Rust：对不同的流使用通用特征类型（tcp 流、tls 流）

Question

在我拥有std::net::TcpStream 或native_tls::TlsStream<TcpStream> 的情况下，我面临动态特征的编译器错误。我的算法思路如下：

1. 在 var "tcp" 中创建 TCP 流
2. 如果方案是 https => 用新的TlsStream<TcpStream> 替换相同的变量。
3. 在流​​上写入和接收数据。

我需要的重要特征是std::io::Read 和std::io::Write。因此我想，我只是将我的“tcp”变量建模为Box<dyn Read + Write>。但无论我如何尝试，我总是遇到编译错误。您可以在下面看到一个无法编译的最小示例。

use std::net::{IpAddr, Ipv4Addr, TcpStream};
use std::io::{Write as IoWrite, Read as IoRead};
use native_tls::TlsConnector;

fn main() {
    let use_https = true;
    // IP of "example.com".
    let ip_addr = std::net::IpAddr::V4(Ipv4Addr::new(85, 13, 155, 159));
    let port = if use_https { 443 } else { 80 };
    let tcp_stream = TcpStream::connect((ip_addr, port)).unwrap();
    let tcp_stream = maybe_connect_to_tls(tcp_stream, use_https);
}

fn maybe_connect_to_tls(tcp: TcpStream, use_https: bool) -> Box<dyn IoRead + IoWrite> {
    if use_https {
        let tls = TlsConnector::new().unwrap();
        let tcp = tls.connect("example.com", tcp).unwrap();
        Box::new(tcp)
    } else {
        Box::new(tcp)
    }
}

Playground link

这是我尝试编译时遇到的错误：

error[E0225]: only auto traits can be used as additional traits in a trait object
  --> src/main.rs:14:78
   |
14 | fn maybe_connect_to_tls(tcp: TcpStream, use_https: bool) -> Box<dyn IoRead + IoWrite> {
   |                                                                     ------   ^^^^^^^ additional non-auto trait
   |                                                                     |
   |                                                                     first non-auto trait
   |
   = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::io::Read + std::io::Write {}`
   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>

我也尝试了maybe_connect_to_tls<T>(tcp: TcpStream, use_https: bool) -> Box<T> where T: IoWrite + IoRead + ?Sized，但无论我尝试什么都会导致其他编译失败..

知道如何解决这个问题吗？我想要一个通用的流对象，它应该对应用程序不可见，具体实现是什么。

Answer 1

For Rust internal reasons，你的 trait 对象只能包含一个非auto trait。但是，您可以简单地创建一个依赖于 Read 和 Write 特征的特征：

use std::net::{IpAddr, Ipv4Addr, TcpStream};
use std::io::{Write as IoWrite, Read as IoRead};
use native_tls::TlsConnector;

trait ReadAndWrite: IoRead + IoWrite {
}

impl <T: IoRead + IoWrite> ReadAndWrite for T {
}

fn main() {
    let use_https = true;
    // IP of "example.com".
    let ip_addr = std::net::IpAddr::V4(Ipv4Addr::new(85, 13, 155, 159));
    let port = if use_https { 443 } else { 80 };
    let tcp_stream = TcpStream::connect((ip_addr, port)).unwrap();
    let tcp_stream = maybe_connect_to_tls(tcp_stream, use_https);
}

fn maybe_connect_to_tls(tcp: TcpStream, use_https: bool) -> Box<dyn ReadAndWrite> {
    if use_https {
        let tls = TlsConnector::new().unwrap();
        let tcp = tls.connect("example.com", tcp).unwrap();
        Box::new(tcp)
    } else {
        Box::new(tcp)
    }
}

Playground link