custom rubocop cop for checking if spec files include `require 'rails_helper'`

redundant_require_rails_helper.rb

# frozen_string_literal: true

# put this file at .rubocop/cop/rspec/redundant_require_rails_helper.rb
# then ensure you have the following in .rubocop.yml:
#
# require:
#   - .rubocop/cop/rspec/redundant_require_rails_helper.rb
# 
# RSpec/RedundantRequireRailsHelper:
#   Enabled: true


module RuboCop
  module Cop
    module RSpec
      # Checks spec files for the presence of `require 'rails_helper'`.
      #
      # @example
      #   # bad
      #   require 'rails_helper'
      #
      #   # good
      #   # No `require 'rails_helper'` at the top of the file
      class RedundantRequireRailsHelper < Cop
        MSG = "Do not require 'rails_helper' in spec files since it's already required by the .rspec file."

        def on_new_investigation
          file_path = processed_source.file_path
          return unless spec_file?(file_path)

          require_nodes = processed_source.ast.each_node(:send).select do |send_node|
            require_rails_helper?(send_node)
          end

          add_offense_to_require_nodes(require_nodes)
        end

        private

        def spec_file?(file_path)
          file_path.end_with?("_spec.rb")
        end

        def require_rails_helper?(send_node)
          send_node.command?(:require) && rails_helper?(send_node.first_argument)
        end

        def rails_helper?(arg_node)
          arg_node.str_type? && ["rails_helper"].include?(arg_node.value)
        end

        def add_offense_to_require_nodes(nodes)
          nodes.each { |node| add_offense(node, message: MSG) }
        end
      end
    end
  end
end