Yeet `propagate_ambiguity` by adwinwhite · Pull Request #162935 · rust-lang/rust

adwinwhite · 2026-09-18T04:51:22Z

Fixes rust-lang/project-assumptions-on-binders#29

When assumptions computation fails, we want to force the goal response to be ambiguous since we can't evaluate placeholder constraints.
We used to do this via LeafRegionConstraint::Ambiguity and propagates it everywhere.

This PR simplifies that by tracking whether we should force ambiguity in a more direct way. We just check whether we have computed assumptions for relevant universes.
This also clarifies the meaning of LeafRegionConstraint::Ambiguity which only represents true ambiguity (forever ambiguity no matter inference progress).

This doesn't solve the problem that we're being conservative about forcing ambiguity. Maybe we can have falses in some universes even if other universes don't have assumptions. We can be smart about this in the future.

Unsure part: we can also have ambiguity from non-lifetime placeholder. Unsure what to do with that. Still trying to understand it.

r? @BoxyUwU

rustbot · 2026-09-18T04:51:26Z

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BoxyUwU

sick! this looks right to me :3 thanks

View changes since this review

BoxyUwU · 2026-09-23T10:47:16Z


-        let constraint = ((smallest_universe + 1)..=largest_universe)
-            .map(|u| UniverseIndex::from_usize(u))
+        if !self


Can you add a comment explaining why we use ambiguity here

BoxyUwU · 2026-09-23T10:48:36Z

@rustbot author

rustbot · 2026-09-23T10:48:41Z

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

adwinwhite · 2026-09-28T06:13:24Z

@rustbot ready

BoxyUwU · 2026-09-28T10:23:30Z

@bors r+ rollup

thx adwin

rust-bors · 2026-09-28T10:23:33Z

📌 Commit c901622 has been approved by BoxyUwU

It is now in the queue for this repository.

@BoxyUwU

Yeet `propagate_ambiguity` Fixes rust-lang/project-assumptions-on-binders#29 When assumptions computation fails, we want to force the goal response to be ambiguous since we can't evaluate placeholder constraints. We used to do this via `LeafRegionConstraint::Ambiguity` and propagates it everywhere. This PR simplifies that by tracking whether we should force ambiguity in a more direct way. We just check whether we have computed assumptions for relevant universes. This also clarifies the meaning of `LeafRegionConstraint::Ambiguity` which only represents true ambiguity (forever ambiguity no matter inference progress). This doesn't solve the problem that we're being conservative about forcing ambiguity. Maybe we can have `false`s in some universes even if other universes don't have assumptions. We can be smart about this in the future. Unsure part: we can also have ambiguity from non-lifetime placeholder. Unsure what to do with that. Still trying to understand it. r? @BoxyUwU

…uwer Rollup of 5 pull requests Successful merges: - #162917 (fix ice for unresolved inherent delegation) - #163209 (x perf takes database path) - #162935 (Yeet `propagate_ambiguity`) - #163289 (Ensure llvm worker threads have sufficient stack space) - #163303 (Remove `PartialOrd`/`Ord` impls for `Span`/`SpanData`)

…uwer Rollup of 9 pull requests Successful merges: - #162915 (Fix maximum `send` length on non-{Mac, Windows} platforms) - #162917 (fix ice for unresolved inherent delegation) - #163209 (x perf takes database path) - #162373 (Move the foreign module #[link] ABI check to attribute parsing) - #162829 (regression test for inherent associated const ICE) - #162935 (Yeet `propagate_ambiguity`) - #163289 (Ensure llvm worker threads have sufficient stack space) - #163303 (Remove `PartialOrd`/`Ord` impls for `Span`/`SpanData`) - #163421 (triagebot: Subscribe me to changes in test-float-parse)

@BoxyUwU

Rollup merge of #162935 - adwinwhite:abby-ambig, r=BoxyUwU Yeet `propagate_ambiguity` Fixes rust-lang/project-assumptions-on-binders#29 When assumptions computation fails, we want to force the goal response to be ambiguous since we can't evaluate placeholder constraints. We used to do this via `LeafRegionConstraint::Ambiguity` and propagates it everywhere. This PR simplifies that by tracking whether we should force ambiguity in a more direct way. We just check whether we have computed assumptions for relevant universes. This also clarifies the meaning of `LeafRegionConstraint::Ambiguity` which only represents true ambiguity (forever ambiguity no matter inference progress). This doesn't solve the problem that we're being conservative about forcing ambiguity. Maybe we can have `false`s in some universes even if other universes don't have assumptions. We can be smart about this in the future. Unsure part: we can also have ambiguity from non-lifetime placeholder. Unsure what to do with that. Still trying to understand it. r? @BoxyUwU

rust-timer · 2026-09-28T18:13:17Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (2be20bd): 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
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.1%]	19
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (secondary -4.0%)

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)	-	-	0
Improvements ✅ (secondary)	-4.0%	[-4.0%, -4.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: missing data
Artifact size: 406.43 MiB -> 406.35 MiB (-0.02%)

…uwer Rollup of 9 pull requests Successful merges: - rust-lang/rust#162915 (Fix maximum `send` length on non-{Mac, Windows} platforms) - rust-lang/rust#162917 (fix ice for unresolved inherent delegation) - rust-lang/rust#163209 (x perf takes database path) - rust-lang/rust#162373 (Move the foreign module #[link] ABI check to attribute parsing) - rust-lang/rust#162829 (regression test for inherent associated const ICE) - rust-lang/rust#162935 (Yeet `propagate_ambiguity`) - rust-lang/rust#163289 (Ensure llvm worker threads have sufficient stack space) - rust-lang/rust#163303 (Remove `PartialOrd`/`Ord` impls for `Span`/`SpanData`) - rust-lang/rust#163421 (triagebot: Subscribe me to changes in test-float-parse)

rustbot assigned BoxyUwU Sep 18, 2026

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adwinwhite force-pushed the abby-ambig branch from cf82761 to f5bfcf0 Compare September 18, 2026 06:19

adwinwhite marked this pull request as ready for review September 18, 2026 06:48

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 Sep 18, 2026

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adwinwhite force-pushed the abby-ambig branch from f5bfcf0 to 94e5351 Compare September 18, 2026 09:06

BoxyUwU reviewed Sep 23, 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 23, 2026

BoxyUwU mentioned this pull request Sep 23, 2026

trait_solver: Preserve concrete candidates in ambiguous ORs #162442

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Dnreikronos mentioned this pull request Sep 23, 2026

assumptions_on_binders: Test mixed ambiguous ORs #163219

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adwinwhite added 4 commits September 28, 2026 09:12

skip registering type outlives when non region infers exist

54e09fa

remove None assumptions

7809ec4

force ambiguity when no assumptions

d1f102f

yeet propagate_ambiguity

c901622

adwinwhite force-pushed the abby-ambig branch from 94e5351 to c901622 Compare September 28, 2026 01:31

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 Sep 28, 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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 28, 2026

JonathanBrouwer mentioned this pull request Sep 28, 2026

Rollup of 5 pull requests #163437

Closed

JonathanBrouwer mentioned this pull request Sep 28, 2026

Rollup of 9 pull requests #163439

Merged

rust-bors Bot merged commit 7987cc7 into rust-lang:main Sep 28, 2026
13 checks passed

rustbot added this to the 1.101.0 milestone Sep 28, 2026

BoxyUwU mentioned this pull request Sep 28, 2026

Next-Gen Region Constraints rust-lang/goals#621

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Yeet `propagate_ambiguity` - #162935

Yeet `propagate_ambiguity`#162935
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