for f := range w.features {
		fs = append(fs, f)
	}
	sort.Strings(fs)
	return
}

var parsedFileCache = make(map[string]*ast.File)

func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
	filename := filepath.Join(dir, file)
	if f := parsedFileCache[filename]; f != nil {
		return f, nil
	}

	f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)