Implement a new flag `-Zdisable-fast-paths` in trait solving by ShoyuVanilla · Pull Request #156172 · rust-lang/rust

ShoyuVanilla · 2026-05-05T04:33:36Z

Discussion: #t-types/trait-system-refactor > ask for help @ 💬

r? lcnr

rustbot · 2026-05-05T04:33:40Z

Some changes occurred to the core trait solver

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

lcnr

nice, r=me after nit

View changes since this review

lcnr · 2026-05-05T08:19:26Z

 themselves (default: no)"),
 direct_access_external_data: Option<bool> = (None, parse_opt_bool, [TRACKED],
 "Direct or use GOT indirect to reference external data symbols"),
+ disable_fast_paths: bool = (false, parse_bool, [UNTRACKED],


i think this in theory impacts behavior, so it should be TRACKED 🤔

Ah, right. I actually got some codes failing to compile with the flags due to the overflow since we don't do compute_goal_fast_path with it but goes into the nested goals

ShoyuVanilla · 2026-05-05T12:14:22Z

@bors r=lcnr

rust-bors · 2026-05-05T12:14:25Z

📌 Commit 4432f6b has been approved by lcnr

It is now in the queue for this repository.

…uwer Rollup of 7 pull requests Successful merges: - #156014 (resolve: Catch "cannot reexport" errors from macros 2.0 better) - #156058 (Print HRTB binders before fn qualifiers) - #156172 (Implement a new flag `-Zdisable-fast-paths` in trait solving) - #156184 (Revert "remove `MethodReceiverExpr` special-casing") - #155957 (Revert const hacks and use const closures in std) - #156127 (Update `askama` version to `0.16.0`) - #156183 (Remove duplicate debug assert)

Rollup merge of #156172 - ShoyuVanilla:slowbro, r=lcnr Implement a new flag `-Zdisable-fast-paths` in trait solving Discussion: [#t-types/trait-system-refactor > ask for help @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/364551-t-types.2Ftrait-system-refactor/topic/ask.20for.20help/near/583953725) r? lcnr

JonathanBrouwer · 2026-05-08T13:43:36Z

@rust-timer build 6cebd3e

rust-timer · 2026-05-08T15:06:00Z

Finished benchmarking commit (6cebd3e): comparison URL.

Overall result: ❌ regressions - 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)	0.1%	[0.1%, 0.1%]	2
Regressions ❌ (secondary)	0.8%	[0.2%, 1.5%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.1%, 0.1%]	2

Max RSS (memory usage)

Results (secondary -0.6%)

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)	-0.6%	[-0.6%, -0.6%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -0.3%)

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)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.6%	[-0.7%, -0.5%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 495.494s -> 495.573s (0.02%)
Artifact size: 394.42 MiB -> 394.43 MiB (0.00%)

JonathanBrouwer · 2026-05-08T15:19:52Z

Caused the regression in
#156190

lqd · 2026-05-08T15:43:08Z

why is the old solver impacted? 🤔

ShoyuVanilla · 2026-05-09T14:51:21Z

This flag also affects some old solver optimization routines. But anyway, it's surprising that this impacts the perf in non-negligible amounts 🤔
Maybe having this impacts some optimizations like branch prediction. I'll try some experiments with that hypothesis

…uwer Rollup of 7 pull requests Successful merges: - rust-lang/rust#156014 (resolve: Catch "cannot reexport" errors from macros 2.0 better) - rust-lang/rust#156058 (Print HRTB binders before fn qualifiers) - rust-lang/rust#156172 (Implement a new flag `-Zdisable-fast-paths` in trait solving) - rust-lang/rust#156184 (Revert "remove `MethodReceiverExpr` special-casing") - rust-lang/rust#155957 (Revert const hacks and use const closures in std) - rust-lang/rust#156127 (Update `askama` version to `0.16.0`) - rust-lang/rust#156183 (Remove duplicate debug assert)

@lcnr

…=BoxyUwU,lcnr obligations_for_self_ty: skip irrelevant goals (recompute sub_root from stalled_vars) Supersedes [#146759](#146759). Since the original PR is having conflict now, I've rebased. This includes 2 commit 1. the original one with rebase 2. above that, I've changed: ``` if stalled_on is Some: ignore sub_roots -> walk stalled_vars -> re-evaluate sub_root else: same with this PR ``` Why: cached sub_roots can be stale if there is a sub-unification merge after stalling ([lcnr proposed on the original PR](#146759 (comment))) so re-evaluating when reads ---> so more conservative Testing: * next-solver / traits tests * we can follow after merging #156172 with -Zdisable-fast-paths Style question: In my preference, inside filter, I used filter_map - matches, but do you prefer if-let chain? Original author: [lcnr](https://github.com/lcnr) [#146759](#146759) r? @lcnr

…uwer Rollup of 7 pull requests Successful merges: - rust-lang/rust#156014 (resolve: Catch "cannot reexport" errors from macros 2.0 better) - rust-lang/rust#156058 (Print HRTB binders before fn qualifiers) - rust-lang/rust#156172 (Implement a new flag `-Zdisable-fast-paths` in trait solving) - rust-lang/rust#156184 (Revert "remove `MethodReceiverExpr` special-casing") - rust-lang/rust#155957 (Revert const hacks and use const closures in std) - rust-lang/rust#156127 (Update `askama` version to `0.16.0`) - rust-lang/rust#156183 (Remove duplicate debug assert)

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 May 5, 2026

rustbot assigned lcnr May 5, 2026

lcnr reviewed May 5, 2026
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lcnr mentioned this pull request May 5, 2026
obligations_for_self_ty: skip irrelevant goals #146759
Closed

Implement a new flag -Zdisable-fast-paths in trait solving
4432f6b

ShoyuVanilla force-pushed the slowbro branch from 3311efa to 4432f6bCompare May 5, 2026 10:46

inq mentioned this pull request May 5, 2026
obligations_for_self_ty: skip irrelevant goals (recompute sub_root from stalled_vars) #156187
Merged

rust-borsBot 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 May 5, 2026

JonathanBrouwer mentioned this pull request May 5, 2026
Rollup of 7 pull requests #156190
Merged

rust-borsBot merged commit b503949 into rust-lang:mainMay 5, 2026
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rustbot added this to the 1.97.0 milestone May 5, 2026

ShoyuVanilla deleted the slowbro branch May 6, 2026 00:02

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rustbot added the perf-regression Performance regression. label May 8, 2026

ShoyuVanilla mentioned this pull request Jun 4, 2026
perf: Hint -Zdisable-fast-paths as false#157441
Closed

lcnr mentioned this pull request Jun 24, 2026
Stabilize the next-generation trait solver rust-lang/goals#113
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Implement a new flag `-Zdisable-fast-paths` in trait solving - #156172

Implement a new flag `-Zdisable-fast-paths` in trait solving#156172
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Overall result: ❌ regressions - please read:

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