Avoid instance variables in helpers

config/default.yml

@@ -103,6 +103,10 @@ Rails/HasManyOrHasOneDependent:
   Include:
     - app/models/**/*.rb
 
+Rails/HelperInstanceVariable:
+  Include:
+    - app/helpers/**/*.rb
+
 Rails/HttpStatus:
   EnforcedStyle: symbolic
   SupportedStyles:

config/enabled.yml

@@ -102,6 +102,10 @@ Rails/HasManyOrHasOneDependent:
   StyleGuide: 'https://github.com/rubocop-hq/rails-style-guide#has_many-has_one-dependent-option'
   Enabled: true
 
+Rails/HelperInstanceVariable:
+  Description: 'Do not use instance variables in helpers'
+  Enabled: true
+
 Rails/HttpPositionalArguments:
   Description: 'Use keyword arguments instead of positional arguments in http method calls.'
   Enabled: true

lib/rubocop/cop/rails/helper_instance_variable.rb

@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # This cop checks for use of the helper methods which reference
+      # instance variables.
+      #
+      # Relying on instance variables makes it difficult to re-use helper
+      # methods.
+      #
+      # If it seems awkward to explicitly pass in each dependent
+      # variable, consider moving the behaviour elsewhere, for
+      # example to a model, decorator or presenter.
+      #
+      # @example
+      #   # bad
+      #   def welcome_message
+      #     "Hello #{@user.name}"
+      #   end
+      #
+      #   # good
+      #   def welcome_message(user)
+      #     "Hello #{user.name}"
+      #   end
+      class HelperInstanceVariable < Cop
+        MSG = 'Do not use instance variables in helpers.'.freeze
+
+        def on_ivar(node)
+          add_offense(node)
+        end
+
+        def on_ivasgn(node)
+          add_offense(node, location: :name)
+        end
+      end
+    end
+  end
+end

lib/rubocop/cop/rails_cops.rb

@@ -29,6 +29,7 @@ module Cop
 require_relative 'rails/find_each'
 require_relative 'rails/has_and_belongs_to_many'
 require_relative 'rails/has_many_or_has_one_dependent'
+require_relative 'rails/helper_instance_variable'
 require_relative 'rails/http_positional_arguments'
 require_relative 'rails/http_status'
 require_relative 'rails/inverse_of'

manual/cops_rails.md

@@ -804,6 +804,42 @@ Include | `app/models/**/*.rb` | Array
 
 * [https://github.com/rubocop-hq/rails-style-guide#has_many-has_one-dependent-option](https://github.com/rubocop-hq/rails-style-guide#has_many-has_one-dependent-option)
 
+## Rails/HelperInstanceVariable
+
+Enabled by default | Supports autocorrection
+--- | ---
+Enabled | No
+
+This cop checks for use of the helper methods which reference
+instance variables.
+
+Relying on instance variables makes it difficult to re-use helper
+methods.
+
+If it seems awkward to explicitly pass in each dependent
+variable, consider moving the behaviour elsewhere, for
+example to a model, decorator or presenter.
+
+### Examples
+
+```ruby
+# bad
+def welcome_message
+  "Hello #{@user.name}"
+end
+
+# good
+def welcome_message(user)
+  "Hello #{user.name}"
+end
+```
+
+### Configurable attributes
+
+Name | Default value | Configurable values
+--- | --- | ---
+Include | `app/helpers/**/*.rb` | Array
+
 ## Rails/HttpPositionalArguments
 
 Enabled by default | Supports autocorrection

spec/rubocop/cop/rails/helper_instance_variable_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::HelperInstanceVariable do
+  subject(:cop) { described_class.new }
+
+  it 'reports uses of instance variables' do
+    expect_offense(<<-'RUBY'.strip_indent)
+      def welcome_message
+        "Hello #{@user.name}"
+                 ^^^^^ Do not use instance variables in helpers.
+      end
+    RUBY
+  end
+
+  it 'reports instance variable assignments' do
+    expect_offense(<<-RUBY.strip_indent)
+      def welcome_message(user)
+        @user_name = user.name
+        ^^^^^^^^^^ Do not use instance variables in helpers.
+      end
+    RUBY
+  end
+
+  specify do
+    expect_no_offenses(<<-'RUBY'.strip_indent)
+      def welcome_message(user)
+        "Hello #{user.name}"
+      end
+    RUBY
+  end
+end