missing DefaultRackup constant breaks existing config files

[indirect]

Running Puma from the current HEAD causes an exception with the Heroku-recommended puma.rb config file. Apparently the DefaultRackup constant has disappeared between 2.11.0 and d3bf7fe.

Here's the exception:

config/puma.rb:7:in `_load_from': uninitialized constant Puma::DSL::DefaultRackup (NameError)
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/dsl.rb:18:in `instance_eval'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/dsl.rb:18:in `_load_from'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/dsl.rb:7:in `block in load'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/dsl.rb:6:in `tap'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/dsl.rb:6:in `load'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/configuration.rb:32:in `load'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/cli.rb:545:in `parse_options'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/lib/puma/cli.rb:190:in `run'
        from /app/.bundle/ruby/2.2.0/bundler/gems/puma-d3bf7fea4bfe/bin/puma:10:in `<top (required)>'
        from /app/.bundle/ruby/2.2.0/bin/puma:23:in `load'
        from /app/.bundle/ruby/2.2.0/bin/puma:23:in `<main>'

And here's the puma.rb file:

workers Integer(ENV['WEB_CONCURRENCY'] || 2)
threads_count = Integer(ENV['MAX_THREADS'] || 5)
threads threads_count, threads_count

preload_app!

rackup      DefaultRackup
port        ENV['PORT']     || 3000
environment ENV['RACK_ENV'] || 'development'

on_worker_boot do
  ActiveSupport.on_load(:active_record) do
    ActiveRecord::Base.establish_connection
  end
end

shutdown_debug

[indirect]

I just discovered that everything works perfectly with the line rackup DefaultRackup removed completely, since (I assume) Puma can find the default config.ru file without being explicitly told to look for it. Still a pretty weird regression, though. :)