rustdoc: Stop unconditionally evaluating the initializer of associated consts #151232

fmease · 2026-01-17T00:17:43Z

See the descriptions of the added tests for details.

Fixes #131625.
Fixes [after beta-1.93 backport] #149635.
Fixes #150312.

Supersedes #150629 IINM.

CC @cuviper (#149635 (comment))
r? @GuillaumeGomez or @yotamofek (#150629)

fmease · 2026-01-17T00:41:53Z

This should theoretically have a positive impact on performance.

@bors try @rust-timer queue

rustdoc: Stop evaluating associated consts

tests/rustdoc-html/constant/assoc-const-has-projection-ty.rs

tests/rustdoc-ui/diverging-assoc-consts.rs

rust-bors · 2026-01-17T02:58:30Z

☀️ Try build successful (CI)
Build commit: 9e129b1 (9e129b1955e6060a281ffcd69c9d4ab002b6384c, parent: f6a07efc81e4b5a9166884f50c2cb1686009c3fd)

rust-timer · 2026-01-17T03:39:57Z

Finished benchmarking commit (9e129b1): comparison URL.

Overall result: ✅ 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
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	3
Improvements ✅ (secondary)	-29.6%	[-29.6%, -29.6%]	1
All ❌✅ (primary)	-0.3%	[-0.3%, -0.2%]	3

Max RSS (memory usage)

Results (primary -2.2%, secondary -4.8%)

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)	-	-	0
Improvements ✅ (primary)	-2.2%	[-3.2%, -1.2%]	2
Improvements ✅ (secondary)	-4.8%	[-4.8%, -4.8%]	1
All ❌✅ (primary)	-2.2%	[-3.2%, -1.2%]	2

Cycles

Results (primary -1.5%, secondary -19.1%)

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)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-19.1%	[-36.2%, -1.9%]	2
All ❌✅ (primary)	-1.5%	[-1.5%, -1.5%]	1

Binary size

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

Bootstrap: 472.76s -> 472.975s (0.05%)
Artifact size: 383.52 MiB -> 383.51 MiB (-0.00%)

yotamofek · 2026-01-17T09:25:05Z

Nice! And simple enough to backport :)
r=me once CI is green.

…d consts

fmease · 2026-01-17T11:14:42Z

CI was green but I've just force-pushed again to tweak the commit message slightly.

r? yotamofek @bors r=yotamofek

rust-bors · 2026-01-17T11:14:45Z

📌 Commit ac8e850 has been approved by yotamofek

It is now in the queue for this repository.

fmease · 2026-01-17T11:30:30Z

Beta backport accepted as per #t-rustdoc > beta-nominated: #151232 @ 💬.

…=fmease [beta] rustdoc: Stop unconditionally evaluating the initializer of associated consts Last minute beta backport of #151232 ([beta backport accepted](https://rust-lang.zulipchat.com/#narrow/channel/266220-t-rustdoc/topic/beta-nominated.3A.20.23151232/near/568543129)). Directly fixes #149635. Not part of some hypothetical future backport rollup because we're under time pressure as the release procedures have commenced already: https://forge.rust-lang.org/#current-release-versions, https://forge.rust-lang.org/release/process.html. r? fmease

rust-bors · 2026-01-17T19:30:06Z

☀️ Test successful - CI
Approved by: yotamofek
Pushing fe98ddc to main...

github-actions · 2026-01-17T19:32:47Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 9f6cd6d (parent) -> fe98ddc (this PR)

Test differences

Show 6 test diffs

Stage 1

[rustdoc-html] tests/rustdoc-html/constant/assoc-const-has-projection-ty.rs: [missing] -> pass (J1)
[ui] tests/rustdoc-ui/diverging-assoc-consts.rs: [missing] -> pass (J1)

Stage 2

[rustdoc-html] tests/rustdoc-html/constant/assoc-const-has-projection-ty.rs: [missing] -> pass (J0)
[ui] tests/rustdoc-ui/diverging-assoc-consts.rs: [missing] -> pass (J0)

Additionally, 2 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard fe98ddcfcfb6f185dbf4adeaf439d8a756da0273 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

aarch64-apple: 7604.5s -> 9471.0s (+24.5%)
dist-aarch64-llvm-mingw: 6698.0s -> 5962.2s (-11.0%)
x86_64-msvc-ext3: 5833.3s -> 6459.0s (+10.7%)
x86_64-gnu-llvm-20-2: 5630.0s -> 5055.2s (-10.2%)
arm-android: 6005.2s -> 5426.5s (-9.6%)
dist-apple-various: 3600.1s -> 3943.8s (+9.5%)
dist-x86_64-apple: 6734.9s -> 7357.9s (+9.3%)
x86_64-rust-for-linux: 2843.9s -> 2583.2s (-9.2%)
dist-s390x-linux: 5209.7s -> 4757.8s (-8.7%)
dist-aarch64-apple: 7561.2s -> 6992.9s (-7.5%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-01-17T20:12:26Z

