASE_PATH}\#{API_VERSION}/users", to: 'users#index'
+        end
+      RUBY
+    end
+
+    it 'registers an offense when defining nested constants' do
+      expect_offense(<<~RUBY, source)
+        Api::VERSION = 'v1'
+        ^^^^^^^^^^^^^^^^^^^ Do not define constants in config/routes.rb. Use a local variable or move this constant somewhere else.
+        Rails.application.routes.draw do
+          get "/users", to: 'users#index'
+        end
+      RUBY
+    end
+
+    it 'registers an offense when defining constants in opened classes' do
+      expect_offense(<<~RUBY, source)
+        class Api
+          VERSION = 'v1'
+          ^^^^^^^^^^^^^^ Do not define constants in config/routes.rb. Use a local variable or move this constant somewhere else.
+        end
+        Rails.application.routes.draw do
+          get "/users", to: 'users#index'
+        end
+      RUBY
+    end
+  end
+end