Print deprecation notices to stderr after all tests have run

test.rb

# I've always disliked the way deprecation notices would appear while the test
# suite is running: they break up the neat little rows of dots (or F's, as it
# may be) which causes noise and makes the notices hard to read and understand.
#
# Adding this snippet to `config/environments/test.rb` will buffer all your
# deprecation notices until the end of the test run and keep your test output
# nice and tidy.
#
Rails.application.config do
  # ...
  
  # Collect deprecation notices in a StringIO buffer while tests are running
  deprecation_buffer = StringIO.new
  config.active_support.deprecation = ->(message, callstack) {
    deprecation_buffer.puts message
    deprecation_buffer.puts callstack.join("\n  ") if ActiveSupport::Deprecation.debug
  }

  # Print deprecation notices to stderr after all tests have run
  at_exit {
    $stderr.puts deprecation_buffer.string
  }
end