Lint unused features by mu001999 · Pull Request #152164 · rust-lang/rust

mu001999 · 2026-02-05T11:39:56Z

This PR records used features through query side effect, then reports unsued features finally.

jyn514

this is neat! in the future, it would be easier to review if you split the rename into a separate commit from the change in behavior.

could you add some tests for this please?

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compiler/rustc_interface/src/passes.rs

mu001999 · 2026-02-05T15:33:26Z

Initial implementation completed. I plan to add more tests and check whether features in rustc are reported as unused correctly or not.

compiler/rustc_feature/src/unstable.rs

Lint unused features

rust-bors · 2026-02-20T17:35:12Z

☀️ Try build successful (CI)
Build commit: c5f63cc (c5f63ccbcca9aa53fca1ca26b806737efa5cf682, parent: 59fd4ef94daa991e6797b5aa6127e824f3067def)

rust-timer · 2026-02-20T18:15:36Z

Finished benchmarking commit (c5f63cc): 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)	0.6%	[0.1%, 1.1%]	158
Regressions ❌ (secondary)	0.6%	[0.1%, 1.9%]	155
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.7%, -0.1%]	3
All ❌✅ (primary)	0.6%	[0.1%, 1.1%]	158

Max RSS (memory usage)

Results (primary 0.1%, secondary 1.6%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[1.9%, 2.5%]	2
Regressions ❌ (secondary)	3.0%	[1.2%, 4.8%]	3
Improvements ✅ (primary)	-1.9%	[-3.3%, -0.6%]	2
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	0.1%	[-3.3%, 2.5%]	4

Cycles

Results (primary 1.6%, 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)	3.1%	[2.2%, 4.7%]	3
Regressions ❌ (secondary)	2.4%	[2.4%, 2.4%]	1
Improvements ✅ (primary)	-2.7%	[-2.7%, -2.7%]	1
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	1.6%	[-2.7%, 4.7%]	4

Binary size

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

Bootstrap: 481.762s -> 483.482s (0.36%)
Artifact size: 397.93 MiB -> 398.06 MiB (0.03%)

JonathanBrouwer · 2026-02-20T21:52:03Z

Perf is a lot better now but it's still quite a regression, I wonder what we can do to fix the last bit of perf loss

bjorn3 · 2026-02-20T21:54:49Z

Maybe we could skip this new code when unstable features are not allowed at all (not nightly and no RUSTC_BOOTSTRAP)?

compiler/rustc_middle/src/queries.rs

compiler/rustc_session/src/session.rs

compiler/rustc_feature/src/unstable.rs

cjgillot

Thanks! This is great!

Do you have an idea where the remaining perf regression comes from?

View changes since this review

compiler/rustc_middle/src/dep_graph/graph.rs

compiler/rustc_feature/src/unstable.rs

cjgillot · 2026-02-21T00:57:45Z

compiler/rustc_session/src/session.rs

    pub mir_opt_bisect_eval_count: AtomicUsize,
+
+    pub used_features: Lock<Option<Arc<WorkerLocal<RefCell<FxHashMap<Symbol, Option<u32>>>>>>>,
+    used_features_weak: Weak<WorkerLocal<RefCell<FxHashMap<Symbol, Option<u32>>>>>,


That's what I have in mind. So we avoid encoding several DepNodes for each feature. Even with that we already have up to 3% increase in dep-graph size!

mu001999 · 2026-02-21T01:28:54Z

Do you have an idea where the remaining perf regression comes from?

I think it might be related to the overhead caused by query side effects and recording used features themselves. After all, we added an extra table and still need to traverse all enabled features during the final reporting phase.

Anyway, I think we could do another perf run first because the new impl is simper than before.

mu001999 · 2026-02-21T03:51:08Z

@JonathanBrouwer Please help do the perf run again, thanks!

JonathanBrouwer · 2026-02-21T09:05:19Z

@bors try @rust-timer queue

rust-timer · 2026-02-21T09:05:22Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

rust-bors · 2026-02-21T09:05:24Z

⌛ Trying commit a283871 with merge eca2935…

To cancel the try build, run the command @bors try cancel.

Workflow: https://github.com/rust-lang/rust/actions/runs/22254092757

Lint unused features

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jyn514 reviewed Feb 5, 2026

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rust-bors bot pushed a commit that referenced this pull request Feb 20, 2026

Auto merge of #152164 - mu001999-contrib:lint/unused_features, r=<try>

c5f63cc

Lint unused features

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 20, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 20, 2026

cjgillot reviewed Feb 20, 2026

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mu001999 added 5 commits February 21, 2026 10:16

Report unused features

19273b7

Add tests for unused-features

18fde19

Allow unused features in tools

39e5f06

Allow unused_features in tests

20334d1

Remove unused features in library tests

a283871

mu001999 force-pushed the lint/unused_features branch from 2d2add7 to a283871 Compare February 21, 2026 02:16

rust-bors bot pushed a commit that referenced this pull request Feb 21, 2026

Auto merge of #152164 - mu001999-contrib:lint/unused_features, r=<try>

eca2935

Lint unused features

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Feb 21, 2026

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rust-timer commented Feb 20, 2026

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

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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JonathanBrouwer commented Feb 20, 2026

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