c-variadic: initialize the copied `VaList` in const-eval by folkertdev · Pull Request #157977 · rust-lang/rust

folkertdev · 2026-06-16T15:34:19Z

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Properly initialize the result of VaList::clone in rustc_const_eval.

r? RalfJung
cc @theemathas

rustbot · 2026-06-16T15:34:23Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

miri is developed in its own repository. If possible, consider making this change to rust-lang/miri instead.

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

rustbot · 2026-06-16T15:34:26Z

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

theemathas · 2026-06-16T15:44:30Z

+fn clone() {
+    pub unsafe extern "C" fn clone_the_va_list(args: ...) {
+        // The implicit `drop` will catch a `VaList` that isn't properly initialized.
+        let _ = args.clone();
+    }
+
+    unsafe { clone_the_va_list(1i32, 2i32) }
+}
+


Could you also add tests to make sure that interleaving next_arg() with clone() in various combinations work correctly (give correct return values in miri)?

View changes since the review

Sure, I added some.

folkertdev · 2026-06-17T09:33:54Z

Hmm, we hit an interesting case here where we do have a fallback body, but that fallback is not valid for miri (but it is for llvm, gcc and cranelift) because it doesn't update the internal bookkeeping. So using -Zmiri-force-intrinsic-fallback fails.

How should I work around this?

RalfJung · 2026-06-17T17:35:44Z

How should I work around this?

Skip those tests under cfg!(force_intrinsic_fallback).

RalfJung

Thanks! I just have some nits and clarification questions.

@rustbot author

View changes since this review

RalfJung · 2026-06-17T17:37:28Z

+fn clone_and_advance() {
+    if cfg!(force_intrinsic_fallback) {
+        // Skip this test when we use the fallback bodies, the fallback body does
+        // not hook into the Miri bookkeeping that we need to detect UB.


This test does not have UB though? I don't understand this comment.

With the flag enabled Miri thinks there is UB though

https://triage.rust-lang.org/gha-logs/rust-lang/rust/81717608108

We then duplicate a VaList by just duplicating the bits (a memcpy) and that is fine for LLVM but for Miri we don't set up the allocations correctly in that case.

Yes, I understand that you do need to skip the test when fallback bodies are on. But the comment currently doesn't make sense.

It's also kind of bad that "force-intrinsic-fallbacks" reports incorrect UB. That's not supposed to happen.

Let's add something like this to the va_copy fallback body

// This fallback body exploits the fact that our codegen backends all just a plain // memcpy to duplicate VaList. This assumption is wrong for Miri. assert!(!cfg!(miri), "fallback body is incorrect under Miri");

We could also remove the fallback body when cfg(miri)? Or does miri use the pre-built core?

There's no nice way to disable the body with cfg.

RalfJung · 2026-06-17T17:38:17Z

This seems like a good test to run natively. Can you add //@run-native at the top of this file?

Adding that annotation makes the test fail, without any good way to debug it. Based on https://triage.rust-lang.org/gha-logs/rust-lang/rust/81980686843 it looks like the assert is triggered, so somehow the native test sets cfg!(miri) to true but also uses the fallback?

Is there some flag I'm missing for seeing the actual cause of the failure?

I think the cause was the other clone test also hitting the assert. Still, the debugging experience here could be better...

This behaves exactly like regular rustc ui tests. So not sure what we could do on the debugging side.

rustbot · 2026-06-17T17:38:41Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-06-17T18:51:07Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

folkertdev

@rustbot ready

View changes since this review

folkertdev · 2026-06-18T10:07:31Z

I think the cause was the other clone test also hitting the assert. Still, the debugging experience here could be better...

RalfJung · 2026-06-20T04:38:41Z

@bors r+

rust-bors · 2026-06-20T04:38:44Z

📌 Commit 7c497ef has been approved by RalfJung

It is now in the queue for this repository.

