Rust lifetimes with closures using hyper

Question

Been learning rust and having a problem with lifetime when passing conn to the request_handler. I get an error saying

error[E0312]: lifetime of reference outlives lifetime of borrowed content...
  --> src/main.rs:33:70
   |
33 |         let request_handler = |req: Request<Body>| async { request_handler(conn, req).await };
   |                                                                            ^^^^
   |
   = note: ...the reference is valid for the static lifetime...

but I am not sure how to handle lifetimes with closures and why/how it is static. I have a loose understanding of lifetimes and borrowing but this seems like a more complex case. I also would just not use a closure, but the return type of one of the closures has a type that is not exported by the hyper crate, so i don't know how i would create a fn without being able to declare the return type.

Also I can confirm if i remove passing conn i can get everything to work, but I want to use the conn object in the request_handler.

use hyper::server::conn::AddrStream;
use hyper::service::make_service_fn;
use hyper::Version;
use hyper::{Body, Error, Method, Request, Response, Server};
use std::net::{IpAddr, Ipv4Addr, SocketAddr};
mod http_models;
mod utils;
use hyper::service::service_fn;

async fn request_handler(
    conn: &'static AddrStream,
    req: Request<Body>,
) -> Result<Response<Body>, hyper::Error> {
    println!("req: {:?}", req);
    if req.method() == Method::CONNECT {
        println!("Connect")
    }
    let res: Response<Body> = Response::builder()
        .status(200)
        .version(Version::HTTP_11)
        .body(Body::empty())
        .unwrap();
    return Ok(res);
}

#[tokio::main]
async fn main() -> Result<(), Error> {
    let ip = IpAddr::V4(Ipv4Addr::new(127, 0, 0, 1));
    let addr = SocketAddr::new(ip, 1337);
    //let client = Client::new();

    let make_service = make_service_fn(|conn: &AddrStream| async {
        let request_handler = |req: Request<Body>| async { request_handler(conn, req).await };
        let service = service_fn(request_handler);
        Ok::<_, Error>(service)
    });

    let server = Server::bind(&addr).serve(make_service);

    println!("Listening on http://{}", addr);

    if let Err(e) = server.await {
        eprintln!("server error: {}", e);
    }
    Ok(())
}

You do not use the `conn` in `request_handler`, if you remove this argument the problem just disappears. Can you do that or your real code is more complex? — rodrigo, Jun 18 '20 at 23:45
I will be using it even though it's not being used right now — Gekctek, Jun 19 '20 at 00:05

score 2 · Accepted Answer · answered Jun 19 '20 at 04:57

The conn argument of the closure passed to make_service_fn only lives as long as the closure body, but the return value of the closure (Ok(service)) references it. The closure must have the type FnMut(&Target) -> impl Future, which means it sadly is not permitted to return a value that references its argument.

The only solution is to copy/clone whatever you need from conn while setting up your request handler, since you cannot keep a reference to it once the closure returns.

Cool. That helps a lot ty. I saw other examples of items being cloned and didn't fully understand their purpose — Gekctek, Jun 19 '20 at 15:22

Rust lifetimes with closures using hyper

1 Answers1