lib/rubocop/cop/style/trailing_comma_in_block_args.rb


# frozen_string_literal: true

module RuboCop
  module Cop
    module Style
      # Checks whether trailing commas in block arguments are
      # required. Blocks with only one argument and a trailing comma require
      # that comma to be present. Blocks with more than one argument never
      # require a trailing comma.
      #
      # @safety
      #   This cop is unsafe because a trailing comma can indicate there are
      #   more parameters that are not used.
      #
      #   For example:
      #   [source,ruby]
      #   ----
      #   # with a trailing comma
      #   {foo: 1, bar: 2, baz: 3}.map {|key,| key }
      #   #=> [:foo, :bar, :baz]
      #
      #   # without a trailing comma
      #   {foo: 1, bar: 2, baz: 3}.map {|key| key }
      #   #=> [[:foo, 1], [:bar, 2], [:baz, 3]]
      #   ----
      #
      #   This can be fixed by replacing the trailing comma with a placeholder
      #   argument (such as `|key, _value|`).
      #
      # @example
      #   # bad
      #   add { |foo, bar,| foo + bar }
      #
      #   # good
      #   add { |foo, bar| foo + bar }
      #
      #   # good
      #   add { |foo,| foo }
      #
      #   # good
      #   add { foo }
      #
      #   # bad
      #   add do |foo, bar,|
      #     foo + bar
      #   end
      #
      #   # good
      #   add do |foo, bar|
      #     foo + bar
      #   end
      #
      #   # good
      #   add do |foo,|
      #     foo
      #   end
      #
      #   # good
      #   add do
      #     foo + bar
      #   end
      class TrailingCommaInBlockArgs < Base
        extend AutoCorrector

        MSG = 'Useless trailing comma present in block arguments.'

        def on_block(node) # rubocop:disable InternalAffairs/NumblockHandler
          # lambda literal (`->`) never has block arguments.
          return if node.send_node.lambda_literal?
          return unless useless_trailing_comma?(node)

          last_comma_pos = last_comma(node).pos

          add_offense(last_comma_pos) { |corrector| corrector.replace(last_comma_pos, '') }
        end

        private

        def useless_trailing_comma?(node)
          arg_count(node) > 1 && trailing_comma?(node)
        end

        def arg_count(node)
          node.arguments.each_descendant(:arg, :optarg, :kwoptarg).to_a.size
        end

        def trailing_comma?(node)
          argument_tokens(node).last.comma?
        end

        def last_comma(node)
          argument_tokens(node).last
        end

        def argument_tokens(node)
          tokens = processed_source.tokens_within(node)
          pipes = tokens.select { |token| token.type == :tPIPE }
          begin_pos, end_pos = pipes.map { |pipe| tokens.index(pipe) }

          tokens[begin_pos + 1..end_pos - 1]
        end
      end
    end
  end
end