Rust中如何在foreach操作变量后继续使用该变量？

问题代码 #

use actix_web::{HttpRequest, HttpResponse, Responder};
use awc::ClientRequest;

pub async fn proxy(req: HttpRequest) -> impl Responder {
    let response = construct_request(req).send().await;

    let body = response.unwrap().body().await.expect("");

    HttpResponse::Ok().body(body)
}

fn construct_request(req: HttpRequest) -> ClientRequest {
    let client = awc::Client::default();

    let mut new_req = client.get("url");

    req.headers().iter().for_each(|headerPair| {
        new_req.append_header((headerPair.0, headerPair.1));
    });
        
    new_req.content_type("application/json")
}

编译器错误信息 #

error[E0507]: cannot move out of `new_req`, a captured variable in an `FnMut` closure
   --> src/routes/proxy.rs:18:9
    |
15  |       let mut new_req = client.get("http://localhost:8001/ping");
    |           ----------- captured outer variable
16  |
17  |       req.headers().iter().for_each(|headerPair| {
    |  ___________________________________-
18  | |         new_req.append_header((headerPair.0, headerPair.1));
    | |         ^^^^^^^ ------------------------------------------- `new_req` moved due to this method call
    | |         |
    | |         move occurs because `new_req` has type `ClientRequest`, which does not implement the `Copy` trait
19  | |     });
    | |_____- captured by this `FnMut` closure
    |
note: this function takes ownership of the receiver `self`, which moves `new_req`
   --> /Users/{sanitized}/awc-3.0.1/src/request.rs:186:30
    |
186 |     pub fn append_header(mut self, header: impl TryIntoHeaderPair) -> Self {
    |                              ^^^^

warning: variable does not need to be mutable
  --> src/routes/proxy.rs:15:9
   |
15 |     let mut new_req = client.get("http://localhost:8001/ping");
   |         ----^^^^^^^
   |         |
   |         help: remove this `mut`
   |
   = note: `#[warn(unused_mut)]` on by default

error[E0382]: use of moved value: `new_req`
  --> src/routes/proxy.rs:21:5
   |
15 |     let mut new_req = client.get("http://localhost:8001/ping");
   |         ----------- move occurs because `new_req` has type `ClientRequest`, which does not implement the `Copy` trait
16 |
17 |     req.headers().iter().for_each(|headerPair| {
   |                                   ------------ value moved into closure here
18 |         new_req.append_header((headerPair.0, headerPair.1));
   |         ------- variable moved due to use in closure
...
21 |     new_req.append_header(("custom", req.headers().get("custom").unwrap()))
   |     ^^^^^^^ value used here after move

问题原因 #

awc::ClientRequest的append_header方法采用获取所有权的设计（接收self而非&mut self），调用后会返回一个新的ClientRequest实例，原实例的所有权会被转移。在for_each闭包中，第一次调用new_req.append_header就会把new_req的所有权移走，后续迭代和循环结束后都无法再使用原变量。

解决方案 #

方法1：使用fold链式累积 #

fold方法可以在迭代过程中累积一个值，正好适配这种需要每次用新实例替换旧实例的场景：

fn construct_request(req: HttpRequest) -> ClientRequest {
    let client = awc::Client::default();

    req.headers()
        .iter()
        .fold(client.get("url"), |current_req, header_pair| {
            current_req.append_header((header_pair.0, header_pair.1))
        })
        .content_type("application/json")
}

这里以初始请求实例为起点，每次迭代将当前请求传入闭包，调用append_header后返回新实例作为下一次迭代的输入，最后直接链式调用content_type完成配置。

方法2：手动for循环赋值 #

如果更习惯显式循环结构，可以用普通for循环，每次将append_header返回的新实例重新赋值给变量：

fn construct_request(req: HttpRequest) -> ClientRequest {
    let client = awc::Client::default();

    let mut new_req = client.get("url");

    for header_pair in req.headers().iter() {
        new_req = new_req.append_header((header_pair.0, header_pair.1));
    }
        
    new_req.content_type("application/json")
}

每次循环都更新new_req为最新的实例，避免所有权移动导致的问题，循环结束后可以正常继续使用变量。

补充说明 #

两种方案都能解决编译器错误，其中fold写法更简洁，而手动循环可读性更强。原代码中的unused_mut警告会自动消失：fold场景不需要可变变量，手动循环场景中new_req确实需要可变属性。

内容的提问来源于stack exchange，提问作者dantdj