Borrowck: Simplify SCC annotation computation, placeholder rewriting by amandasystems · Pull Request #151696 · rust-lang/rust

amandasystems · 2026-01-26T16:10:16Z

This change backports some changes from the now abandoned #142623.

Notably, it simplifies the PlaceholderReachability enum by replacing the case when no placeholders were reached with a standard Option::None.

It also rewrites the API for scc::Annotations to be update-mut rather than a more Functional programming style. This showed some slight performance impact in early tests of the PR and definitely makes the implementation simpler.

This probably wants a perf run just for good measure.

r? @lcnr

lcnr · 2026-01-27T09:25:27Z

merge conflict :3

amandasystems · 2026-01-27T13:21:21Z

merge conflict :3

WITH UPSTREAM!

I must have accidentally based this on an old fork or something.

This simplifies the `PlaceholderReachability` `enum` by replacing the case when no placeholders were reached with a standard `Option::None`. It also rewrites the API for `scc::Annotations` to be update-mut rather than a more Functional programming style. This showed some slight performance impact in early tests of the PR and definitely makes the implementation simpler.

rustbot · 2026-01-27T13:23:25Z

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.

lqd · 2026-01-27T14:03:17Z

This probably wants a perf run just for good measure.

@bors try @rust-timer queue

Borrowck: Simplify SCC annotation computation, placeholder rewriting

rust-bors · 2026-01-27T16:17:52Z

☀️ Try build successful (CI)
Build commit: f2cb709 (f2cb709a16d35d694e66c79b47d4bf630cfbe07d, parent: 94a0cd15f5976fa35e5e6784e621c04e9f958e57)

rust-timer · 2026-01-27T17:34:11Z

Finished benchmarking commit (f2cb709): comparison URL.

Overall result: ❌✅ regressions and improvements - 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

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)	0.1%	[0.0%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -2.3%, secondary -2.0%)

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)	-2.3%	[-2.4%, -2.2%]	2
Improvements ✅ (secondary)	-2.0%	[-2.8%, -1.6%]	3
All ❌✅ (primary)	-2.3%	[-2.4%, -2.2%]	2

Cycles

Results (secondary 0.7%)

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)	3.4%	[2.1%, 4.9%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.6%	[-7.0%, -2.7%]	3
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 473.018s -> 472.115s (-0.19%)
Artifact size: 385.68 MiB -> 383.70 MiB (-0.52%)

amandasystems · 2026-01-27T18:18:44Z

Uuuh ok so I wasn’t excepting that kind of memory use improvement and it might be a fluke but I’LL TAKE IT.

I guess it’s possible an option may sometimes be cheaper than a 3-value enum, who knows compilers are weird 🤷‍♀️

lcnr · 2026-01-28T09:50:48Z

@bors r+

rust-bors · 2026-01-28T09:50:51Z

📌 Commit 4bdccab has been approved by lcnr

It is now in the queue for this repository.

lcnr · 2026-01-28T09:51:19Z

No idea how serious to take memory use changes here, I treat this PR as mostly perf neutral. The code is nicer though :3

lqd · 2026-01-28T10:12:59Z

No idea how serious to take memory use changes here

They look like the usual variance/noise, yes. So this is safe to rollup.

@bors rollup=maybe

…uwer Rollup of 12 pull requests Successful merges: - #150491 (resolve: Mark items under exported ambiguous imports as exported) - #150720 (Do not suggest `derive` if there is already an impl) - #150968 (compiler-builtins: Remove the no-f16-f128 feature) - #151493 ([RFC] rustc_parse: improve the error diagnostic for "missing let in let chain") - #151660 (Bump `std`'s `backtrace`'s `rustc-demangle`) - #151696 (Borrowck: Simplify SCC annotation computation, placeholder rewriting) - #151704 (Implement `set_output_kind` for Emscripten linker) - #151706 (Remove Fuchsia from target OS list in unix.rs for sleep) - #151769 (fix undefined behavior in VecDeque::splice) - #151779 (stdarch subtree update) - #151449 ([rustdoc] Add regression test for #151411) - #151773 (clean up checks for constant promotion of integer division/remainder operations)

@lcnr

Rollup merge of #151696 - amandasystems:scc-annotations-update, r=lcnr Borrowck: Simplify SCC annotation computation, placeholder rewriting This change backports some changes from the now abandoned #142623. Notably, it simplifies the `PlaceholderReachability` `enum` by replacing the case when no placeholders were reached with a standard `Option::None`. It also rewrites the API for `scc::Annotations` to be update-mut rather than a more Functional programming style. This showed some slight performance impact in early tests of the PR and definitely makes the implementation simpler. This probably wants a perf run just for good measure. r? @lcnr

…uwer Rollup of 12 pull requests Successful merges: - rust-lang/rust#150491 (resolve: Mark items under exported ambiguous imports as exported) - rust-lang/rust#150720 (Do not suggest `derive` if there is already an impl) - rust-lang/rust#150968 (compiler-builtins: Remove the no-f16-f128 feature) - rust-lang/rust#151493 ([RFC] rustc_parse: improve the error diagnostic for "missing let in let chain") - rust-lang/rust#151660 (Bump `std`'s `backtrace`'s `rustc-demangle`) - rust-lang/rust#151696 (Borrowck: Simplify SCC annotation computation, placeholder rewriting) - rust-lang/rust#151704 (Implement `set_output_kind` for Emscripten linker) - rust-lang/rust#151706 (Remove Fuchsia from target OS list in unix.rs for sleep) - rust-lang/rust#151769 (fix undefined behavior in VecDeque::splice) - rust-lang/rust#151779 (stdarch subtree update) - rust-lang/rust#151449 ([rustdoc] Add regression test for rust-lang/rust#151411) - rust-lang/rust#151773 (clean up checks for constant promotion of integer division/remainder operations)

rustbot assigned lcnr Jan 26, 2026

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 Jan 26, 2026

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amandasystems force-pushed the scc-annotations-update branch from 3989564 to 56873b6 Compare January 26, 2026 16:15

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JayanAXHF added the A-borrow-checker Area: The borrow checker label Jan 26, 2026

lcnr approved these changes Jan 27, 2026

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amandasystems force-pushed the scc-annotations-update branch from 56873b6 to 4bdccab Compare January 27, 2026 13:23

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rust-bors bot pushed a commit that referenced this pull request Jan 27, 2026

Auto merge of #151696 - amandasystems:scc-annotations-update, r=<try>

f2cb709

Borrowck: Simplify SCC annotation computation, placeholder rewriting

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 27, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jan 27, 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 Jan 28, 2026

JonathanBrouwer mentioned this pull request Jan 28, 2026

Rollup of 12 pull requests #151794

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rust-bors bot merged commit d0a9dac into rust-lang:main Jan 29, 2026
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rustbot added this to the 1.95.0 milestone Jan 29, 2026

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rust-timer commented Jan 27, 2026

Overall result: ❌✅ regressions and improvements - no action needed

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