module Minitest::Spec::DSL

def let name, &block
  name = name.to_s
  pre, post = "let '#{name}' cannot ", ". Please use another name."
  methods = Minitest::Spec.instance_methods.map(&:to_s) - %w[subject]
  raise ArgumentError, "#{pre}begin with 'test'#{post}" if
    name =~ /\Atest/
  raise ArgumentError, "#{pre}override a method in Minitest::Spec#{post}" if
    methods.include? name
  define_method name do
    @_memoized ||= {}
    @_memoized.fetch(name) { |k| @_memoized[k] = instance_eval(&block) }
  end
end