Region inference: Use outlives-static constraints in constraint search #140737

amandasystems · 2025-05-07T11:12:09Z

Revise the extra r: 'static constraints added upon universe issues to add an explanation, and use that explanation during constraint blame search. This greatly simplifies the region inference logic, which now does not need to reverse-engineer the event that caused a region to outlive 'static.

This cosmetically changes the output of two UI tests. I blessed them i separate commits with separate motivations, but that can of course be squashed as desired. We probably want that.

The PR was extracted out of #130227 and consists of one-third of its functional payload.

r? lcnr

bors · 2025-06-05T09:38:19Z

☔ The latest upstream changes (presumably #140466) made this pull request unmergeable. Please resolve the merge conflicts.

amandasystems · 2025-06-09T11:48:37Z

(We may also want a perf run to see if I messed something up badly)

lcnr · 2025-06-09T11:49:59Z

@bors try @rust-timer queue

Region inference: Use outlives-static constraints in constraint search Revise the extra `r: 'static` constraints added upon universe issues to add an explanation, and use that explanation during constraint blame search. This greatly simplifies the region inference logic, which now does not need to reverse-engineer the event that caused a region to outlive `'static`. This cosmetically changes the output of two UI tests. I blessed them i separate commits with separate motivations, but that can of course be squashed as desired. We probably want that. The PR was extracted out of #130227 and consists of one-third of its functional payload. It is based on #140466, so that has to land first. We probably want a perf run of this. It shouldn't have much of an impact and a positive one if any, but I have been wrong before. In particular, SCC annotations are heavier now. r? lcnr

bors · 2025-06-09T11:51:10Z

⌛ Trying commit a209255 with merge 0d3d480...

bors · 2025-06-09T14:19:00Z

☀️ Try build successful - checks-actions
Build commit: 0d3d480 (0d3d48082efe47a20c953415945cc721105e5f85)

rust-timer · 2025-06-09T15:46:52Z

Finished benchmarking commit (0d3d480): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.2%	[0.1%, 0.3%]	12
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.5%]	6
All ❌✅ (primary)	0.2%	[0.1%, 0.3%]	12

Max RSS (memory usage)

Results (primary 0.4%, secondary -1.0%)

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

	mean	range	count
Regressions ❌ (primary)	1.4%	[1.3%, 1.5%]	2
Regressions ❌ (secondary)	3.0%	[3.0%, 3.0%]	1
Improvements ✅ (primary)	-1.7%	[-1.7%, -1.7%]	1
Improvements ✅ (secondary)	-1.9%	[-2.4%, -0.6%]	4
All ❌✅ (primary)	0.4%	[-1.7%, 1.5%]	3

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 753.257s -> 756.017s (0.37%)
Artifact size: 372.34 MiB -> 372.33 MiB (-0.00%)

amandasystems · 2025-08-08T09:30:34Z

@rustbot blocked waiting for #145041 and #144988 to land!

(#144988 is not technically necessary, but would remove some code I added during development and so would be kind of nice to have).

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc `@amandasystems`

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ``@amandasystems``

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ```@amandasystems```

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc `@amandasystems`

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ``@amandasystems``

…=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in rust-lang#140737 (comment). r? types cc ```@amandasystems```

Rollup merge of #145041 - lcnr:borrowck-limitations-error, r=BoxyUwU rework GAT borrowck limitation error The old one depends on the `ConstraintCategory` of the constraint which meant we did not emit this note if we had to prove the higher ranked trait bound due to e.g. normalization. This made it annoying brittle and caused MIR borrowck errors to be order dependent, fixes the issue in #140737 (comment). r? types cc ```@amandasystems```

Revise the extra `r: 'static` constraints added upon universe issues to add an explanation, and use that explanation during constraint blame search. This greatly simplifies the region inference logic, which now does not need to reverse-engineer the event that caused a region to outlive 'static.

rustbot · 2025-08-19T10:40:02Z

This PR was rebased onto a different master 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.

amandasystems · 2025-08-19T10:41:49Z

@rustbot ready

amandasystems · 2025-08-19T10:43:08Z

We may also want a new perf run since I have changed the code that runs during SCC construction which might affect performance (positively, I hope)

lcnr · 2025-08-19T11:59:19Z

@bors try @rust-timer queue

Region inference: Use outlives-static constraints in constraint search

lcnr · 2025-08-19T12:06:44Z

compiler/rustc_borrowck/src/region_infer/mod.rs

+            }
+        });
+
+        // Edge case: it's possible that `'from_region` is an unnameable placeholder.


what exactly is this edge case and when does it occur, also

// This the false argument is what prevents circular reasoning here!

