Make region equality emits Eq constraints by ShoyuVanilla · Pull Request #155258 · rust-lang/rust

ShoyuVanilla · 2026-04-13T20:48:07Z

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For context, see..

The box named coroutine witness Send lifetime requirements now considered by leakcheck from this roadmap
#t-types/trait-system-refactor > A question on #251 @ 💬
Comments on rustc_type_ir::predicate::RegionEqPredicate

ShoyuVanilla · 2026-04-13T20:52:11Z

r? lcnr

rustbot · 2026-04-13T20:52:18Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

ShoyuVanilla · 2026-04-13T20:54:28Z

+        //~| ERROR mismatched `self` parameter type
+        //~| NOTE expected struct `Foo<'a, 'b>`
+        //~| NOTE found struct `Foo<'b, 'a>`
+        //~| NOTE lifetime mismatch


View changes since the review

Other tests are untouched but this has been affected somehow (I guess this might be due to having some duplication with both 'a: 'b, 'b: 'a and 'a == 'b)
I think this might be okay as it's unchanged anyway modulo diagnostics deduplication

seems fine 🤷

This somehow got returned back again while destructuring eq bounds 😂

ShoyuVanilla · 2026-04-14T03:36:52Z

@bors try @rust-timer queue

Make region equality emits Eq constraints

rust-bors · 2026-04-14T05:44:56Z

☀️ Try build successful (CI)
Build commit: 52727e5 (52727e55499277ba04069f2dfd8ca8cbbc4e76b6, parent: 338dff3e3a375cb4a3c68be825058c582262655a)

rust-timer · 2026-04-14T06:25:09Z

Finished benchmarking commit (52727e5): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never
@rustbot label: -S-waiting-on-perf +perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.2%	[0.0%, 0.3%]	6
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.2%]	23
Improvements ✅ (secondary)	-0.5%	[-1.3%, -0.0%]	27
All ❌✅ (primary)	-0.3%	[-0.5%, -0.2%]	23

Max RSS (memory usage)

Results (secondary -0.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.3%	[0.9%, 5.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.6%, -2.8%]	4
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -2.4%, secondary -5.3%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.4%	[-2.4%, -2.4%]	1
Improvements ✅ (secondary)	-5.3%	[-8.9%, -1.9%]	10
All ❌✅ (primary)	-2.4%	[-2.4%, -2.4%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 489.33s -> 490.845s (0.31%)
Artifact size: 394.21 MiB -> 394.03 MiB (-0.04%)

ShoyuVanilla · 2026-04-14T06:28:04Z

Oh, looks like having less number of constraints by collapsing into Eq helps the perf a bit

lcnr

I would keep assumptions as bidirectional outlives and never have Eq in there

can you make sure we always exhaustively match on ConstraintKind 🤔 I do dislike constraint kind as a concept and feel like we should just match on the actual region instead of separately storing that in the ConstraintKind

View changes since this review

lcnr · 2026-04-14T09:48:12Z

+/// - We want to uplift bidirectional constraints to the caller instead of unifying them
+///   when solving nested goals, otherwise we often lose implied bounds.
+/// - But we still want to know they are equal from the caller. This is crucial when we
+///   are proving other nested goals which are sensitive to region equalities.


this comment is slightly confusing/unhelpful to me right now. It is hard to express why exactly we need this, because really, it is a hack required by the better solution not being possible

"otherwise we often lose implied bounds", what do you mean, what is often, link to a relevant example

I think the framing here isn't quite right. For me the issue is that having existential regions which aren't actually existential is not something which we can support.

Also, "know they are equal" seems slightly odd to me. Know they are equal means "unify existential regions with regions they are equal to, to resolve them to the same thing" here I think? Maybe make that more clear

I also think our conversation on zulip has been useful here, so maybe just link to that 😁

ShoyuVanilla · 2026-04-14T09:57:54Z

I would keep assumptions as bidirectional outlives and never have Eq in there

Okay, I'll keep them as they are.

can you make sure we always exhaustively match on ConstraintKind 🤔 I do dislike constraint kind as a concept and feel like we should just match on the actual region instead of separately storing that in the ConstraintKind

Yeah, I also thought ConstraintKind is very iffy while working on this 😅 Maybe it would be great to clean them up as a follow up. For now, I'll make sure they are exhaustively matched

JonathanBrouwer · 2026-04-18T09:46:04Z

@bors p=6
Scheduling this before the next rollup

Make region equality emits Eq constraints For context, see.. - The box named *coroutine witness Send lifetime requirements now considered by leakcheck* from [this roadmap](https://raw.githubusercontent.com/hexcatnl/roadmap/6f23e638f65249ef02af86a5454275103a71552d/next-solver.svg) - [#t-types/trait-system-refactor > A question on #251 @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/A.20question.20on.20.23251/near/584166935) - Comments on `rustc_type_ir::predicate::RegionEqPredicate`

rust-bors · 2026-04-18T12:57:42Z

💔 Test for e297e30 failed: CI. Failed job:

auto - dist-apple-various (web logs, enhanced plaintext logs)

ShoyuVanilla · 2026-04-18T13:00:10Z

I'm so cursed 🙃

rustbot · 2026-04-18T13:03:31Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

ShoyuVanilla · 2026-04-18T14:35:40Z

@bors retry

rust-bors · 2026-04-18T14:35:43Z

❗ You can only retry pull requests that are approved and have a previously failed auto build.

