class Rack::Builder

def self.new_from_string(builder_script, path = "(rackup)", **options)

a Rack::Builder block, returning a Rack application.
Evaluate the given +builder_script+ string in the context of 
def self.new_from_string(builder_script, path = "(rackup)", **options)
  builder = self.new(**options)
  # We want to build a variant of TOPLEVEL_BINDING with self as a Rack::Builder instance.
  # We cannot use instance_eval(String) as that would resolve constants differently.
  binding = BUILDER_TOPLEVEL_BINDING.call(builder)
  eval(builder_script, binding, path)
  return builder.to_app
end