Rubocop's cop to check if gem has version declared in Gemfile

.rubocop.yml

require:
  - ./lib/no_gem_version.rb

no_gem_version.rb

module RuboCop
  module Cop
    module Style
      # Check that gems have version.
      #
      # Gems without versioning can cause a lot of truble when updated without notice
      #
      # @example
      #   # bad
      #   gem 'rubocop'
      #
      #   # good
      #   gem 'rubocop', '~> 0.50'
      class NoGemVersion < Cop
        GEMFILE_FILE_NAME = 'Gemfile'.freeze
        MSG = 'Gem `%<gem_name>s` does not have a version set.'.freeze

        def_node_matcher :defining_dependency?, <<-PATTERN
          (send _ :gem ...)
        PATTERN

        def initialize(config = nil, options = nil)
          super(config, options)
          @in_gemfile = false
        end

        def investigate(processed_source)
          @in_gemfile = true if processed_source.buffer.name.split('/').last == GEMFILE_FILE_NAME
        end

        def on_send(node)
          return unless @in_gemfile
          if defining_dependency?(node) && !has_version?(node)
            add_offense(node, :expression, format(MSG, gem_name: node.arguments[0].children[0]))
          end
        end

        private

        # node.node_parts => [nil, :gem, s(:str, "some_gem"), s(:str, "some_version")]
        def has_version?(node)
          node.node_parts.size >= 4
        end
      end
    end
  end
end