请教大神们，GO中冒号":"的作用是什么啊？

&Client{
    url:       u,
    userAgent: conf.UserAgent,
    httpClient: &http.Client{
        Timeout:   conf.Timeout,
        Transport: tr,
    },
    credentials: conf.Credentials,
}

请问这冒号代表的是什么意思？