RuboCop / Sorbet sigil rule

sorbet_sigil.rb

# typed: true
# @prettier

require 'sorbet-runtime'

# Put this somewhere like lib/rubocop/cop/lint/sorbet_sigil.rb and then
# require it in .rubocop.yml:
#
#     require:
#       - ./lib/rubocop/cop/lint/sorbet_sigil.rb
#
# If you use Rails etc, make sure to place it somewhere that isn't loaded in
# production (as most likely RuboCop won't be available so it'll error)

module RuboCop
  module Cop
    module Lint
      # This cop ensures the first line of the file contains a valid Sorbet sigil
      #
      # @example
      #
      #  # bad
      #  # (start of file)
      #  # class Foo; end
      #
      #  # bad
      #  # (start of file)
      #  # # typed: no
      #
      #  # good
      #  # (start of file)
      #  # # typed: true
      class SorbetSigil < Cop
        extend T::Sig

        MSG = 'File must have a Sorbet sigil'.freeze

        sig { params(processed_source: RuboCop::ProcessedSource).void }
        def investigate(processed_source)
          token = processed_source.tokens.first
          return if token.nil? || valid_sorbet_sigil(token)

          add_offense(token, location: token.pos)
        end

        sig { params(token: RuboCop::Token).returns(T::Boolean) }
        def valid_sorbet_sigil(token)
          token.type == :tCOMMENT &&
            !token.text.to_s.strip.match(
              /^#\s*typed:\s*(ignore|false|true|strict|strong)$/,
            )
              .nil?
        end
      end
    end
  end
end