validate_with.rb

RSpec::Matchers.define :validate_with do |expected_validator|
  match do |subject|
    @validator = current_validator(subject, expected_validator)

    if @validator.options.present?
      @validator.options.all? do |_, condition|
        subject.instance_eval(&condition)
      end
    end
  end

  def current_validator(subject, expected_validator)
    subject.class.validators.find do |validator|
      validator.class == expected_validator
    end
  end

  description do
    "validates with #{expected}"
  end

  failure_message do
    "expected to validate with #{expected}"
  end

  failure_message_when_negated do
    "expected to not validate with #{expected}"
  end
end