关于Rust的From特性的尝试

在The Rust Programming Language一书中，第 9.2 节Recoverable Errors with Result中有如下：

For example, we could change the read_username_from_file function in Listing 9-7 to return a custom error type named OurError that we define. If we also define impl From<io::Error> for OurError to construct an instance of OurError from an io::Error, then the ? operator calls in the body of read_username_from_file will call from and convert the error types without needing to add any more code to the function.

这里提到的From特性，并提到可以使用自定义的OurError，并为它实现impl From<io::Error> for OurError特性，我觉得我有时间可以尝试一下。

下面是我的尝试代码及输出：

#![allow(unused)]
use std::fs::File;
use std::io::{self, Read};#[derive(Debug)]
struct OurError {desc: String,
}impl From<io::Error> for OurError {fn from(value: io::Error) -> Self {OurError {desc: format!("From io::Error {} to OurError.", value),}}
}fn read_username_from_file() -> Result<String, OurError> {let mut username_file = File::open("hello.txt")?;let mut username = String::new();username_file.read_to_string(&mut username)?;Ok(username)
}fn main() {read_username_from_file().unwrap();
}

程序输出：

thread 'main' panicked at src\main.rs:26:31:
called `Result::unwrap()` on an `Err` value: OurError { desc: "From io::Error 系统找不到指定的文件。 (os error 2) to OurError." }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\temp.exe` (exit code: 101)