Rust更換Crates源

Rust編譯時遇到如下問題：

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 )

解決辦法：更換Crates源

Rust開發時有時使用官方的源太慢，可以考慮更換使用國內中科大的源。更換方法如下：

在 $HOME/.cargo/config 中添加如下內容：

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

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

如果所處的環境中不允許使用 git 協議，可以把上述地址改為：

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

為什么這么配置可以參考The Cargo Book/Source Replacement.

參考文檔：
The Cargo Book/Source Replacement
The Cargo Book/Configuration
Rust Crates 源使用幫助