impl Trait not allowed after updating tracing-attributes:0.1.12

[FintanH]

Bug Report

Hey 👋 thanks for taking the time to look at the following issue 😊

Version

The versions are:
tracing = 0.1.23
tracing-attributes = 0.1.12
tracing-core = 0.1.17

Here's the Cargo.lock of the reproducible.

Platform

Linux haptop 5.10.0-rc6 #1-NixOS SMP Sun Nov 29 23:50:50 UTC 2020 x86_64 GNU/Linux

Description

We're using a combination of tracing::instrument while also returning a trait object, i.e. impl. When this combined with Result and using err in tracing::instrument then we get the following compiler error:

error[E0562]: `impl Trait` not allowed outside of function and inherent method return types
  --> src/main.rs:22:30
   |
22 |     fn test(&self) -> Result<impl Iterator<Item = u32>, Error> {
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^

This was working before and it seems that an update of tracing-attributes caused the code not to compile. It's probably worth noting that this isn't an issue without the Result and err pragma.

Here's a minimal example of a project not compiling https://github.com/radicle-dev/tracing-bug. If you switch to the works branch, https://github.com/radicle-dev/tracing-bug/tree/works, then you can see that using tracing-attributes = "=0.1.11" allows the code to compile again.

If there's any more detail that would be helpful, please let me know ✌️ Thanks again!

[hawkw]

Thanks for the report! I think the issue is due to PR #1006, which changed #[instrument(err)] to handle early error returns by wrapping the function body in a closure that's immediately invoked.

You'll notice here that we annotate that closure with the function's return type:

tracing/tracing-attributes/src/lib.rs

Line 472 in 70ee64a

match move || #return_type #block () {

In the case where that return type contains an impl Trait, we can't use that type in a type annotation in this context, so this is invalid.

I think we should just be able to remove the type annotation and the compiler should still be able to infer the function's return type. If that isn't the case, and this is necessary to infer the type, I think we have to change the implementation to generate an inner function with the same prototype as the outer function, and call that, rather than using a closure. That might be somewhat more complicated, so hopefully removing the type annotation is sufficient.

Thanks again for the report — the repo case should be really helpful so we can easily write a test for this.

[hawkw]

@FintanH just wanted to let you know I've published tracing v0.1.24 with a fix for this! Thanks again for the helpful reproduction!

[FintanH]

Amazing, thanks for keeping me in the loop and the quick turnaround @hawkw 😊 And also thank you for the great library ✌️