问题

以下是您要的翻译部分：

我正在尝试在Rust中编写基本的网络编程代码，但遇到了一个我不理解的错误。到目前为止，我一直在Rust中使用match语句来检查错误，但当我尝试检查TcpStream::connect()时，出现了一个意外的错误：

我的代码：
```rust
use std::net::TcpStream;

fn main() {
    let mut server = match TcpStream::connect("127.0.0.1:23456"){
        Ok(x) => x,
        Err(x) => println!("无法连接到服务器: {x}"),
    };
}

编译器错误：

error[E0308]: 'match' 分支具有不兼容的类型
 --> src/main.rs:8:19
  |
6 |       let mut server = match TcpStream::connect("127.0.0.1:23456"){
  |  ______________________-
7 | |         Ok(x) => x,
  | |                  - 这被确定为类型 `TcpStream`
8 | |         Err(x) => println!("无法连接到服务器: {x}"),
  | |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ 
                        预期结构 `TcpStream`，找到 `()`
9 | |     };
  | |_____- 'match' 分支具有不兼容的类型
  |

我每次使用match语句时，都允许我将Result类型解构为OK情况下的返回值（如上所示），或者在错误情况下返回错误字符串。

TcpStream::connect()确实返回TcpStream，但为什么编译器坚持要求错误情况也返回TcpStream？

<details>
<summary>英文:</summary>

I&#39;m trying to write basic networking code in Rust, but running into an error I don&#39;t understand. I have been using match statements to error check everything in Rust so far, but when I try to error check TcpStream::connect(), I get an unexpected error:

My code:

use std::net::TcpStream;

fn main() {
let mut server = match TcpStream::connect("127.0.0.1:23456"){
Ok(x) => x,
Err(x) => println!("Could not connect to server: {x}"),
};
}

The compiler error:

error[E0308]: match arms have incompatible types
--> src/main.rs:8:19
|
6 | let mut server = match TcpStream::connect("127.0.0.1:23456"){
| ______________________-
7 | | Ok(x) => x,
| | - this is found to be of type TcpStream
8 | | Err(x) => println!("Could not connect to server: {x}"),
| | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
expected struct TcpStream, found ()
9 | | };
| |_____- match arms have incompatible types
|

Every other time I use a match statement it allows me to destructure the Result type into a return value in the OK case (as above), or an error string in the error case.

It is the case that TcpStream::connect() returns a TcpStream, but why is the compiler insisting that the error case also needs to return a TcpStream?

</details>


# 答案1
**得分**: 2

The value of the `match` statement gets assigned to `server`.

However, both branches of your `match` statement return a different type.

- `Ok(x)` returns `x`, which is of type `TcpStream`.
- `Err(x)` returns the result of `println!()`, which has the return value `()`.

`TcpStream` and `()` are incompatible.

Just think about the code after the match statement. What should the `server` variable be? You don't stop the execution when the error happens, you simply `println!()` and continue. So **something** has to be written to the `server` variable.

If you `panic!()` instead of `println!()`, meaning print and abort, then it compiles because it knows that the `Err` case won't continue afterwards:

```rust
use std::net::TcpStream;

fn main() {
    let mut server = match TcpStream::connect("127.0.0.1:23456") {
        Ok(x) => x,
        Err(x) => panic!("Could not connect to server: {x}"),
    };
}

thread 'main' panicked at 'Could not connect to server: Connection refused (os error 111)', src/main.rs:6:19

That said, if this is your desired behavior, there is a short form for it:

use std::net::TcpStream;

fn main() {
    let mut server = TcpStream::connect("127.0.0.1:23456").expect("Could not connect to server");
}

thread 'main' panicked at 'Could not connect to server: Os { code: 111, kind: ConnectionRefused, message: "Connection refused" }', src/main.rs:4:60

I recommend handling the error properly, though.

There are many ways to do that, so this part will be opinionated.

I personally like miette (an alternative would be anyhow):

use miette::{Context, IntoDiagnostic};
use std::net::TcpStream;

fn main() -> miette::Result<()> {
    let mut _server = TcpStream::connect("127.0.0.1:23456")
        .into_diagnostic()
        .wrap_err("Could not connect to server.")?;

    // Do something with server

    Ok(())
}

Error: 
  × Could not connect to server.
  ╰─▶ Connection refused (os error 111)

use std::net::TcpStream;

fn main() {
    let mut server = match TcpStream::connect("127.0.0.1:23456") {
        Ok(x) => x,
        Err(x) => panic!("Could not connect to server: {x}"),
    };
}

thread 'main' panicked at 'Could not connect to server: Connection refused (os error 111)', src/main.rs:6:19

use std::net::TcpStream;

fn main() {
    let mut server = TcpStream::connect("127.0.0.1:23456").expect("Could not connect to server");
}

thread 'main' panicked at 'Could not connect to server: Os { code: 111, kind: ConnectionRefused, message: "Connection refused" }', src/main.rs:4:60

use miette::{Context, IntoDiagnostic};
use std::net::TcpStream;

fn main() -> miette::Result<()> {
    let mut _server = TcpStream::connect("127.0.0.1:23456")
        .into_diagnostic()
        .wrap_err("Could not connect to server.")?;

    // Do something with server

    Ok(())
}

Error: 
  × Could not connect to server.
  ╰─▶ Connection refused (os error 111)

答案2

得分: 2

Your server variable can't be both TcpStream and the unit type ()

尝试传播错误而不是这样。

这里有来自官方文档的示例：

use std::io::prelude::*;
use std::net::TcpStream;

fn main() -> std::io::Result<()> {
    let mut stream = TcpStream::connect("127.0.0.1:34254")?;

    stream.write(&[1])?;
    stream.read(&mut [0; 128])?;
    Ok(())
}

如果它无法连接到服务器，它会发出错误消息

Error: Os { code: 111, kind: ConnectionRefused, message: "Connection refused" }

并设置一个非零的退出码

$ echo $?
1

use std::io::prelude::*;
use std::net::TcpStream;

fn main() -> std::io::Result<()> {
    let mut stream = TcpStream::connect("127.0.0.1:34254")?;

    stream.write(&[1])?;
    stream.read(&mut [0; 128])?;
    Ok(())
}

Error: Os { code: 111, kind: ConnectionRefused, message: "Connection refused" }

$ echo $?
1