Rust编译时遇到如下问题：

    1. Downloading futures v0.1.19
    2. warning: spurious network error (2 tries remaining): [28] Timeout was reached (Operation timed out after 30857 milliseconds with 0 out of 0 bytes received)
    3. error: unable to get packages from source                                       
    5. Caused by:
    6.   [35] SSL connect error (OpenSSL SSL_connect: SSL_ERROR_SYSCALL in connection to static.crates.io:443 )

    解决办法：更换Crates源
    Rust开发时有时使用官方的源太慢，可以考虑更换使用国内中科大的源。更换方法如下：
    在 $HOME/.cargo/config 中添加如下内容：

    1. [source.crates-io]
    2. replace-with = 'ustc'
    4. [source.ustc]
    5. registry = "git://mirrors.ustc.edu.cn/crates.io-index"

    如果所处的环境中不允许使用 git 协议，可以把上述地址改为：
    registry = "https://mirrors.ustc.edu.cn/crates.io-index"
    为什么这么配置可以参考The Cargo Book/Source Replacement.
    参考文档：
    The Cargo Book/Source Replacement
    The Cargo Book/Configuration
    Rust Crates 源使用帮助