Find routes that will raise a routing error when requested

unroutable_routes.rake

desc 'Find routes that will raise a routing error when requested'
task unroutable_routes: :environment do
  # A lot of this code was taken from how `rake routes` works
  # https://github.com/rails/rails/blob/f95c0b7e96eb36bc3efc0c5beffbb9e84ea664e4/railties/lib/rails/commands/routes/routes_command.rb

  require 'action_dispatch/routing/inspector'

  unroutables = Rails.application.routes.routes.
     map { |r| ActionDispatch::Routing::RouteWrapper.new(r) }.
     reject { |r| r.internal? || r.engine? || r.path.starts_with?('/rails/') || !r.controller }.
     reject do |r|
       controller = "#{r.controller}_controller".classify.safe_constantize

       controller && (
         controller.method_defined?(r.action) ||
         Rails.root.join('app/views', r.controller).glob("#{r.action}.*").any?
       )
     end.
     map { |r| r.__getobj__ }

  if unroutables.present?
    inspector = ActionDispatch::Routing::RoutesInspector.new(unroutables)

    formatter = if ENV['EXPANDED']
      ActionDispatch::Routing::ConsoleFormatter::Expanded.new
    else
      ActionDispatch::Routing::ConsoleFormatter::Sheet.new
    end

    puts inspector.format(formatter)
  end

  puts "#{unroutables.size} unroutable routes"
end