Tracking Issue for #[track_caller] on closures

[Aaron1011]

This is a tracking issue for the RFC "XXX" (rust-lang/rfcs#NNN).
The feature gate for the issue is #![feature(closure_track_caller)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the RFC
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

• Should async {}, etc support track caller on their lowered closures?

Implementation history

• Initial implementation in Support #[track_caller] on closures and generators #87064

[kartva]

What's the status of this issue? I'd be happy to try and pursue stabilization if it's ready.

[Aaron1011]

Even if this is stabilized, you'll still need the unstable stmt_expr_attributes to actually write it on a closure expression: #15701

[mpfaff]

Even if this is stabilized, you'll still need the unstable stmt_expr_attributes to actually write it on a closure expression: #15701

That seems to be the case only if the closure is not passed immediately as a function argument:

// This works
let x = std::convert::identity(#[track_caller] || {});
// But this does not
let x = #[track_caller] || {};

Maybe I'm misunderstanding the terminology, but in the case that does work, the closure is still an expression, no?

[Aaron1011]

You're right - this seems like a bug in the stmt_expr_attributes feature gate (or maybe an intentional partial stabilization).

[tmandry]

Nominating for lang team discussion. We may not be ready to approve attributes on closures generally. However, this being unstable led to the backing out of#[track_caller] support on async fn in #104741 (originally implemented in #104219) unless the closure_track_caller feature gate is enabled, because it uses that feature under the hood.

@rustbot label I-lang-nominated I-async-nominated

@rust-lang/lang, can we get consensus on stabilizing support for #[track_caller] on async fn only, and does that need an FCP?

The only possible question I see is whether caller tracking should occur during initial call or subsequent polling; this follows the approach adopted and FCP'd in #87064 and adds it to poll. One implication of this is that combinators would need to add #[track_caller] to their poll implementations to be able to "see through" them to user code. @rust-lang/wg-async in case there are any concerns about this.

Additionally, how close are we to consensus on stabilizing closure_track_caller if a stabilization PR were to come up?

[nikomatsakis]

One clarification @tmandry -- why exactly did we back things out? Was it because we were afraid of committing to something that would require a feature gate, or because the compiler didn't accept the patch because of the feature gate being needed on stable? (If the latter, there's a way to deal with that.)

That said, I feel fairly comfortable with approving attributes on closures.

[tmandry]

One clarification @tmandry -- why exactly did we back things out? Was it because we were afraid of committing to something that would require a feature gate, or because the compiler didn't accept the patch because of the feature gate being needed on stable? (If the latter, there's a way to deal with that.)

This wasn't entirely clear to me, but I think it was more the latter - or more specifically, because the original patch had a bug that caused an error to appear on stable when #[track_caller] was used (previously it was accepted as a no-op.) @bryangarza may be able to add more background.

[bryangarza]

We reused the closure_track_caller because it was already in the code, but I think we could make a separate flag for async fn only if we want to stabilize just that part.

[tmandry]

We discussed this in the t-lang meeting (notes).

We agreed that we should make a separate flag for #[track_caller] on async fn and then FCP that. The rest of this comment can be copied over to the new tracking issue once it is created to kick off the FCP.

---

It's not 100% clear that tracking the poller, rather than the original caller, is the ideal behavior. That said, we may still be able to resolve the question without an RFC.

Arguments in favor of tracking the poller (current behavior):

• The user might expect to see the call that's happening now rather than one that happened in the past.
• Prevents us from having to store the caller in the Future type.
• Usually poller and caller are the same function, anyway, so we should do the simpler and more performant thing.

Arguments in favor of tracking the original caller instead of the poller:

• User might expect to see the actual caller; especially given the name track_caller, showing the poller can confusing.
• A &'static Location is only one pointer; storing it is not so bad.
• Combinators won't need to add #[track_caller] to "thread through" the caller information.
• If we tracked the poller, #[track_caller] on a spawned task would show the executor.
  • There's no way today for the executor to save off caller location information (e.g. in spawn) and re-provide it to the polled function, even if it wanted to. We could fix this with a set_caller API.
  • Tracking won't work for &dyn Future. Executors can easily work around this if they have an API with access to the underlying type before coercing to dyn, as most do (playground). But if you happen to already have a dyn Future, you'll be stuck.

[tmandry]

@bryangarza Would you be interested in making a separate feature flag and (optionally) opening a new tracking issue?

[bryangarza]

@tmandry sure, I can do both of those

[RalfJung]

This is a tracking issue for the RFC "XXX" (rust-lang/rfcs#NNN).

Is there an RFC? Or is this just noise that should be removed from the issue?
I was looking for a high-level description of the design here, where can I find that?