privacy: cache for trait ref in projection #146128

bvanjoi · 2025-09-02T16:21:28Z

Performance test results from local for #145741:

time cargo clean && cargo build: 20.60s
time cargo +stage1 clean && cargo +stage1 build: 11.29s

I'm uncertain if this is a completely correct fix, as I've just reviewed the privacy update logic and it appears that a single update should suffice in the alias term. Feel free to close this if there are any algorithmic inaccuracies, and I'll investigate further.

rustbot · 2025-09-02T16:21:33Z

r? @SparrowLii

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lqd · 2025-09-02T17:09:48Z

@bors try @rust-timer queue

privacy: cache for trait ref in projection

rust-bors · 2025-09-02T19:30:56Z

☀️ Try build successful (CI)
Build commit: 97bca8b (97bca8b377025fbbb752947ef69ee3c8bc9ec6f5, parent: a2c8b0b92c14b02f0b3f96a0d5296f1090dc286b)

rust-timer · 2025-09-02T20:40:16Z

Finished benchmarking commit (97bca8b): 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)	0.1%	[0.1%, 0.2%]	12
Regressions ❌ (secondary)	0.1%	[0.1%, 0.2%]	5
Improvements ✅ (primary)	-0.3%	[-0.4%, -0.1%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.0%	[-0.4%, 0.2%]	19

Max RSS (memory usage)

Results (primary -0.1%)

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

	mean	range	count
Regressions ❌ (primary)	1.3%	[1.3%, 1.3%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.6%	[-1.6%, -1.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-1.6%, 1.3%]	2

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: 465.026s -> 467.402s (0.51%)
Artifact size: 388.33 MiB -> 388.34 MiB (0.00%)

SparrowLii · 2025-09-30T06:24:17Z

r? @petrochenkov Could you have a look of this, Vadim?

petrochenkov · 2025-10-08T15:58:41Z

compiler/rustc_privacy/src/lib.rs

        let (trait_ref, assoc_args) = projection.trait_ref_and_own_args(tcx);
-        try_visit!(self.visit_trait(trait_ref));
+        if self.visited_trait_ref_in_projection.insert(trait_ref) {
+            try_visit!(self.visit_trait(trait_ref));


This is only correct to do if in all concrete DefIdVisitors visit_trait always does the same thing for the same trait_ref and that thing is not affected by mutable state in the visitor.

TypePrivacyVisitor and SearchInterfaceForPrivateItemsVisitor clearly satisfy that condition by not having any mutable state, but I'm not so sure about ReachEverythingInTheInterfaceVisitor and ReachableContext.

ReachEverythingInTheInterfaceVisitor and ReachableContext seem to also be idempotent.

petrochenkov · 2025-10-08T17:03:04Z

I want to check some different caching strategies for this in #147486.
@rustbot blocked

From local testing, caching at DefId level doesn't help with #145741, you have to skip larger pieces of code to improve compilation times for it.

petrochenkov · 2025-10-21T12:22:13Z

Caching in this PR is overfit to one specific test case, #147486 (comment) shows that you can achieve the same effect in a more general way by caching all ty::Aliases.
(In my local testing ty::Alias caching still sped up the example from #145741 more than 2x.)

You could apply the changes from #147486 to this PR, or I can just land #147486 instead.
@rustbot author

rustbot · 2025-10-21T12:22:17Z

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

petrochenkov · 2025-10-21T12:24:28Z

tests/ui/privacy/enormous-trait-ref-in-projection-term.rs

+    <<Mask as TypeTupleAccess>::_5 as MaskBit>::Pick<Pick<5>>,
+>;
+
+fn main() {}


I don't think it makes sense to add this to UI tests, UI tests do not tests whether something compiles fast or slow, so it will mostly just slow down the testing.
It could potentially be added as a stress benchmark to https://github.com/rust-lang/rustc-perf though.

It could potentially be added as a stress benchmark to https://github.com/rust-lang/rustc-perf though.

This example seems like a good fit for the secondary benchmark suite. 🤔

The benchmark is currently too slow for rustc-perf, it takes ~25s just to check, and we run a lot of configs three times, so this would take many minutes to benchmark.

In case you didn't build this manually, but used some code generator, could you generate a faster version? E.g. with less items in the TypeTuple.

bvanjoi · 2025-10-22T02:43:19Z

or I can just land #147486 instead.

I think that works since #147386 is already good enough, so I'll close this one.

privacy: Introduce some caching to type visiting in `DefIdVisitorSkeleton` The caching fixes compilation speed issues in special cases like #145741, without introducing too much overhead in general cases. I tried to cache more, but it caused regressions from the caching overhead, like it can be seen from benchmark runs below. Inspired by #146128. Closes #145741.

privacy: Introduce some caching to type visiting in `DefIdVisitorSkeleton` The caching fixes compilation speed issues in special cases like rust-lang/rust#145741, without introducing too much overhead in general cases. I tried to cache more, but it caused regressions from the caching overhead, like it can be seen from benchmark runs below. Inspired by rust-lang/rust#146128. Closes rust-lang/rust#145741.

privacy: cache for trait ref in projection

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rustbot assigned SparrowLii Sep 2, 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 Sep 2, 2025

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rust-bors bot added a commit that referenced this pull request Sep 2, 2025

Auto merge of #146128 - bvanjoi:issue-145741, r=<try>

97bca8b

privacy: cache for trait ref in projection

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 2, 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 Sep 2, 2025

rustbot assigned petrochenkov and unassigned SparrowLii Sep 30, 2025

petrochenkov reviewed Oct 8, 2025

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privacy: Introduce some caching to type visiting in DefIdVisitorSkeleton #147486

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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-review Status: Awaiting review from the assignee but also interested parties. labels Oct 8, 2025

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bvanjoi closed this Oct 22, 2025

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

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