[FIX #1376] Add new Rails/Env cop

Author: cdudas17

changelog/new_env_cop.md

@@ -0,0 +1 @@
+* [#1376](https://github.com/rubocop/rubocop-rails/issues/1376): Add new `Rails/Env` cop. ([@cdudas17][])

config/default.yml

@@ -441,6 +441,11 @@ Rails/EnumUniqueness:
   Include:
     - app/models/**/*.rb
 
+Rails/Env:
+  Description: 'Use Feature Flags or config instead of `Rails.env`.'
+  Enabled: false
+  VersionAdded: '<<next>>'
+
 Rails/EnvLocal:
   Description: 'Use `Rails.env.local?` instead of `Rails.env.development? || Rails.env.test?`.'
   Enabled: pending

lib/rubocop/cop/rails/env.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks for usage of `Rails.env` which can be replaced with Feature Flags
+      #
+      # @example
+      #
+      #   # bad
+      #   Rails.env.production? || Rails.env.local?
+      #
+      #   # good
+      #   if FeatureFlag.enabled?(:new_feature)
+      #     # new feature code
+      #   end
+      #
+      class Env < Base
+        MSG = 'Use Feature Flags or config instead of `Rails.env`.'
+        RESTRICT_ON_SEND = %i[env].freeze
+        # This allow list is derived from:
+        # (Rails.env.methods - Object.instance_methods).select { |m| m.to_s.end_with?('?') }
+        # and then removing the environment specific methods like development?, test?, production?, local?
+        ALLOWED_LIST = Set.new(
+          %i[
+            unicode_normalized?
+            exclude?
+            empty?
+            acts_like_string?
+            include?
+            is_utf8?
+            casecmp?
+            match?
+            starts_with?
+            ends_with?
+            start_with?
+            end_with?
+            valid_encoding?
+            ascii_only?
+            between?
+          ]
+        ).freeze
+
+        def on_send(node)
+          return unless node.receiver&.const_name == 'Rails'
+
+          parent = node.parent
+          return unless parent&.predicate_method?
+
+          return if ALLOWED_LIST.include?(parent.method_name)
+
+          add_offense(parent)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -48,6 +48,7 @@
 require_relative 'rails/enum_hash'
 require_relative 'rails/enum_syntax'
 require_relative 'rails/enum_uniqueness'
+require_relative 'rails/env'
 require_relative 'rails/env_local'
 require_relative 'rails/environment_comparison'
 require_relative 'rails/environment_variable_access'

spec/rubocop/cop/rails/env_spec.rb

@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::Env, :config do
+  it 'registers an offense for `Rails.env.development? || Rails.env.test?`' do
+    expect_offense(<<~RUBY)
+      Rails.env.development? || Rails.env.test?
+                                ^^^^^^^^^^^^^^^ Use Feature Flags or config instead of `Rails.env`.
+      ^^^^^^^^^^^^^^^^^^^^^^ Use Feature Flags or config instead of `Rails.env`.
+    RUBY
+  end
+
+  it 'registers an offense for `Rails.env.production?`' do
+    expect_offense(<<~RUBY)
+      Rails.env.production?
+      ^^^^^^^^^^^^^^^^^^^^^ Use Feature Flags or config instead of `Rails.env`.
+    RUBY
+  end
+
+  it 'does not register an offense for `Rails.env`' do
+    expect_no_offenses(<<~RUBY)
+      Rails.env
+    RUBY
+  end
+
+  it 'does not register an offense for valid Rails.env methods' do
+    expect_no_offenses(<<~RUBY)
+      Rails.env.capitalize
+      Rails.env.empty?
+    RUBY
+  end
+
+  it 'does not register an offense for unrelated config' do
+    expect_no_offenses(<<~RUBY)
+      Rails.environment
+    RUBY
+  end
+end