使用 withUnsafeMutablePointer 函数和 UnsafeMutablePointer struct来检索数据，如下所示：

Use withUnsafeMutablePointer function and UnsafeMutablePointer struct to retrieving the data, such as the following:

var result: AnyObject?
var status = withUnsafeMutablePointer(&result) { SecItemCopyMatching(queryAttributes, UnsafeMutablePointer($0)) }

if status == errSecSuccess {
    if let data = result as NSData? {
        if let string = NSString(data: data, encoding: NSUTF8StringEncoding) {
            // ...
        }
    }
}

它在发布（最快[-O]）版本时效果很好。

it works fine with release (Fastest [-O]) build.