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)
	if err != nil {
		return nil, err
	}
	parsedFileCache[filename] = f

	return f, nil
}