sockets - 循环中匹配臂中的变量来自哪里？

Question

我正在尝试使用 this as a starting point 在 Rust 中实现一个 HTTP 客户端.我被 rust-lang.org 网站通过他们的 TcpStream 页面中的 rust-by-example 建议之一转到了这个链接。我正在研究如何从 TcpStream 中读取数据。我正在尝试遵循此代码:

fn handle_client(mut stream: TcpStream) {
    // read 20 bytes at a time from stream echoing back to stream
    loop {
        let mut read = [0; 1028];
        match stream.read(&mut read) {
            Ok(n) => {
                if n == 0 { 
                    // connection was closed
                    break;
                }
                stream.write(&read[0..n]).unwrap();
            }
            Err(err) => {
                panic!(err);
            }
        }
    }
}

n 变量从何而来？它到底是什么？作者说一次读取20个字节；这是从哪里来的？

我还没有真正尝试过任何东西，因为我想在尝试之前先了解一下。

Answer 1

我强烈建议您阅读所用工具的文档。在这种情况下，The match Control Flow Operator来自 Rust 编程语言 解释了您需要了解的内容。

来自Patterns that Bind to Values部分:

In the match expression for this code, we add a variable called state to the pattern that matches values of the variant Coin::Quarter. When a Coin::Quarter matches, the state variable will bind to the value of that quarter’s state. Then we can use state in the code for that arm, like so:

fn value_in_cents(coin: Coin) -> u8 {
    match coin {
        Coin::Penny => 1,
        Coin::Nickel => 5,
        Coin::Dime => 10,
        Coin::Quarter(state) => {
            println!("State quarter from {:?}!", state);
            25
        },
    }
}

If we were to call value_in_cents(Coin::Quarter(UsState::Alaska)), coin would be Coin::Quarter(UsState::Alaska). When we compare that value with each of the match arms, none of them match until we reach Coin::Quarter(state). At that point, the binding for state will be the value UsState::Alaska. We can then use that binding in the println! expression, thus getting the inner state value out of the Coin enum variant for Quarter.

有an entire chapter关于可用的模式匹配语法及其使用位置。