fix(infer): stop skipping Empty lifetime checks - #161935

im-lunex · 2026-08-28T15:04:59Z

stop skipping the check in lexical_region_resolve/mod.rs and actually validate Empty lifetimes..

follow-up - make note_and_explain.rs handle ReVar and ReErased so it prints a normal error message instead of exploding.. - (added ui-test and .stderr)

fixes: #161864

r? @lcnr

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rustbot · 2026-08-28T15:05:05Z

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lcnr · 2026-09-03T09:49:14Z

This doesn't feel 100% sufficient and don't have the capacity to fully dive into this myself rn, started a zulip thread about it https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/yeet.20.27empty.20in.20region.20handling/with/621318591

adwinwhite · 2026-09-14T09:16:18Z

+            ty::ReVar(_) => (alt_span, "revar", region.to_string()),
+
+            ty::ReBound(..) => {
                bug!("unexpected region for DescriptionCtx: {:?}", region);


any idea why we have ReVar/ReErases now? I kinda lost track of this.

View changes since the review

added separate handling for ReVar and ReErased because they represent different kinds of regions... so thought differentiating those would be better...

So nothing blows up if we don't change them? Better keep them as is then. We want them to fail loud.

This actually the upstream panics when it gets a ReErased region... so keeping it for now

what code triggers the panic? some ui test?

Oh, this is annoying. It has never been actually called before because it's a fallback for try_report_nice_region_error. And the diagnostics type must outlive [reerased] is confusing.

Would you like to investigate why try_report_trait_placeholder_mismatch doesn't work and try to make it work so that this fallback doesn't get triggered? The error kind we handle here is RegionResolutionError::UpperBoundUniverseConflict if I'm not mistaken.

Yeah I think the blocker is that RelateParamBound doesn't carry the TypeTrace/ValuePairs data that try_report_trait_placeholder_mismatch needs.. it's (Span, Ty, Option) a completely different shape. we can't just add a match arm..

That's a separate, larger task with its own design and review. [This issue is truly annoying while working on this we just encountered 3 more]

I guess we have to create a new diagnostics on our own then. Are you interested in doing that? 😃
It probably amounts to adding a new method try_report_placeholder_leak in fn try_report(), which would handle RegionResolutionError::UpperBoundUniverseConflict and emit a custom error.
The wording is likely similiar to TypeError::RegionsPlaceholderMismatch, plus regions' spans.

yeah i will give it a try..

Thanks! Feel free to ask if anything's unclear.

adwinwhite

Thanks for working on this!

@rustbot author

View changes since this review

adwinwhite · 2026-09-16T02:43:06Z

+            ty::ReVar(_) => (alt_span, "revar", region.to_string()),
+
+            ty::ReBound(..) => {
                bug!("unexpected region for DescriptionCtx: {:?}", region);


So nothing blows up if we don't change them? Better keep them as is then. We want them to fail loud.

rustbot · 2026-09-16T02:48:19Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

adwinwhite · 2026-09-17T06:25:05Z

+// Regression test for https://github.com/rust-lang/rust/issues/161864
+//@ revisions: current next
+//[next] compile-flags: -Znext-solver=coherence
+//@ compile-flags: -Znext-solver


Suggested change

// Regression test for https://github.com/rust-lang/rust/issues/161864

//@ revisions: current next

//[next] compile-flags: -Znext-solver=coherence

//@ compile-flags: -Znext-solver

//@ revisions: current next

//@[next] compile-flags: -Znext-solver

// Regression test for https://github.com/rust-lang/rust/issues/161864

// We used to allow placeholders to outlive `'empty` in any universe,

// which can be exploited to extend lifetimes.

// `'empty` is a misleading name because the region can cover function body

// for the callee.

View changes since the review

adwinwhite · 2026-09-21T02:17:37Z

r? me so I don't forget about this.

rustbot · 2026-09-21T14:25:56Z

Requested reviewer is already assigned to this pull request.

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infer: route all VarValue variants through sub_region_values in colle…

a1d5757

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rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Aug 28, 2026

rustbot assigned mu001999 Aug 28, 2026

rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Aug 28, 2026

rustbot assigned lcnr and unassigned mu001999 Aug 28, 2026

adwinwhite reviewed Sep 14, 2026

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adwinwhite reviewed Sep 16, 2026

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 16, 2026

improve test and undo changes in note_and_explain.rs

d7c3839

im-lunex requested a review from adwinwhite September 16, 2026 09:21

adwinwhite reviewed Sep 17, 2026

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rustbot assigned adwinwhite and unassigned lcnr Sep 21, 2026

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