Rust：对泛型参数的引用不满足 trait bound

Question

我正在尝试用 Rust 编写一个简单的 TCP 客户端，询问用户是否希望他们的连接是 SSL。我正在尝试尽可能多地抽象代码，因此我正在尝试编写一个“communicate_with_server”函数，该函数应该能够接收SslStream 或TcpStream。

fn communicate_with_server<T: std::io::Read + std::io::Write>(mut stream: T) {
    let initial_message = format!("hello world");
    let mut buffer: Vec<u8> = Vec::new();
    let _bytes_written = stream.write_all(initial_message.as_bytes());
    let _flush = stream.flush();

    loop {
        let mut stream_reader = BufReader::new(&stream);
        stream_reader.read_until(b'\n', &mut buffer).expect("Reading into buffer failed");
        // ... Wait for the server to respond and do some analysis on the response ...
    }
}

当我尝试编译这段代码时，我得到以下响应：

error[E0277]: the trait bound `&T: std::io::Read` is not satisfied
  --> src\main.rs:47:48
   |
47 |         let mut stream_reader = BufReader::new(&stream);
   |                                                -^^^^^^
   |                                                |
   |                                                the trait `std::io::Read` is not implemented for `&T`
   |                                                help: consider removing the leading `&`-reference
   |
   = note: required by `BufReader::<R>::new`

error[E0599]: no method named `read_until` found for struct `BufReader<&T>` in the current scope
  --> src\main.rs:49:23
   |
49 |         stream_reader.read_until(b'\n', &mut buffer).expect("Reading into buffer failed");
   |                       ^^^^^^^^^^ method not found in `BufReader<&T>`
   | 

考虑到第一个错误，第二个错误是有道理的，但我不太确定为什么 &T 没有实现 Read 特征。

Answer 1

&T 其中T: Read 不包含Read，因为许多Read trait 方法需要改变T，而&T 是不可能的，即不可变参考T。

要解决您的问题，您应该将 可变 引用（即 &mut stream）传递给 BufReader::new，如下所示：

use std::io::{ Read, Write, BufReader, BufRead };

fn communicate_with_server<T: Read + Write>(mut stream: T) {
    let initial_message = format!("hello world");
    let mut buffer: Vec<u8> = Vec::new();
    let _bytes_written = stream.write_all(initial_message.as_bytes());
    let _flush = stream.flush();

    loop {
        let mut stream_reader = BufReader::new(&mut stream);
        stream_reader.read_until(b'\n', &mut buffer).expect("Reading into buffer failed");
        // ... Wait for the server to respond and do some analysis on the response ...
    }
}

playground