for dirname := range c.fsDirnames() {
		if c.IndexDirectory != nil && !c.IndexDirectory(dirname) {
			continue
		}
		dirGate <- true
		wg.Add(1)
		go func(dirname string) {
			defer func() { <-dirGate }()
			defer wg.Done()

			list, err := c.fs.ReadDir(dirname)
			if err != nil {
				log.Printf("ReadDir(%q): %v; skipping directory", dirname, err)
				return // ignore this directory
			}
			for _, fi := range list {
				wg.Add(1)
				go func(fi os.FileInfo) {
					defer wg.Done()
					x.visitFile(dirname, fi)
				}(fi)
			}
		}(dirname)
	}
	wg.Wait()

func (b *treeBuilder) newDirTree(fset *token.FileSet, path, name string, depth int) *Directory {
	// ...
        for _, d := range list {
		filename := pathpkg.Join(path, d.Name())
		switch {
		case isPkgDir(d):
			ch := make(chan *Directory, 1)
			dirchs = append(dirchs, ch)
			name := d.Name()
			go func() {
				ch <- b.newDirTree(fset, filename, name, depth+1)
			}()