Sorry; was babying too hard to think with computer about this.

I meant that there is a risk of this being infinitely recursive if this passes through the same outlives static constraint again. Sometimes that's even a shortcut, since static leads everywhere, in which case we're at risk of blaming boring constraints or recursing forever. It's also nicer from a purely logical standpoint to do the search in the original graph before rewriting it (ie without the new edges) since it's closer to the original set of constraints.

As for the other case, this would happen when we have an SCC representative r for SCC S whose computed universe was lowered. I think this always happens because it shares SCC with an existential e with a lower universe. In that case r would be the unnameable region, which in some cases triggers a search for a path from r to r, which ICEs, so I blame e instead.

This should become unnecessary once the placeholder outlives existential check is enabled in the next PR, but for now this is the only way to detect that issue.

Presumably this could also happen if some other region that’s a less desirable representative ended up in the same SCC, but I don’t know if that ever happens for regions which aren’t either placeholders or existentialis both of which are errors.

rust-bors · 2025-08-19T14:16:49Z

☀️ Try build successful (CI)
Build commit: 834fbf1 (834fbf1e2aa01b56f03cb70cc1f290e146189916, parent: 8365fcb2b840c95eeb0bc377af8bd498fad22245)

rust-timer · 2025-08-19T15:26:17Z

Finished benchmarking commit (834fbf1): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	4.7%	[4.7%, 4.7%]	1
Regressions ❌ (secondary)	0.4%	[0.2%, 0.6%]	5
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.6%, -0.2%]	14
All ❌✅ (primary)	4.7%	[4.7%, 4.7%]	1

Max RSS (memory usage)

Results (primary 1.7%, secondary -2.8%)

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

	mean	range	count
Regressions ❌ (primary)	1.7%	[1.7%, 1.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-5.0%, -1.9%]	4
All ❌✅ (primary)	1.7%	[1.7%, 1.7%]	1

Cycles

Results (primary 3.8%, secondary 43.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.8%	[3.8%, 3.8%]	1
Regressions ❌ (secondary)	43.8%	[39.1%, 49.9%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.8%	[3.8%, 3.8%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 471.447s -> 469.972s (-0.31%)
Artifact size: 378.21 MiB -> 378.17 MiB (-0.01%)

rustbot assigned lcnr May 7, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels May 7, 2025

amandasystems force-pushed the revised-constraint-search branch from f4af776 to 72e81ea Compare May 16, 2025 10:14

amandasystems mentioned this pull request May 26, 2025

Move placeholder handling to a proper preprocessing step #140466

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amandasystems force-pushed the revised-constraint-search branch from 72e81ea to 9a1face Compare May 27, 2025 10:09

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lqd mentioned this pull request May 27, 2025

Scalable Polonius support on nightly rust-lang/rust-project-goals#118

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amandasystems force-pushed the revised-constraint-search branch from 7902ae9 to 6539053 Compare May 28, 2025 17:05

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amandasystems force-pushed the revised-constraint-search branch from 6539053 to a209255 Compare June 5, 2025 11:13

amandasystems changed the title ~~[WIP] Region inference: Use outlives-static constraints in constraint search~~ Region inference: Use outlives-static constraints in constraint search Jun 9, 2025

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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 Jun 9, 2025

amandasystems mentioned this pull request Jun 17, 2025

Move placeholder error handling to before region inference #142623

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amandasystems force-pushed the revised-constraint-search branch from a209255 to 335fa61 Compare June 18, 2025 10:21

amandasystems force-pushed the revised-constraint-search branch from 335fa61 to 6a325fd Compare June 27, 2025 15:44

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lcnr mentioned this pull request Aug 7, 2025

rework GAT borrowck limitation error #145041

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rustbot added S-blocked Status: Blocked on something else such as an RFC or other implementation work. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Aug 8, 2025

amandasystems added 2 commits August 19, 2025 12:32

Bless changes in squiggly line lengths

c11f113

amandasystems force-pushed the revised-constraint-search branch from 5a4a082 to c11f113 Compare August 19, 2025 10:39

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-blocked Status: Blocked on something else such as an RFC or other implementation work. labels Aug 19, 2025

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rust-bors bot added a commit that referenced this pull request Aug 19, 2025

Auto merge of #140737 - amandasystems:revised-constraint-search, r=<try>

834fbf1

Region inference: Use outlives-static constraints in constraint search

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 19, 2025

lcnr reviewed Aug 19, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 19, 2025

Region inference: Use outlives-static constraints in constraint search #140737

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Region inference: Use outlives-static constraints in constraint search #140737

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ❌✅ regressions and improvements - please read the text below

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