fix rebasing cycle heads when not reaching a fixpoint #147061

lcnr · 2025-09-26T11:49:49Z

fixes rust-lang/trait-system-refactor-initiative#232

annoyingly subtle, imagine the following proof tree

A (no cycle head usages, final result Y)
- ignored B (depends on A with provisional result X)
  - A (cycle, provisional result X)
B (using the cache entry here incorrectly assumes A has final result X)

lcnr · 2025-09-26T11:54:38Z

compiler/rustc_type_ir/src/search_graph/mod.rs


+                let Some(new_highest_head_index) = heads.opt_highest_cycle_head_index() else {
+                    return false;
+                };


moved earlier for potential perf gains. no need to do anything more involved if we need to pop anyways

compiler/rustc_type_ir/src/search_graph/mod.rs

lcnr · 2025-09-26T11:55:20Z

@bors try @rust-timer queue

fix rebasing cycle heads when not reaching a fixpoint

rustbot · 2025-09-26T11:58:16Z

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.

lcnr · 2025-09-26T12:14:18Z

@bors cancel

@bors try

fix rebasing cycle heads when not reaching a fixpoint

BoxyUwU · 2025-09-26T13:21:40Z

r=me if ci and perf good

rust-bors · 2025-09-26T14:26:24Z

☀️ Try build successful (CI)
Build commit: a7a478b (a7a478bb1c36795d5c8eb82ffec98b5b4200479d, parent: 5b9007bfc358817cf066ee27c6b440440727d3a7)

rust-timer · 2025-09-26T15:37:32Z

Finished benchmarking commit (a7a478b): comparison URL.

Overall result: no relevant changes - no action needed

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.

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

Instruction count

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

Max RSS (memory usage)

Results (primary 1.2%, secondary 2.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.9%	[2.9%, 3.0%]	2
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-2.2%	[-2.2%, -2.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.2%	[-2.2%, 3.0%]	3

Cycles

Results (primary -1.8%, secondary -3.9%)

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

	mean	range	count
Regressions ❌ (primary)	4.0%	[2.0%, 5.4%]	6
Regressions ❌ (secondary)	4.0%	[3.3%, 4.8%]	6
Improvements ✅ (primary)	-4.0%	[-9.8%, -2.1%]	16
Improvements ✅ (secondary)	-6.5%	[-19.9%, -1.0%]	18
All ❌✅ (primary)	-1.8%	[-9.8%, 5.4%]	22

Binary size

Results (secondary 0.0%)

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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Bootstrap: 478.827s -> 470.609s (-1.72%)
Artifact size: 388.10 MiB -> 388.06 MiB (-0.01%)

BoxyUwU · 2025-09-26T16:43:23Z

@bors r+

bors · 2025-09-26T16:43:26Z

📌 Commit 4e44d58 has been approved by BoxyUwU

It is now in the queue for this repository.

lcnr · 2025-09-27T15:37:41Z

@bors rollup

…xyUwU fix rebasing cycle heads when not reaching a fixpoint fixes rust-lang/trait-system-refactor-initiative#232 annoyingly subtle, imagine the following proof tree - A (no cycle head usages, final result Y) - *ignored* B (depends on A with provisional result X) - A (cycle, provisional result X) - B (using the cache entry here incorrectly assumes A has final result X) r? `@BoxyUwU`

Rollup of 16 pull requests Successful merges: - #142139 (Include additional hashes in src/stage0) - #146745 (Clarified error note for usize range matching) - #146763 (cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 5)) - #146788 (chore: removes deprecated discord.) - #146942 ([rustdoc] Finish getting rid of usages `write_str`) - #147002 (rustdoc-search: stringdex update with more packing) - #147061 (fix rebasing cycle heads when not reaching a fixpoint) - #147066 (Fix tracking issue number for feature(macro_attr)) - #147081 (doc: fix a typo in platform-support.md) - #147082 (formatting_options: fix alternate docs 0b/0o mixup) - #147086 (compiletest: Use `PanicHookInfo::payload_as_str` now that it's stable in beta) - #147092 (Do not compute optimized MIR if code does not type-check.) - #147093 (redox: switch to colon as path separator) - #147095 (Library: Remove remaining private `#[repr]` workarounds) - #147098 (Add auto extra-checks in pre-push hook) - #147110 (Fix typo) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 14 pull requests Successful merges: - #142139 (Include additional hashes in src/stage0) - #146745 (Clarified error note for usize range matching) - #146763 (cg_llvm: Replace some DIBuilder wrappers with LLVM-C API bindings (part 5)) - #146788 (chore: removes deprecated discord.) - #146942 ([rustdoc] Finish getting rid of usages `write_str`) - #147061 (fix rebasing cycle heads when not reaching a fixpoint) - #147066 (Fix tracking issue number for feature(macro_attr)) - #147081 (doc: fix a typo in platform-support.md) - #147082 (formatting_options: fix alternate docs 0b/0o mixup) - #147086 (compiletest: Use `PanicHookInfo::payload_as_str` now that it's stable in beta) - #147093 (redox: switch to colon as path separator) - #147095 (Library: Remove remaining private `#[repr]` workarounds) - #147098 (Add auto extra-checks in pre-push hook) - #147110 (Fix typo) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #147061 - lcnr:provisional-cache-woops, r=BoxyUwU fix rebasing cycle heads when not reaching a fixpoint fixes rust-lang/trait-system-refactor-initiative#232 annoyingly subtle, imagine the following proof tree - A (no cycle head usages, final result Y) - *ignored* B (depends on A with provisional result X) - A (cycle, provisional result X) - B (using the cache entry here incorrectly assumes A has final result X) r? ``@BoxyUwU``

rustbot assigned BoxyUwU Sep 26, 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. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Sep 26, 2025

lcnr requested a review from Copilot September 26, 2025 11:49

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lcnr force-pushed the provisional-cache-woops branch 2 times, most recently from 19c2f98 to a09eb21 Compare September 26, 2025 11:54

lcnr commented Sep 26, 2025

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compiler/rustc_type_ir/src/search_graph/mod.rs Outdated Show resolved Hide resolved