Finished benchmarking commit (fe98ddc): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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)	-0.3%	[-0.4%, -0.3%]	2
Improvements ✅ (secondary)	-14.8%	[-29.6%, -0.0%]	2
All ❌✅ (primary)	-0.3%	[-0.4%, -0.3%]	2

Max RSS (memory usage)

Results (primary -0.9%, secondary -4.5%)

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)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-4.5%	[-4.5%, -4.5%]	1
All ❌✅ (primary)	-0.9%	[-0.9%, -0.9%]	1

Cycles

Results (primary -2.1%, secondary -14.9%)

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)	-2.1%	[-2.1%, -2.1%]	2
Improvements ✅ (secondary)	-14.9%	[-36.5%, -2.9%]	3
All ❌✅ (primary)	-2.1%	[-2.1%, -2.1%]	2

Binary size

Results (primary 0.1%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.1%]	53
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	24
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.0%, 0.1%]	53

Bootstrap: 472.543s -> 473.229s (0.15%)
Artifact size: 383.42 MiB -> 383.52 MiB (0.03%)

fmease added the beta-nominated Nominated for backporting to the compiler in the beta channel. label Jan 17, 2026

rustbot assigned GuillaumeGomez Jan 17, 2026

fmease added beta-nominated Nominated for backporting to the compiler in the beta channel. and removed beta-nominated Nominated for backporting to the compiler in the beta channel. labels Jan 17, 2026

fmease force-pushed the rustdoc-dont-eval-assoc-consts branch from ab8d394 to 1aacbfc Compare January 17, 2026 00:32

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rust-bors bot pushed a commit that referenced this pull request Jan 17, 2026

Auto merge of #151232 - fmease:rustdoc-dont-eval-assoc-consts, r=<try>

9e129b1

rustdoc: Stop evaluating associated consts

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

fmease commented Jan 17, 2026

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tests/rustdoc-html/constant/assoc-const-has-projection-ty.rs Outdated Show resolved Hide resolved

tests/rustdoc-ui/diverging-assoc-consts.rs Outdated Show resolved Hide resolved

fmease commented Jan 17, 2026

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

fmease force-pushed the rustdoc-dont-eval-assoc-consts branch from 1aacbfc to 1175147 Compare January 17, 2026 09:05

fmease changed the title ~~rustdoc: Stop evaluating associated consts~~ rustdoc: Stop unconditionally evaluating the initializer of associated consts Jan 17, 2026

rustdoc: Stop unconditionally evaluating the initializer of associate…

ac8e850

…d consts

fmease force-pushed the rustdoc-dont-eval-assoc-consts branch from 1175147 to ac8e850 Compare January 17, 2026 11:13

rustbot assigned yotamofek Jan 17, 2026

rust-bors bot added the S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. label Jan 17, 2026

rustbot unassigned GuillaumeGomez Jan 17, 2026

rust-bors bot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 17, 2026

fmease mentioned this pull request Jan 17, 2026

[beta] rustdoc: Stop unconditionally evaluating the initializer of associated consts #151241

Merged

fmease added the beta-accepted Accepted for backporting to the compiler in the beta channel. label Jan 17, 2026

fmease added this to the 1.93.0 milestone Jan 17, 2026

fmease removed the beta-nominated Nominated for backporting to the compiler in the beta channel. label Jan 17, 2026

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rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jan 17, 2026

rust-bors bot merged commit fe98ddc into rust-lang:main Jan 17, 2026
12 checks passed

rustbot modified the milestones: 1.93.0, 1.94.0 Jan 17, 2026

fmease deleted the rustdoc-dont-eval-assoc-consts branch January 17, 2026 19:34

fmease mentioned this pull request Jan 17, 2026

panic in constant evaluation while documenting default associated constant #149635

Closed

cuviper modified the milestones: 1.94.0, 1.93.0 Jan 17, 2026

rust-bors bot mentioned this pull request Jan 17, 2026

rustdoc: render doc(hidden) as a code attribute #151001

Open

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rustdoc: Stop unconditionally evaluating the initializer of associated consts #151232

rustdoc: Stop unconditionally evaluating the initializer of associated consts #151232

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Overall result: ✅ improvements - no action needed

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Test differences

Stage 1

Stage 2

Job duration changes

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Overall result: ✅ improvements - no action needed

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