迭代原因

它通常用于调试目的，以找出错误的根本原因。为了检查实现 std::error::Error 的错误值：

use std::error::Error;

let orig_error = call_returning_error();

// Use an Option<&Error>. This is the return type of Error.cause().
let mut err = Some(&orig_error as &Error);

// Print each error's cause until the cause is None.
while let Some(e) = err {
    println!("{}", e);
    err = e.cause();
}