ShoyuVanilla · 2026-04-18T14:36:04Z

@bors r=lcnr

rust-bors · 2026-04-18T14:36:06Z

📌 Commit ace3aa3 has been approved by lcnr

It is now in the queue for this repository.

rust-bors · 2026-04-18T19:15:55Z

☀️ Test successful - CI
Approved by: lcnr
Duration: 3h 11m 24s
Pushing b2f1ccf to main...

github-actions · 2026-04-18T19:18:42Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 3142bea (parent) -> b2f1ccf (this PR)

Test differences

Show 24 test diffs

24 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard b2f1ccf524a3a4cf9c34545167cc23b659cf1cbd --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

x86_64-gnu-llvm-21-3: 1h 25m -> 2h 2m (+44.5%)
dist-x86_64-llvm-mingw: 1h 29m -> 2h 5m (+40.0%)
x86_64-msvc-ext2: 1h 22m -> 1h 48m (+31.3%)
x86_64-gnu-debug: 1h 36m -> 2h 5m (+30.2%)
dist-powerpc-linux: 1h 12m -> 1h 30m (+25.5%)
x86_64-gnu-distcheck: 1h 54m -> 2h 20m (+23.4%)
dist-various-1: 1h 13m -> 56m 56s (-22.7%)
dist-ohos-armv7: 1h 12m -> 57m 27s (-20.9%)
dist-armv7-linux: 1h 30m -> 1h 12m (-20.2%)
i686-msvc-1: 2h 54m -> 2h 21m (-18.8%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-04-18T19:56:31Z

Finished benchmarking commit (b2f1ccf): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.2%]	21
Improvements ✅ (secondary)	-0.5%	[-1.2%, -0.1%]	26
All ❌✅ (primary)	-0.3%	[-0.4%, -0.2%]	21

Max RSS (memory usage)

Results (primary -6.3%, secondary 1.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	1.7%	[0.6%, 5.1%]	4
Improvements ✅ (primary)	-6.3%	[-7.0%, -5.6%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-6.3%	[-7.0%, -5.6%]	2

Cycles

Results (secondary -7.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-11.5%	[-17.6%, -3.2%]	3
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 492.552s -> 492.524s (-0.01%)
Artifact size: 394.41 MiB -> 394.41 MiB (0.00%)

rustbot assigned lcnr Apr 13, 2026

ShoyuVanilla marked this pull request as ready for review April 13, 2026 20:52

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 13, 2026

ShoyuVanilla commented Apr 13, 2026

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ShoyuVanilla force-pushed the eq-constraint branch from 49b0929 to 8d8e4f3 Compare April 13, 2026 20:56

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ShoyuVanilla force-pushed the eq-constraint branch from 8d8e4f3 to 4119383 Compare April 14, 2026 03:34

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rust-bors Bot pushed a commit that referenced this pull request Apr 14, 2026

Auto merge of #155258 - ShoyuVanilla:eq-constraint, r=<try>

52727e5

Make region equality emits Eq constraints

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 14, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Apr 14, 2026

lcnr reviewed Apr 14, 2026

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Comment thread compiler/rustc_borrowck/src/type_check/constraint_conversion.rs Outdated

lcnr reviewed Apr 14, 2026

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Comment thread compiler/rustc_infer/src/infer/canonical/query_response.rs Outdated

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rust-bors Bot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Apr 18, 2026

Make region equality emits Eq constraints

ace3aa3

ShoyuVanilla force-pushed the eq-constraint branch from 69d9237 to ace3aa3 Compare April 18, 2026 13:03

rust-bors Bot 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 Apr 18, 2026

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Apr 18, 2026

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rust-bors Bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Apr 18, 2026

rust-bors Bot merged commit b2f1ccf into rust-lang:main Apr 18, 2026
12 checks passed

rustbot added this to the 1.97.0 milestone Apr 18, 2026

rustbot removed the perf-regression Performance regression. label Apr 18, 2026

ShoyuVanilla deleted the eq-constraint branch April 18, 2026 23:18

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Overall result: ❌✅ regressions and improvements - please read:

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Test differences

Job duration changes

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Overall result: ✅ improvements - no action needed

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