Check coroutine upvars in dtorck constraint

[compiler-errors]

Fix #144155.

This PR fixes an unsoundness where we were not considering coroutine upvars as drop-live if the coroutine interior types (witness types) had nothing which required drop.

In the case that the coroutine does not have any interior types that need to be dropped, then we don't need to treat all of the upvars as use-live; instead, this PR uses the same logic as closures, and descends into the upvar types to collect anything that must be drop-live. The rest of this PR is reworking the comment to explain the behavior here.

r? @lcnr or reassign 😸

---

Just some thoughts --- a proper fix for this whole situation would be to consider TypingMode in the needs_drop function, and just calling coroutine_ty.needs_drop(tcx, typing_env) in the dtorck constraint check.

During MIR building, we should probably use a typing mode that stalls the local coroutines and considers them to be unconditionally drop, or perhaps just stall all coroutines in analysis mode. Then in borrowck mode, we can re-check needs_drop but descend into witness types properly. #144158 implements this experimentally.

This is a pretty involved fix, and conflicts with some in-flight changes (#144157) that I have around removing coroutine witnesses altogether. I'm happy to add a FIXME to rework this whole approach, but I don't want to block this quick fix since it's obviously more correct than the status-quo.

[compiler-errors]

Yeah, I guess the resume ty doesn't need to be checked since although it can contain lifetimes, it would need to be stored in a witness ty to actually be dropped (since it doesn't have its own storage) and so it's redundant wrt the witness part of this drop check.

[compiler-errors]

Tweaked the comment a bit more and implemented the modified check that only requires the upvars be drop-live if there are no drops in the interior.

[compiler-errors]

Also tweaked this comment to use "interior" rather than flopping between "witness" and "interior". TBH i kinda prefer that name over witness, and at least for the purposes of this explanation it makes it clear that it's types that are local to the body of the coroutine.

[compiler-errors]

Also added a test for the upvar breakage.

[lcnr]

Hi @rust-lang/types, I previously assumed that every type stored in a coroutine is part of its witness. This assumption caused us to only require things to be live when dropping the coroutine if the witness required drop in #117134.

This is not the case as we may capture upvars by value. These upvars are not considered part of the witness, but still need to get dropped when dropping the coroutine. This PR updates the comment to better explain what's going on and fixes this issue.

We now require all upvars to be drop-live regardless of whether the coroutine witness has anything that needs to be drop.

@rfcbot fcp merge

I think we should still do a crater run?

[rfcbot]

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

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[oli-obk]

@bors2 try

[rust-bors]

⌛ Trying commit 9b0b433 with merge c4ca246…

To cancel the try build, run the command @bors2 try cancel.

[rust-bors]

☀️ Try build successful (CI)
Build commit: c4ca246 (c4ca24658d934444a28db36abebef17a8651701a, parent: 9748d87dc70a9a6725c5dbd76ce29d04752b4f90)

[lcnr]

@craterbot check

[craterbot]

👌 Experiment pr-144156 created and queued.
🤖 Automatically detected try build c4ca246
🔍 You can check out the queue and this experiment's details.

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[bors]

☔ The latest upstream changes (presumably #144458) made this pull request unmergeable. Please resolve the merge conflicts.

[rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[lcnr]

@rustbot author

[compiler-errors]

@rustbot ready

[lcnr]

@bors r+

[bors]

📌 Commit 560e5dc has been approved by lcnr

It is now in the queue for this repository.

[bors]

⌛ Testing commit 560e5dc with merge 5771665...