testing - 为什么Rust在循环后重置一些变量？

Question

我前一段时间读完《 Rust Programming Languange》，为了创建自己的Rust，我正在创建一个简单的http服务器。
在旨在解析&[u8]并创建HttpRequest对象的函数中，我注意到应该解析 header 的循环实际上并没有更新一些用于跟踪解析状态的变量:

pub fn parse_request(buffer: &[u8]) -> Result<HttpRequest, &'static str> {
    let (mut line, mut curr_req_reader_pos) = next_req_line(buffer, 0);

    let (method, path, version) = parse_method_path_and_version(&line)?;

    let (line, curr_req_reader_pos) = next_req_line(buffer, curr_req_reader_pos);
    //eprintln!("--- got next line: {}, {} ---", curr_req_reader_pos, String::from_utf8_lossy(line));
    let mut headers = vec![];
    let mut lel = 0;
    while line.len() > 0 {
        //eprintln!("LOOP");
        let header = parse_header(&line);
        eprintln!("--- parsed header: {} ---", String::from_utf8_lossy(line));
        headers.push(header);
        let (line, curr_req_reader_pos) = next_req_line(buffer, curr_req_reader_pos);
        //eprintln!("--- got next line: {}, {} ---", curr_req_reader_pos, String::from_utf8_lossy(line));
        let (line, curr_req_reader_pos) = next_req_line(buffer, curr_req_reader_pos);
        //eprintln!("--- got next line: {}, {} ---", curr_req_reader_pos, String::from_utf8_lossy(line));
        //eprintln!("LOOP");
        lel += 10;
        //break;
    }

    let (line, curr_req_reader_pos) = next_req_line(buffer, curr_req_reader_pos);
    //eprintln!("--- got next line: {}, {} ---", curr_req_reader_pos, String::from_utf8_lossy(line));
    //eprintln!("{}", lel);

    let has_body;
    match method.as_ref() {
        "POST" | "PUT" | "PATCH" => has_body = true,
        _ => has_body = true
    };

    let body;
    if has_body {
        let (line, curr_req_reader_pos) = next_req_line(buffer, curr_req_reader_pos);
        body = String::from_utf8_lossy(line).to_string();
    } else  {
        body = String::new();
    }

    Ok(HttpRequest {
        method,
        path,
        version,
        headers,
        has_body,
        body
    })
}

每种方法也有测试:

#[test]
fn test_next_req_line() {
    let req = "GET / HTTP/1.1\r\nContent-Length: 3\r\n\r\n\r\n\r\nlel".as_bytes();

    let (mut lel, mut pos) = next_req_line(req, 0);
    assert_eq!("GET / HTTP/1.1", String::from_utf8_lossy(lel));
    assert!(lel.len() > 0);
    assert_eq!(16, pos);

    let (lel, pos) = next_req_line(req, pos);
    assert_eq!("Content-Length: 3", String::from_utf8_lossy(lel));
    assert!(lel.len() > 0);
    assert_eq!(35, pos);

    let (lel, pos) = next_req_line(req, pos);
    assert_eq!("", String::from_utf8_lossy(lel));
    assert!(lel.len() == 0);
    assert_eq!(37, pos);

    let (lel, pos) = next_req_line(req, pos);
    assert_eq!("", String::from_utf8_lossy(lel));
    assert!(lel.len() == 0);

    let (lel, pos) = next_req_line(req, pos);
    assert_eq!("", String::from_utf8_lossy(lel));
    assert!(lel.len() == 0);

    let (lel, pos) = next_req_line(req, pos);
    assert_eq!("lel", String::from_utf8_lossy(lel));
    assert!(lel.len() == 3);
}

    #[test]
    fn test_parse_request() {
        let mut request = String::from("GET / HTTP/1.1\r\n");
        request.push_str("Content-Length: 3\r\n");
        request.push_str("\r\n");
        request.push_str("lel");
        let request = request.as_bytes();

        match parse_request(&request) {
            Ok(http_request) => {
                assert_eq!(http_request.method, "GET");
                assert_eq!(http_request.path, "/");
                assert_eq!(http_request.version, "HTTP/1.1");
            },
            Err(_) => {
                println!("Failed");
            }
        }
    }

带注释的行只是为了显示正在发生的事情，这就是我看到此输出的地方:

    Finished test [unoptimized + debuginfo] target(s) in 0.53s
     Running target/debug/deps/rweb-6bbc2a3130f7e3d9

running 4 tests
test http::testing::test_next_req_line ... ok
test http::testing::test_parse_header ... ok
test http::testing::test_parse_method_path_and_version ... ok
--- parsed method path version  ---
--- got next line: 35, Content-Length: 3 ---
LOOP
--- parsed header: Content-Length: 3 ---
--- got next line: 37,  ---
--- got next line: 39, lel ---
LOOP
--- got next line: 37,  ---
10
test http::testing::test_parse_request ... ok

test result: ok. 4 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

看来while循环并没有重新分配line和curr_req_reader_pos变量，但是对我来说，期望该循环将更新这些变量并解析每个 header 是很有意义的。但是，正如任何人在测试中看到的那样，它可以完美地在循环外运行。
我不知道为什么在我目前对Rust的理解不足的情况下会发生这种情况，为什么会发生呢？

Answer 1

您不是在循环中“重新分配”变量，而是创建新的变量以暂时遮盖外部变量，然后在迭代结束时超出范围。
以下是简化伪代码中发生的情况的说明:

let x0 = 0
{
    let x1 = x0 + 1 // original x gets shadowed, x = 1
    let x2 = x1 + 1 // x1 gets shadowed, x = 2
} // both x1 and x2 go out of scope
let x1 = x0 + 1 // x = 1