Move recursion out of `MatchPairTree::for_pattern` helpers by Zalathar · Pull Request #154943 · rust-lang/rust

Zalathar · 2026-04-07T10:39:50Z

The helper functions now just iterate over the relevant subpatterns, while leaving recursion up to the main function.

This avoids passing parameters that were only used for recursive plumbing, and consolidates all recursive calls into for_pattern itself, which should make it easier to experiment with changes to the recursive structure.

There should be no change to compiler behaviour.

rustbot · 2026-04-07T10:39:59Z

Some changes occurred in match lowering

cc @Nadrieril

rustbot · 2026-04-07T10:40:01Z

r? @nnethercote

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nnethercote

The first commit description says:

consolidates all recursive calls into for_pattern itself

but for_pattern isn't changed?

View changes since this review

nnethercote · 2026-04-08T05:21:05Z

+fn for_each_field_subpat<'tcx>(
+    place: &PlaceBuilder<'tcx>,
+    subpatterns: &[FieldPat<'tcx>],
+    mut callback_fn: impl FnMut(PlaceBuilder<'tcx>, &Pat<'tcx>),


Does this need to be a closure? For every occurrence the callback just calls for_pattern. Seems more general than necessary.

nnethercote · 2026-04-08T05:21:12Z

+    prefix: &[Pat<'tcx>],
+    opt_middle: &Option<Box<Pat<'tcx>>>,
+    suffix: &[Pat<'tcx>],
+    mut callback_fn: impl FnMut(PlaceBuilder<'tcx>, &Pat<'tcx>),


Zalathar · 2026-04-08T06:22:32Z

Thinking about this some more, it might be worth just fully inlining both helpers.

The field-subpat helper is trivial, and much of the complexity in the array/slice helper comes from generalising over array and slice patterns, which have significantly different indexing needs.

Nadrieril · 2026-04-08T10:14:12Z

much of the complexity in the array/slice helper comes from generalising over array and slice patterns, which have significantly different indexing needs.

You can try and see but I think keeping them together is clearer. I'm thinking, how about returning an impl Iterator<Item=(ProjectionElem, Pat)> and iterating over that, to avoid the callback?

The field-subpat helper is trivial

True, we could inline that one

(Happy to take the r ? on this one, it's code I'm very familiar with)

nnethercote · 2026-04-08T11:24:01Z

Ok, thanks!

r? @Nadrieril

rustbot · 2026-04-08T11:24:06Z

Nadrieril is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2026-04-11T08:44:07Z

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.

Zalathar · 2026-04-11T08:46:22Z

I've pushed a brand-new set of commits that:

Inlines field_match_pairs into its callers
Makes prefix_slice_suffix return a Vec<(PlaceBuilder, &Pat)> that callers then iterate over

This is what I'm currently using as a base for #155144, though I think these changes are a reasonable cleanup on their own.

@rustbot ready

Nadrieril · 2026-04-17T08:55:26Z

@bors try @rust-timer queue

Move recursion out of `MatchPairTree::for_pattern` helpers

rust-bors · 2026-04-17T11:04:30Z

☀️ Try build successful (CI)
Build commit: 9e54c30 (9e54c303588dbc58381f83b5260498c5465bff28, parent: 1b8f2e46e14b08208a53585570edd9206374aae8)

rust-timer · 2026-04-17T11:45:33Z

Finished benchmarking commit (9e54c30): comparison URL.

Overall result: ✅ improvements - no action needed

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.

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

Max RSS (memory usage)

Results (secondary 6.8%)

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)	6.8%	[6.8%, 6.8%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (primary 7.4%, 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)	7.4%	[6.9%, 7.8%]	2
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.8%	[-4.3%, -3.3%]	2
All ❌✅ (primary)	7.4%	[6.9%, 7.8%]	2

Binary size

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

Bootstrap: 504.175s -> 489.73s (-2.87%)
Artifact size: 394.15 MiB -> 394.16 MiB (0.00%)

Nadrieril · 2026-04-17T14:21:17Z

Nice

@bors r+

rust-bors · 2026-04-17T14:21:21Z

📌 Commit 4a5c24f has been approved by Nadrieril

It is now in the queue for this repository.

JonathanBrouwer · 2026-04-17T14:23:59Z

@bors rollup
Perf is probably noise and the queue is big, so lets roll this up

Move recursion out of `MatchPairTree::for_pattern` helpers The helper functions now just iterate over the relevant subpatterns, while leaving recursion up to the main function. This avoids passing parameters that were only used for recursive plumbing, and consolidates all recursive calls into `for_pattern` itself, which should make it easier to experiment with changes to the recursive structure. There should be no change to compiler behaviour.

Rollup of 12 pull requests Successful merges: - #147811 (naked functions: respect `function-sections`) - #154935 (Add Sized supertrait for CoerceUnsized and DispatchFromDyn) - #139690 (`impl Default for RepeatN`) - #153511 (`std::any::TypeId`: remove misplaced "and" in `Unique<T>` example) - #154943 (Move recursion out of `MatchPairTree::for_pattern` helpers ) - #155295 (Fix misleading "borrowed data escapes outside of function" diagnostic) - #155427 (ptr: update text in intro text to one in with_addr doc) - #155428 (Fix ICE in borrowck mutability suggestion with multi-byte ref sigil) - #155435 (rustdoc: Fix `redundant_explicit_links` incorrectly firing (or not firing) under certain scenarios) - #155450 (Remove unnecessary safety conditions related to unchecked uint arithmetic) - #155454 (docs: Fix typo in std/src/thready/scoped.rs) - #155467 (`std::error::Request`: clean up documentation)

Rollup merge of #154943 - Zalathar:for-each-subpat, r=Nadrieril Move recursion out of `MatchPairTree::for_pattern` helpers The helper functions now just iterate over the relevant subpatterns, while leaving recursion up to the main function. This avoids passing parameters that were only used for recursive plumbing, and consolidates all recursive calls into `for_pattern` itself, which should make it easier to experiment with changes to the recursive structure. There should be no change to compiler behaviour.

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 Apr 7, 2026

rustbot assigned nnethercote Apr 7, 2026

nnethercote reviewed Apr 8, 2026

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nnethercote 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 8, 2026

rustbot assigned Nadrieril and unassigned nnethercote Apr 8, 2026

Zalathar added 2 commits April 11, 2026 11:53

Inline field_match_pairs into its callers

80bb813

Move the recursive step out of prefix_slice_suffix

4a5c24f

Zalathar force-pushed the for-each-subpat branch from 17f0b0a to 4a5c24f Compare April 11, 2026 08:44

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 11, 2026

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rust-bors Bot pushed a commit that referenced this pull request Apr 17, 2026

Auto merge of #154943 - Zalathar:for-each-subpat, r=<try>

9e54c30

Move recursion out of `MatchPairTree::for_pattern` helpers

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 17, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Apr 17, 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 Apr 17, 2026

This was referenced Apr 18, 2026

Rollup of 8 pull requests #155471

Closed

Rollup of 12 pull requests #155472

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rust-bors Bot merged commit eb18553 into rust-lang:main Apr 18, 2026
12 checks passed

rustbot added this to the 1.97.0 milestone Apr 18, 2026

Zalathar deleted the for-each-subpat branch April 18, 2026 08:38

dianne mentioned this pull request Apr 25, 2026

match: Use an aggregate equality comparison for constant array/slice patterns when possible #155216

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Overall result: ✅ improvements - no action needed

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