Rust更换Crates源

Rust编译时如果遇到如下问题：

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，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：

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

[source.ustc]
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.

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参考文档：
The Cargo Book/Source Replacement
The Cargo Book/Configuration
Rust Crates 源使用帮助