Fix the wasm32-unknown-unknown target feature/cfg bug by nnethercote · Pull Request #161718 · rust-lang/rust

nnethercote · 2026-08-25T05:04:27Z

Due to some bad ordering of session/config initialization code, cfg(target_has_threads) fails to be set for the wasm32-unknown-unknown platform when -Ctarget-feature=+atomics is specified. This PR fixes the problem. Details in individual commits.

r? @Mark-Simulacrum

Due to some bad ordering of session/config initialization code -- more about that in subsequent commits -- `cfg(target_has_threads)` fails to be set for the `wasm32-unknown-unknown` platform when `-Ctarget-feature=+atomics` is specified. This commit modifies a test to demonstrate the bug; as written the test passes.

- `parse_check_cfg` has a single call site and is followed by a call to `fill_well_known`. - `parse_cfg` has two call sites and in both cases is followed by a call to `build_configuration`. This commit moves the follow-up calls into the functions, simplifying `run_compiler`.

Currently it modifies the `Session` and the `Cfg` (and the `Cfg` afterwards is put into the `Session`). It also takes a `CodegenBackend`. Those are some heavyweight arguments. This commit moves the `Session` modifications to the caller so the `&mut Session` isn't necessary, and passes in the `TargetConfig` instead of the whole `CodegenBackend`, plus some other small arguments. `add_configuration` ends up more clearly about modifying the `Cfg`. This is a step towards untangling session/backend initialization.

nnethercote · 2026-08-25T05:18:49Z

LLM disclosure: an LLM identified this bug and made some suggestions on how to fix it and test the fix. I wrote all the code and text myself.

RalfJung · 2026-08-25T06:19:04Z

Ah nice, with my recent refactors here we can now funnel everything through sess.internal_target_features. Good catch!

Currently, `parse_cfg` calls `build_configuration`, which calls `default_configuration`, which calls `sess.target.singlethread(&sess.internal_target_features)`. But `sess.internal_target_features` hasn't been set at this point and is empty! This commit moves the setting of `sess.internal_target_features` before the `parse_cfg` call to fix this ordering bug. This results in the `cfg(target_has_threads)` being correctly set on `wasm32-unknown-unknown` when `-Ctarget-feature=+atomics` is specified. Note: I have plans to make this kind of ordering bug difficult/impossible in a follow-up (e.g. rust-lang#161432).

rustbot · 2026-08-25T10:53:31Z

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- 62578df

Urgau

I was nerd snipped by the changes to check-cfg.

Looks good to me. r=me with/without the nits or Mark's review

View changes since this review

Urgau · 2026-08-25T16:57:58Z


 /// Converts strings provided as `--check-cfg [specs]` into a `CheckCfg`.
-pub(crate) fn parse_check_cfg(dcx: DiagCtxtHandle<'_>, specs: Vec<String>) -> CheckCfg {
+pub(crate) fn parse_check_cfg(sess: &Session, specs: Vec<String>) -> CheckCfg {


A bit surprising that we take a Session to do some parsing, maybe the functions should be renamed to parse_and_build_...?

The only things used from the Session are .dcx() and .target. I thought about just passing those in, but #161432 will introduce an EarlySession type that just contains dcx, target, host, and opts and that will probably be what gets passed to parse_check_cfg.

nnethercote · 2026-08-25T22:40:38Z

@bors r=Urgau

rust-bors · 2026-08-25T22:40:41Z

📌 Commit 62578df has been approved by Urgau

It is now in the queue for this repository.

@Mark-Simulacrum

Fix the wasm32-unknown-unknown target feature/cfg bug Due to some bad ordering of session/config initialization code, `cfg(target_has_threads)` fails to be set for the `wasm32-unknown-unknown` platform when `-Ctarget-feature=+atomics` is specified. This PR fixes the problem. Details in individual commits. r? @Mark-Simulacrum

…uwer Rollup of 7 pull requests Successful merges: - #157036 (lint against repeated repr attributes) - #160183 (panic_unwind: Use global_asm! for IMGREL relocations) - #161718 (Fix the wasm32-unknown-unknown target feature/cfg bug) - #161673 (Fix broken link to lang_items.rs in unstable book) - #161744 (Remove `RawDefPathHash`) - #161747 (explicitly state that allocations cannot grow to the left) - #161796 (Remove dead parse error recovery (underscores in expressions))

@Mark-Simulacrum

Rollup merge of #161718 - nnethercote:parse_cfg-stuff, r=Urgau Fix the wasm32-unknown-unknown target feature/cfg bug Due to some bad ordering of session/config initialization code, `cfg(target_has_threads)` fails to be set for the `wasm32-unknown-unknown` platform when `-Ctarget-feature=+atomics` is specified. This PR fixes the problem. Details in individual commits. r? @Mark-Simulacrum

rust-timer · 2026-08-26T18:39:45Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (8a6ec0c): comparison URL.

Overall result: ❌ regressions - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

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%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	3

Max RSS (memory usage)

Results (primary 0.5%, secondary -1.9%)

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

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.5%, 0.5%]	1
Regressions ❌ (secondary)	3.0%	[1.8%, 3.6%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-4.7%	[-7.8%, -3.3%]	7
All ❌✅ (primary)	0.5%	[0.5%, 0.5%]	1

Cycles

Results (secondary -0.9%)

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)	2.0%	[2.0%, 2.0%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.7%, -2.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (secondary -0.0%)

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)	-	-	0
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	1
All ❌✅ (primary)	-	-	0

Bootstrap: 475.812s -> 479.967s (0.87%)
Artifact size: 403.37 MiB -> 402.68 MiB (-0.17%)

panstromek · 2026-08-26T19:29:09Z

Looks like this caused the regression in #161801

This seems quite unexpected, any idea where this is coming from?

nnethercote · 2026-08-26T20:30:02Z

I think the regression isn't real.

This PR did some very minor rearrangement of startup code that only runs once.
It's a 0.2% regression on three of the libc runs, i.e. very small.
I can't reproduce it on my machine with a local build.
I can't even reproduce it on my machine using the downloaded artifacts. (I got a 0.007% icount increase, not a 0.2% increase.) I don't remember ever seeing that before.

I'm out of ideas! I don't think it's worth investigating any further.

nnethercote added 3 commits August 25, 2026 15:01

rustbot assigned Mark-Simulacrum Aug 25, 2026

rustbot added A-run-make Area: port run-make Makefiles to rmake.rs 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 Aug 25, 2026

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nnethercote mentioned this pull request Aug 25, 2026

Improve Session/CodegenBackend construction #161432

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RalfJung reviewed Aug 25, 2026

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Comment thread compiler/rustc_interface/src/interface.rs

nnethercote force-pushed the parse_cfg-stuff branch from d075ae3 to 62578df Compare August 25, 2026 10:53

Urgau approved these changes Aug 25, 2026

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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 Aug 25, 2026

jhpratt mentioned this pull request Aug 26, 2026

Rollup of 4 pull requests #161799

Closed

JonathanBrouwer mentioned this pull request Aug 26, 2026

Rollup of 7 pull requests #161801

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rust-bors Bot merged commit abfd8e5 into rust-lang:main Aug 26, 2026
13 checks passed

rustbot added this to the 1.100.0 milestone Aug 26, 2026

rustbot added the perf-regression Performance regression. label Aug 26, 2026

nnethercote deleted the parse_cfg-stuff branch August 26, 2026 20:06

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Fix the wasm32-unknown-unknown target feature/cfg bug - #161718

Fix the wasm32-unknown-unknown target feature/cfg bug#161718
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