Bogus `higher-ranked lifetime error` in an async block

[sfackler]

Running stable 1.64.0.

Unfortunately this happened in a complex bit of code that I can't easily reduce :(

            task::spawn({
                let handle_service = handle_service.clone();
                async move {
                    if let Err(e) = handle_service.call(connection).await {
                        debug!("http connection terminated", error: e);
                    }
                }
            });

error: higher-ranked lifetime error
   --> witchcraft-server/src/server.rs:118:13
    |
118 | /             task::spawn({
119 | |                 let handle_service = handle_service.clone();
120 | |                 async move {
121 | |                     if let Err(e) = handle_service.call(connection).await {
...   |
124 | |                 }
125 | |             });
    | |______________^
    |
    = note: could not prove `for<'r> impl for<'r> futures_util::Future<Output = ()>: std::marker::Send`

The future returned by handle_service.call(connection) is definitely Send, and I can work around the failure by boxing it:

            task::spawn({
                let handle_service = handle_service.clone();
                async move {
                    // The compiler hits a `higher-ranked lifetime error` if we don't box this future :/
                    let f: Pin<Box<dyn Future<Output = Result<(), Error>> + Send>> =
                        Box::pin(handle_service.call(connection));
                    if let Err(e) = f.await {
                        debug!("http connection terminated", error: e);
                    }
                }
            });

[eholk]

We discussed this in the wg-async triage today. We think the best course of action is to try to get a minimal test case for this. Afterwards, we'll probably want to get T-Types input on this. @vincenzopalazzo is going to try to reproduce this and minimize the test case.

[eholk]

@rustbot label AsyncAwait-Triaged

[vincenzopalazzo]

@rustbot claim

[danielhenrymantilla]

I wonder if the following snippet wouldn't be a reduction:

struct Type<'a, 'b>(&'a &'b (), ::core::marker::PhantomData<*mut ()>);

// Note: if this had an *implicit* `'b : 'a` things would work.
unsafe impl<'b : 'a, 'a> Send for Type<'a, 'b> {}

fn foo() -> impl Send { async {
    let local = ();
    let r = &&local;
    async {}.await;
    let it = Type(r, <_>::default());
    async {}.await;
}}

• from Discord

[eholk]

@sfackler - Would it be possible to share more of the surrounding code for this example so we can see more of what's going on?

[sfackler]

I'm working on minimizing it.

[sfackler]

Here is a semi-minimized repro. It still depends on Hyper, but it could probably be replaced with some PhantomData inserts with some more work: https://gist.github.com/sfackler/a7f3aaf2ce3f5ff4c99e1a3d67f828ed

error: higher-ranked lifetime error
   --> witchcraft-server/src/lib.rs:162:5
    |
162 | /     spawn(async move {
163 | |         let _ = handle_service.call(stream).await;
164 | |     });
    | |______^
    |
    = note: could not prove `for<'r> impl for<'r> futures_util::Future<Output = ()>: Send`

[sfackler]

For whatever reason, the presence of the AuditLogService in the future seems to be a trigger. If you change line 160 to just let request_service = HandlerService it compiles.

[vincenzopalazzo]

Thank @sfackler,

I think the core concept missed in your previous report are:

Using a wrapper around Future like

pub struct AdaptorFuture<F> {
    inner: F,
}

impl<F, T> Future for AdaptorFuture<F>
where
    F: Future<Output = T>,
{
    type Output = Result<T, io::Error>;

    fn poll(self: Pin<&mut Self>, _: &mut Context<'_>) -> Poll<Self::Output> {
        panic!()
    }
}

and your handle_service implementation

impl<S, R> hyper::service::Service<R> for AdaptorService<S>
where
    S: Service<R>,
{
    type Response = S::Response;

    type Error = io::Error;

    type Future = AdaptorFuture<S::Future>;

    fn poll_ready(&mut self, _: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        Poll::Ready(Ok(()))
    }

    fn call(&mut self, req: R) -> Self::Future {
        AdaptorFuture {
            inner: self.inner.call(req),
        }
    }
}