Finding where deprecation warnings are coming from (Capybara, Rails, etc, anything that calls Kernel#warn)

find_warn.rb

TracePoint.trace(:c_call) do |tp|
  # the various methods on tp tell you where warn is defined, but we want to know about the call stack, 
  # in particular the test code that the test library didn't like and decided to warn us about, 
  # so let's Array#grep the caller strings, filtering out the rspec library lines
  if tp.method_id == :warn
    app_lines = caller(0).grep(%r{enough-of-your-app-code-path-to-match/})
    app_lines_without_reporter = app_lines.grep_v(/name-of-your-rspec-support-file-holding-this-code/)
    p app_lines_without_reporter.join("\n")
    p
  end
end