# File lib/bundler/runtime.rb, line 149
    def clean(dry_run = false)
      gem_bins             = Dir["#{Gem.dir}/bin/*"]
      git_dirs             = Dir["#{Gem.dir}/bundler/gems/*"]
      git_cache_dirs       = Dir["#{Gem.dir}/cache/bundler/git/*"]
      gem_dirs             = Dir["#{Gem.dir}/gems/*"]
      gem_files            = Dir["#{Gem.dir}/cache/*.gem"]
      gemspec_files        = Dir["#{Gem.dir}/specifications/*.gemspec"]
      spec_gem_paths       = []
      # need to keep git sources around
      spec_git_paths       = @definition.spec_git_paths
      spec_git_cache_dirs  = []
      spec_gem_executables = []
      spec_cache_paths     = []
      spec_gemspec_paths   = []
      specs.each do |spec|
        spec_gem_paths << spec.full_gem_path
        # need to check here in case gems are nested like for the rails git repo
        md = %r{(.+bundler/gems/.+-[a-f0-9]{7,12})}.match(spec.full_gem_path)
        spec_git_paths << md[1] if md
        spec_gem_executables << spec.executables.collect do |executable|
          e = "#{Bundler.rubygems.gem_bindir}/#{executable}"
          [e, "#{e}.bat"]
        end
        spec_cache_paths << spec.cache_file
        spec_gemspec_paths << spec.spec_file
        spec_git_cache_dirs << spec.source.cache_path.to_s if spec.source.is_a?(Bundler::Source::Git)
      end
      spec_gem_paths.uniq!
      spec_gem_executables.flatten!

      stale_gem_bins       = gem_bins - spec_gem_executables
      stale_git_dirs       = git_dirs - spec_git_paths - ["#{Gem.dir}/bundler/gems/extensions"]
      stale_git_cache_dirs = git_cache_dirs - spec_git_cache_dirs
      stale_gem_dirs       = gem_dirs - spec_gem_paths
      stale_gem_files      = gem_files - spec_cache_paths
      stale_gemspec_files  = gemspec_files - spec_gemspec_paths

      removed_stale_gem_dirs = stale_gem_dirs.collect {|dir| remove_dir(dir, dry_run) }
      removed_stale_git_dirs = stale_git_dirs.collect {|dir| remove_dir(dir, dry_run) }
      output = removed_stale_gem_dirs + removed_stale_git_dirs

      unless dry_run
        stale_files = stale_gem_bins + stale_gem_files + stale_gemspec_files
        stale_files.each {|file| FileUtils.rm(file) if File.exist?(file) }
        stale_git_cache_dirs.each {|dir| FileUtils.rm_rf(dir) if File.exist?(dir) }
      end

      output
    end