Hack to speed up Guard::RSpec runs with Guardfile and spec_helper.rb

Guardfile

# Signal to interested subprocesses that they are being run from Guard
ENV["GUARD"] = "fast"

guard 'rspec', version: 2, cli: "--color --format Fuubar" do
  watch(%r{^spec/.+_spec\.rb})
  watch(%r{^lib/(.+)\.rb})     { |m| "spec/lib/#{m[1]}_spec.rb" }
  watch('spec/spec_helper.rb') { "spec" }
end

# Only run this once to stop Guardfile re-evaluation making a hash of things
unless ::Guard::RSpec.instance_methods.include?(:run_on_change_orig)
  # There's a "hook" branch of Guard that will make this class patch
  # unnecessary: https://github.com/guard/guard/tree/hook
  class ::Guard::RSpec
    # Monkey-patch to indicate to RSpec that it can run slow tests if
    # only one file was changed
    alias_method :run_on_change_orig, :run_on_change
    def run_on_change(paths)
      if paths.length == 1 && File.read(paths.first).include?("adapter: :slow")
        ENV["GUARD"] = "all"
      end
      run_on_change_orig(paths)
    ensure
      ENV["GUARD"] = "fast"
    end
  end
end

spec_helper.rb

PROJECT_ROOT = File.expand_path(File.join(File.dirname(__FILE__), "..")).freeze

GUARD = ENV["GUARD"]

RSpec.configure do |config|
  config.filter_run(focus: true)
  config.filter_run_excluding(adapter: :slow) if GUARD == "fast"
  config.run_all_when_everything_filtered = true
end