instance_variable.rb

# frozen_string_literal: true

module RuboCop
  module Cop
    module RSpec
      # Checks for instance variable usage in specs.
      #
      # This cop can be configured with the option `AssignmentOnly` which
      # will configure the cop to only register offenses on instance
      # variable usage if the instance variable is also assigned within
      # the spec
      #
      # @example
      #   # bad
      #   describe MyClass do
      #     before { @foo = [] }
      #     it { expect(@foo).to be_empty }
      #   end
      #
      #   # good
      #   describe MyClass do
      #     let(:foo) { [] }
      #     it { expect(foo).to be_empty }
      #   end
      #
      # @example with AssignmentOnly configuration
      #   # rubocop.yml
      #   # RSpec/InstanceVariable:
      #   #   AssignmentOnly: false
      #
      #   # bad
      #   describe MyClass do
      #     before { @foo = [] }
      #     it { expect(@foo).to be_empty }
      #   end
      #
      #   # allowed
      #   describe MyClass do
      #     it { expect(@foo).to be_empty }
      #   end
      #
      #   # good
      #   describe MyClass do
      #     let(:foo) { [] }
      #     it { expect(foo).to be_empty }
      #   end
      #
      class InstanceVariable < Base
        include TopLevelGroup

        MSG = 'Avoid instance variables – use let, ' \
              'a method call, or a local variable (if possible).'

        # @!method dynamic_class?(node)
        def_node_matcher :dynamic_class?, <<-PATTERN
          (block (send (const nil? :Class) :new ...) ...)
        PATTERN

        # @!method custom_matcher?(node)
        def_node_matcher :custom_matcher?, <<-PATTERN
          (block {
            (send nil? :matcher sym)
            (send (const (const nil? :RSpec) :Matchers) :define sym)
          } ...)
        PATTERN

        # @!method ivar_usage(node)
        def_node_search :ivar_usage, '$(ivar $_)'

        # @!method ivar_assigned?(node)
        def_node_search :ivar_assigned?, '(ivasgn % ...)'

        def on_top_level_group(node)
          ivar_usage(node) do |ivar, name|
            next if valid_usage?(ivar)
            next if assignment_only? && !ivar_assigned?(node, name)

            add_offense(ivar)
          end
        end

        private

        def valid_usage?(node)
          node.each_ancestor(:block).any? do |block|
            dynamic_class?(block) || custom_matcher?(block)
          end
        end

        def assignment_only?
          cop_config['AssignmentOnly']
        end
      end
    end
  end
end