#[track_caller] does nothing on async fns

[ghost]

I tried:

#[track_caller]
async fn panic() -> ! {
    panic!();
}

fn main() {
    futures::executor::block_on(async { panic().await });
}

I expected the seventh line to be the panic location, but I got this:

thread 'main' panicked at 'explicit panic', src/main.rs:3:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

With RUST_BACKTRACE=1

thread 'main' panicked at 'explicit panic', src/main.rs:3:5
stack backtrace:
   0: std::panicking::begin_panic
             at /rustc/a601302ff0217b91589b5a7310a8a23adb843fdc/library/std/src/panicking.rs:521:12
   1: playground::panic::{{closure}}
             at ./src/main.rs:3:5
   2: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/a601302ff0217b91589b5a7310a8a23adb843fdc/library/core/src/future/mod.rs:80:19
   3: playground::main::{{closure}}
             at ./src/main.rs:7:41
   4: <core::future::from_generator::GenFuture<T> as core::future::future::Future>::poll
             at /rustc/a601302ff0217b91589b5a7310a8a23adb843fdc/library/core/src/future/mod.rs:80:19
   5: futures_executor::local_pool::block_on::{{closure}}
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/futures-executor-0.3.6/src/local_pool.rs:317:23
   6: futures_executor::local_pool::run_executor::{{closure}}
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/futures-executor-0.3.6/src/local_pool.rs:87:37
   7: std::thread::local::LocalKey<T>::try_with
             at /rustc/a601302ff0217b91589b5a7310a8a23adb843fdc/library/std/src/thread/local.rs:272:16
   8: std::thread::local::LocalKey<T>::with
             at /rustc/a601302ff0217b91589b5a7310a8a23adb843fdc/library/std/src/thread/local.rs:248:9
   9: futures_executor::local_pool::run_executor
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/futures-executor-0.3.6/src/local_pool.rs:83:5
  10: futures_executor::local_pool::block_on
             at ./.cargo/registry/src/github.com-1ecc6299db9ec823/futures-executor-0.3.6/src/local_pool.rs:317:5
  11: playground::main
             at ./src/main.rs:7:5
  12: core::ops::function::FnOnce::call_once
             at /rustc/a601302ff0217b91589b5a7310a8a23adb843fdc/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Playground (tried using nightly 2020-11-06 a601302)

I can achieve the result I expected by implementing a Future manually:

#![feature(never_type)]

use std::{
    future::Future,
    pin::Pin,
    task::{Context, Poll},
};

struct Panic;

impl Future for Panic {
    type Output = !;

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

fn main() {
    futures::executor::block_on(async { Panic.await });
}

Output:

thread 'main' panicked at 'explicit panic', src/main.rs:21:41
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Playground

I noticed that (in the backtrace) the compiler generated a closure (Generator?) for the async fn, so #74042 may be related.

@rustbot modify labels: A-async-await A-attributes

[jonas-schievink]

This seems to be working correctly. The call to panic does not actually panic, so the #[track_caller] does nothing. panic just returns a future that, when polled, will panic.

[ghost]

This seems to be working correctly. The call to panic does not actually panic, so the #[track_caller] does nothing. panic just returns a future that, when polled, will panic.

But I expect it to work further: automatically generates the Future I implemented manually, with #[track_caller] on Future::poll and the generated closure/Generator.

(Feature request) It's even better if #[track_caller] could work on async blocks as well.

[Aaron1011]

Since the call to an async fn can never panic (it always returns an impl Future), I think forwarding the attribute to the returned async closure would make sense.

This would require impementing #74042 (if only on nightly), so that the desugaring can forward #[track_caller] as needed.

[tmandry]

Forwarding to the poll function does seem reasonable to me.

@anp any thoughts on this?

[anp]

Forwarding to the generated future SGTM.