Capture and print app warnings on test run

Configure not to fail the test, for now

test/capture_warnings.rb

@@ -0,0 +1,57 @@
+# https://raw.githubusercontent.com/metric_fu/metric_fu/master/spec/capture_warnings.rb
+require "tempfile"
+require "fileutils"
+
+class CaptureWarnings
+  def initialize(fail_on_warnings = true)
+    @fail_on_warnings = fail_on_warnings
+    @stderr_file = Tempfile.new("app.stderr")
+    @app_root ||= Dir.pwd
+    @output_dir = File.join(app_root, "tmp")
+    FileUtils.mkdir_p(output_dir)
+    @bundle_dir = File.join(app_root, "bundle")
+  end
+
+  def before_tests
+    $stderr.reopen(stderr_file.path)
+    $VERBOSE = true
+    at_exit { $stderr.reopen(STDERR) }
+  end
+
+  def after_tests
+    stderr_file.rewind
+    lines = stderr_file.read.split("\n").uniq
+    stderr_file.close!
+
+    $stderr.reopen(STDERR)
+
+    app_warnings, other_warnings = lines.partition { |line|
+      line.include?(app_root) && !line.include?(bundle_dir)
+    }
+
+    if app_warnings.any?
+      puts <<-WARNINGS
+#{'-' * 30} app warnings: #{'-' * 30}
+
+#{app_warnings.join("\n")}
+
+#{'-' * 75}
+      WARNINGS
+    end
+
+    if other_warnings.any?
+      File.write(File.join(output_dir, "warnings.txt"), other_warnings.join("\n") << "\n")
+      puts
+      puts "Non-app warnings written to tmp/warnings.txt"
+      puts
+    end
+
+    # fail the build...
+    if fail_on_warnings && app_warnings.any?
+      abort "Failing build due to app warnings: #{app_warnings.inspect}"
+    end
+  end
+
+  private
+  attr_reader :stderr_file, :app_root, :output_dir, :bundle_dir, :fail_on_warnings
+end

test/test_helper.rb

@@ -11,6 +11,12 @@
 # Ensure backward compatibility with Minitest 4
 Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)
 
+require "capture_warnings"
+@capture_warnings = CaptureWarnings.new(fail_build = false)
+@capture_warnings.before_tests
+at_exit do
+  @capture_warnings.after_tests
+end
 require 'active_model_serializers'
 
 class Foo < Rails::Application