Rust更换Crates源

Rust编译时遇到以下问题：

Downloading futures v0.1.19
warning: spurious network error (2 tries remaining): [28] Timeout was reached (Operation timed out after 30857 milliseconds with 0 out of 0 bytes received)
error: unable to get packages from source                                       

Caused by:
  [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，能够考虑更换使用国内中科大的源。更换方法以下：

在 $HOME/.cargo/config 中添加以下内容：

[source.crates-io]
replace-with = 'ustc'

[source.ustc]
registry = "git://mirrors.ustc.edu.cn/crates.io-index"

若是所处的环境中不容许使用 git 协议，能够把上述地址改成：

registry = "https://mirrors.ustc.edu.cn/crates.io-index"

为何这么配置能够参考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