Add new `misleading_cfg_in_build_script` lint by GuillaumeGomez · Pull Request #153721 · rust-lang/rust

GuillaumeGomez · 2026-03-11T14:15:49Z

Fixes #125441.
Fixes rust-lang/rust-clippy#9419.

Take-over of rust-lang/rust-clippy#12862.

It was originally implemented in clippy but the clippy thought it made for sense for such a lint to be directly implemented in rustc. I applied all suggestions made on the original PR and also improved the code overall (added docs too).

So on a build.rs from cargo crate, we will check for any target_*, unix and windows cfg and warn about them.

Since you made the original review on clippy.

r? @Urgau

rustbot · 2026-03-11T14:15:53Z

Some changes occurred in check-cfg diagnostics

cc @Urgau

GuillaumeGomez · 2026-03-11T14:35:29Z

Added a ui test to ensure this lint is only emitted on cargo build.rs scripts.

Urgau · 2026-03-11T18:06:22Z

+fn is_build_script(cx: &EarlyContext<'_>) -> bool {
+    rustc_session::utils::was_invoked_from_cargo()
+        && cx.sess().opts.crate_name.as_deref() == Some("build_script_build")
+}


This seems quite hacky and brittle for a lint.

Is there a better to detect if we are compiling a Cargo build script? If there isn't, we could maybe consider having the lint allowed by-default, but have Cargo implicitly set it to warn for build-scripts.

cc @rust-lang/cargo

Agreed, not great. :-/

I was kinda hoping that we had a "build script" crate kind but we don't (which is logical but still, not fun).

Just put it in t-cargo meeting agenda :)
next meeting time: Tue, Mar 17, 2026, 10:00 UTC-4

https://hackmd.io/-vJMV3isQfqu2d2J91t1eA

GuillaumeGomez · 2026-03-12T13:21:44Z

CI is finally happy. ^^'

ehuss · 2026-03-17T16:24:55Z

The cargo team discussed this today (notes). Some comments from that discussion:

We were largely OK with the approach of making this allow-by-default and having cargo set it to a warning when building a build script.
I was concerned about the possible amount of false-positives this would generate. Has there been a crater run or other analysis of what kind of false-positives this might hit? For example, if some build script has "cfg(windows) ... run executable that only exists on windows", it seems like this would be a false positive.
I assume this would need lang-team approval. It might be good to get this on their radar.
As an outsider looking at the diff, I am rather surprised to see yet another cfg parser being implemented here. I'm curious why this isn't using the standard cfg parser?

GuillaumeGomez · 2026-03-17T16:33:49Z

Sounds good.
It's a valid concern, to which I don't really have an answer to. Only answer is to run a crater run to see the impact it would have.
We're not clear which team is "in charge" here.
Unless I missed something, the cfg parser requires data I don't have? I'd love to be proven wrong though, would reduce the code quite a lot!

Urgau · 2026-03-17T18:26:14Z

I assume this would need lang-team approval. It might be good to get this on their radar.

I would expect a T-cargo FCP, with maybe T-compiler, but not T-lang. The lint proposed here has nothing to do with the language, it's purely a compiler and Cargo affair.

To elaborate a bit more, this lint could be in cargo it-self if we had a custom linting system, and it wouldn't need T-lang approval for that, so I don't think it needs one. We also (T-compiler) didn't ask T-lang to FCP the #[cfg]/--cfg lints.

As an outsider looking at the diff, I am rather surprised to see yet another cfg parser being implemented here. I'm curious why this isn't using the standard cfg parser?

I wouldn't look to closely at the impl. I haven't yet done a review of it, I wanted to wait for T-cargo input first.

Urgau

I share the concerns from Eric, about having another cfg parser.

Fortunately, this lint is simple enough that it can use the same infra as the unexpected_cfgs lint and perform the verification directly when parsing the cfgs them-selves in rustc_attr_parsing.

This will require some code re-shuffling between different crates (rustc_attr_parsing, rustc_lint_defs, rustc_lint, ...). You can follow what has been done for the UNEXPECTED_CFGS lint.

View changes since this review

ehuss · 2026-03-23T23:03:50Z

I would expect a T-cargo FCP, with maybe T-compiler, but not T-lang. The lint proposed here has nothing to do with the language, it's purely a compiler and Cargo affair.

I don't think that's how lints have been handled in the past. They all need to be approved by the lang team. I'm not aware of any exclusions.

GuillaumeGomez · 2026-03-23T23:05:29Z

I'm surprised: I added lints in the past without needing t-lang approval (although it might have been a few years for the last one... ^^').

