Why a borrowed error appears if I use a non mutable variable in various place of my code?

Question

I'm learning rust and I'm struggling to understand why the following code does not compile with some "Error[E0505]: cannot move out of req because it is borrowed" error on the req.into_body(). If I remove the println! or move it before the match, it runs.

async fn hello_world(req: Request<Body>) -> Result<Response<Body>, Infallible> {
  let mut response = Response::new(Body::empty());

  let method = req.method();
  let path = req.uri().path();

  match (method, path) {
    (&Method::GET, "/") => {
      *response.body_mut() = Body::from("Try POSTing data to /echo");
    }
    (&Method::POST, "/echo") => {
      *response.body_mut() = req.into_body();
    }
    _ => {
      *response.status_mut() = StatusCode::NOT_FOUND;
    }
  };
  println!("{} {}", method, path);

  Ok(response)
}

I understand some of the borrowing, but I don't understand why the position of the println changes anything, as I'm already defining non mutable variables that are used successfully in the match.

Try req.method().clone() and req.uri().path().clone() in the beginning in case those methods return references on request, not standalone objects — Alexey Larionov

Kitsu Kitsu · Accepted Answer · 2020-07-09T11:42:37

You cannot have mutable and immutable borrow at the same time. For your case, would be a converting the borrowed types (return of method() and uri().path()) to owned ones. You can do it via ToOwned::to_owned, or via explicit conversions (playground):

async fn hello_world(req: Request<Body>) -> Result<Response<Body>, Infallible> {
    let mut response = Response::new(Body::empty());

    let method = req.method().clone(); // &Method -> Method
    let path = req.uri().path().to_string(); // &str -> String
    // `req` is not borrowed at this point, so it can be mutated

    match (method.clone(), path.as_str()) {
        (Method::GET, "/") => {
            *response.body_mut() = Body::from("Try POSTing data to /echo");
        }
        (Method::POST, "/echo") => {
            *response.body_mut() = req.into_body();
        }
        _ => {
            *response.status_mut() = StatusCode::NOT_FOUND;
        }
    };
    println!("{} {}", method, path);

    Ok(response)
}

Why a borrowed error appears if I use a non mutable variable in various place of my code?

1 Answers