Soutaro (major Ruby typing contributor) has launched a new RBS::Inline project that enables Ruby type annotations in comments instead of a companion RBS file:

# rbs_inline: enabled

class Person
  attr_reader :name #:: String

  attr_reader :addresses #:: Array[String]

  # @rbs name: String
  # @rbs addresses: Array[String]
  # @rbs returns void
  def initialize(name:, addresses:)
    @name = name
    @addresses = addresses
  end

  def to_s #:: String
    "Person(name = #{name}, addresses = #{addresses.join(", ")})"
  end

  # @rbs yields (String) -> void
  def each_address(&block) #:: void
    addresses.each(&block)
  end
end

The README notes that if all goes well, this will be merged back upstream into the main RBS gem and become a first class feature. During a recent trial run of RBS I found having to have two tabs open for every Ruby file (the .rb and its companion .rbs) to be somewhat painful, and having an alternative is a welcome addition.