…, r=<try> Add new `misleading_cfg_in_build_script` lint

rust-bors · 2026-05-11T19:17:52Z

☀️ Try build successful (CI)
Build commit: 174774e (174774e66d501beae7194df11e0b825ba98f06ab, parent: 64a965e9013a9d14e83c4d370af26f6be6bf96fb)

rust-timer · 2026-05-11T20:06:08Z

Error occured while categorizing benchmark run:

could not find start commit for bound Commit("64a965e9013a9d14e83c4d370af26f6be6bf96fb")

GuillaumeGomez · 2026-05-11T21:56:45Z

Failed again? O.o

Let's try a full thing just in case. ^^'

@bors try @rust-timer queue

…, r=<try> Add new `misleading_cfg_in_build_script` lint

Kobzol · 2026-05-11T22:00:10Z

rust-lang/triagebot#2401 should fix the rustc-perf error.

rust-bors · 2026-05-12T00:08:03Z

☀️ Try build successful (CI)
Build commit: be965f1 (be965f1eed9ef118145a9e32331b9ce1e57874b2, parent: 64a965e9013a9d14e83c4d370af26f6be6bf96fb)

rust-timer · 2026-05-12T00:49:08Z

Error occured while categorizing benchmark run:

could not find start commit for bound Commit("64a965e9013a9d14e83c4d370af26f6be6bf96fb")

GuillaumeGomez · 2026-05-12T01:09:39Z

I'll wait for it to fixed. Please restart it once fixed on your side @Kobzol. ;)

Kobzol · 2026-05-12T05:00:53Z

@bors try @rust-timer queue

…, r=<try> Add new `misleading_cfg_in_build_script` lint

rust-bors · 2026-05-12T07:10:18Z

☀️ Try build successful (CI)
Build commit: f7e8e16 (f7e8e162edaeb4d30b8bb9761903d167e58b3844, parent: 29b7590130c83542a095cdf1323ed0f78eec2bb8)

rust-timer · 2026-05-12T07:51:09Z

Finished benchmarking commit (f7e8e16): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

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.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.2%	[0.2%, 0.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.9%	[-1.0%, -0.8%]	6
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	1

Max RSS (memory usage)

Results (primary 2.6%, secondary -1.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.6%	[2.6%, 2.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	1
All ❌✅ (primary)	2.6%	[2.6%, 2.6%]	1

Cycles

Results (secondary -1.0%)

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)	3.2%	[2.4%, 4.3%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.4%	[-4.0%, -2.3%]	5
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 517.584s -> 509.112s (-1.64%)
Artifact size: 398.01 MiB -> 398.05 MiB (0.01%)

GuillaumeGomez · 2026-05-12T22:27:54Z

@Urgau How does it look to you?

Urgau · 2026-05-13T19:36:27Z

How does it look to you?

I haven't done yet a deep-dive review, but that looks pretty good.

Happy to see the logic moved directly in the parsing, it seems to simplify a bunch of things. Even perf seems to be happy.

I will try to do a review before RustWeek, otherwise it will after.

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 Mar 11, 2026

rustbot assigned Urgau Mar 11, 2026

GuillaumeGomez force-pushed the misleading_cfg_in_build_script branch from 262d2e2 to 3e99ab7 Compare March 11, 2026 14:35

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ehuss added the I-cargo-nominated Nominated for discussion during a cargo team meeting. label Mar 13, 2026

ehuss added this to Cargo status tracker Mar 17, 2026

ehuss moved this to Nominated in Cargo status tracker Mar 17, 2026

Urgau reviewed Mar 18, 2026

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Comment thread compiler/rustc_lint/src/misleading_cfg_in_build_script.rs Outdated

Comment thread compiler/rustc_lint/src/misleading_cfg_in_build_script.rs Outdated

Comment thread compiler/rustc_lint/src/misleading_cfg_in_build_script.rs Outdated

GuillaumeGomez force-pushed the misleading_cfg_in_build_script branch from 2d14a6b to 06e2527 Compare March 19, 2026 13:34

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traviscross added T-lang Relevant to the language team I-lang-nominated Nominated for discussion during a lang team meeting. labels Mar 23, 2026

rust-bors Bot pushed a commit that referenced this pull request May 11, 2026

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…, r=<try> Add new `misleading_cfg_in_build_script` lint

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Auto merge of #153721 - GuillaumeGomez:misleading_cfg_in_build_script…

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…, r=<try> Add new `misleading_cfg_in_build_script` lint

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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 May 12, 2026

Urgau reviewed May 13, 2026

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Comment thread compiler/rustc_ast/src/attr/mod.rs Outdated

Urgau 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. perf-regression Performance regression. labels May 13, 2026

Move misleading_cfg_in_build_script directly into attribute parsing

a114a87

GuillaumeGomez force-pushed the misleading_cfg_in_build_script branch from 0ecdc4f to a114a87 Compare May 14, 2026 00:42

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Overall result: ❌✅ regressions and improvements - please read:

Instruction count

Max RSS (memory usage)

Cycles

Binary size

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