@theemathas

c-variadic: initialize the copied `VaList` in const-eval Properly initialize the result of `VaList::clone` in `rustc_const_eval`. r? RalfJung cc @theemathas

rust-bors · 2026-06-20T04:44:27Z

💔 Test for 4f5af41 failed: CI. Failed job:

auto - x86_64-msvc-ext3 (web logs, enhanced plaintext logs)

rust-log-analyzer · 2026-06-20T04:50:49Z

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  NO_DOWNLOAD_CI_LLVM: 1
  WIX: /d/a/rust/rust/wix
##[endgroup]
    Updating crates.io index
error: failed to get `icu_normalizer` as a dependency of package `idna_adapter v1.2.0`
    ... which satisfies dependency `idna_adapter = "^1"` of package `idna v1.0.3`
    ... which satisfies dependency `idna = "^1.0"` of package `cookie_store v0.21.1`
    ... which satisfies dependency `cookie_store = "^0.21.1"` of package `ureq v3.0.8`
    ... which satisfies dependency `ureq = "^3"` of package `citool v0.1.0 (D:\a\rust\rust\src\ci\citool)`

Caused by:
  failed to load source for dependency `icu_normalizer`

Caused by:
  unable to update registry `crates-io`

Caused by:
  download of ic/u_/icu_normalizer failed

Caused by:
  curl failed

Caused by:

RalfJung · 2026-06-20T04:54:05Z

@bors retry

rust-bors · 2026-06-20T08:04:10Z

☀️ Test successful - CI
Approved by: RalfJung
Duration: 3h 9m 48s
Pushing 4008bbd to main...

github-actions · 2026-06-20T08:07:36Z

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 8c75e93 (parent) -> 4008bbd (this PR)

Test differences

No test diffs found

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 4008bbdf34b583ce44a467043aae3703afc0a76f --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

x86_64-gnu-gcc-core-tests: 13m 25s -> 5m 46s (-57.0%)
dist-x86_64-freebsd: 1h 33m -> 56m 47s (-39.4%)
x86_64-rust-for-linux: 50m 27s -> 34m 28s (-31.7%)
dist-aarch64-apple: 1h 36m -> 2h 6m (+31.2%)
x86_64-gnu-stable: 1h 50m -> 2h 21m (+28.0%)
dist-x86_64-msvc: 2h 10m -> 1h 35m (-27.1%)
dist-powerpc64-linux-musl: 1h 15m -> 1h 34m (+24.7%)
dist-armv7-linux: 1h 12m -> 1h 30m (+24.6%)
dist-x86_64-msvc-alt: 2h 5m -> 2h 32m (+21.9%)
x86_64-gnu-nopt: 2h 22m -> 1h 53m (-20.2%)

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 · 2026-06-20T08:49:07Z

Finished benchmarking commit (4008bbd): comparison URL.

Overall result: ❌ regressions - 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)	0.3%	[0.2%, 0.3%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

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

Binary size

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

Bootstrap: 480.613s -> 487.822s (1.50%)
Artifact size: 390.68 MiB -> 391.12 MiB (0.11%)

folkertdev added the F-c_variadic `#![feature(c_variadic)]` label Jun 16, 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 Jun 16, 2026

rustbot assigned RalfJung Jun 16, 2026

folkertdev mentioned this pull request Jun 16, 2026

Stabilize c-variadic function definitions #155697

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theemathas reviewed Jun 16, 2026

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folkertdev force-pushed the miri-zero-valist branch from 6af9b67 to eb8f617 Compare June 16, 2026 17:00

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folkertdev force-pushed the miri-zero-valist branch from eb8f617 to 867b361 Compare June 17, 2026 11:11

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folkertdev force-pushed the miri-zero-valist branch from 867b361 to 29c674b Compare June 17, 2026 18:51

folkertdev force-pushed the miri-zero-valist branch from 29c674b to ba67985 Compare June 17, 2026 19:15

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c-variadic: initialize the copied VaList

7c497ef

folkertdev force-pushed the miri-zero-valist branch from db11cc5 to 7c497ef Compare June 18, 2026 10:06

folkertdev commented Jun 18, 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 Jun 20, 2026

rust-bors Bot merged commit 4008bbd into rust-lang:main Jun 20, 2026
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c-variadic: test that we use equality up to free lifetimes #158039

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Test differences

Job duration changes

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rust-timer commented Jun 20, 2026

Overall result: ❌ regressions - no action needed

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