add c-variadic function definitions by folkertdev · Pull Request #2177 · rust-lang/reference

folkertdev · 2026-02-17T21:27:24Z

I think this has all of the raw material, but needs polishing.

Here is a draft of the stabilization report, for additional context: https://hackmd.io/@Q66MPiW4T7yNTKOCaEb-Lw/S1iI3WIwZg

Tracking issue: rust-lang/rust#44930

folkertdev · 2026-02-17T21:33:21Z

+> [!WARNING]
+> Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].


copied from

https://doc.rust-lang.org/reference/items/external-blocks.html?highlight=variadic#r-items.extern.variadic

basically, the responsibility for passing valid arguments is on the caller.

folkertdev · 2026-02-17T21:35:01Z

+unsafe extern "C" fn example() -> i32 {
+    let mut storage = MaybeUninit::<VaList<'_>>::uninit();
+    va_start(storage.as_mut_ptr()); // Initializes the VaList.
+    let mut ap: &mut VaList<'_> = ap.assume_init_mut();
+
+    unsafe { ap.arg::<i32>() }
+
+    va_end(ap)
+}


va_start and va_end are C functions/concepts. We could instead handwave and say

let mut ap: VaList<'_> = /* ... */; // Initializes the VaList.

the va_end is called by the VaList Drop implementation.

ehuss

I think the rule items.fn.params.varargs will need to be updated, since it says it can only be used in an external block.

View changes since this review

ehuss · 2026-02-27T15:18:06Z

+> Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].
+
+r[items.fn.async.desugar-brief]
+A c-variadic function definition is roughly equivalent to a function operating on a [`VaList`].


What is "roughly" about it?

In how arguments are actually passed, these signatures are (very) different

fn foo(ap: ...) { /* ... */ } fn bar(ap: VaList) { /* ... */ }

Those two functions could have exactly the same body though.

Looking at it again, I'm not sure this desugaring answers more questions than it raises? Because really it is not a desugaring, we're hooking into a bunch of custom compiler magic to make this work. We could provide an equivalent C function perhaps to illustrate what parts the rust compiler handles automatically in comparison.

i.e. it is equivalent to

// Assuming C23, so `...` as the only argument is accepted. int example(...) { va_list ap; va_start(ap); int x = va_arg(ap, int); va_end(ap); return x; }

@ghost

…-in-deps, r=mati865 report the `varargs_without_pattern` lint in deps tracking issue: rust-lang#44930 After discussion in rust-lang/reference#2177 (comment). Based on rust-lang#143619 (comment) there was only one actual impacted crate https://crates.io/crates/binrw. The issue was fixed in jam1garner/binrw#342, and has since been released jam1garner/binrw#342 (comment). Hence we may as well report this loudly. r? @ghost

@ghost

…-in-deps, r=mati865 report the `varargs_without_pattern` lint in deps tracking issue: rust-lang#44930 After discussion in rust-lang/reference#2177 (comment). Based on rust-lang#143619 (comment) there was only one actual impacted crate https://crates.io/crates/binrw. The issue was fixed in jam1garner/binrw#342, and has since been released jam1garner/binrw#342 (comment). Hence we may as well report this loudly. r? @ghost

@ghost

…-in-deps, r=mati865 report the `varargs_without_pattern` lint in deps tracking issue: rust-lang#44930 After discussion in rust-lang/reference#2177 (comment). Based on rust-lang#143619 (comment) there was only one actual impacted crate https://crates.io/crates/binrw. The issue was fixed in jam1garner/binrw#342, and has since been released jam1garner/binrw#342 (comment). Hence we may as well report this loudly. r? @ghost

@ghost

Rollup merge of #154599 - folkertdev:varargs-without-pattern-in-deps, r=mati865 report the `varargs_without_pattern` lint in deps tracking issue: #44930 After discussion in rust-lang/reference#2177 (comment). Based on #143619 (comment) there was only one actual impacted crate https://crates.io/crates/binrw. The issue was fixed in jam1garner/binrw#342, and has since been released jam1garner/binrw#342 (comment). Hence we may as well report this loudly. r? @ghost

folkertdev · 2026-04-23T15:29:03Z

The varargs_without_pattern is now reported in dependencies (on nightly).

I've pushed some tweaks, I'm now planning to submit a stabilization PR in coming days, so if you could look at this again that would be helpful.

ehuss · 2026-04-30T02:45:37Z

It looks like from rust-lang/rust#155974 that there is an intent to make this unavailable on certain targets. I don't think we've ever done something like that, and I'm not sure how we're going to document that. I suppose there will be a rule. It will need to be careful to distinguish that it is a compile error during validation. Unfortunately we don't define that as a specific phase in the reference, so I'm not sure how we should approach that.

folkertdev · 2026-04-30T08:55:01Z

I don't think this is very different from https://doc.rust-lang.org/beta/unstable-book/language-features/asm-experimental-arch.html. Consequently the page on inline assembly specifies https://doc.rust-lang.org/nightly/reference/inline-assembly.html?highlight=assemb#r-asm.stable-targets.

So we could have

Support for c-variadic function definitions is stable on the following architectures:

x86 and x86-64

arm

aarch64 and arm64ec

...

The compiler will emit an error if ... is used in a function definition on an unsupported target.

Also spirv and bpf just fundamentally do not support this feature, so there you'd always get an error.

Co-authored-by: Tshepang Mbambo <hopsi@tuta.io>

rustbot · 2026-05-13T12:13:56Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

tgross35 · 2026-07-10T12:00:27Z

+- x86 and x86-64
+- ARM
+- AArch64 and Arm64EC
+- RISC-V (except when using the ilp32e ABI)
+- LoongArch
+- s390x
+- PowerPC and PowerPC64
+- AmdGpu and Nvptx64
+- wasm32 and wasm64
+- csky
+- xtensa
+- hexagon
+- sparc64
+- mips


Nit: This mixes lowercase with the arches' name casing (C-SKY, Xtensa, Hexagon, SPARC64, MIPS, Wasm). Also some platforms are explicit about both 32- and 64-bit (x86, ppc) while others aren't (loongarch, mips)

View changes since the review

It is based on the list of stable architectures for asm!, I've tried to correct the names.

tgross35 · 2026-07-10T12:12:58Z

Is there somewhere it should be mentioned that variadics are allowed in traits?

View changes since the review

I get the sense that this sort of thing is usually left implicit , but up to our reference maintainers.

folkertdev

The merge of the stabilization PR is now only blocked on the additions to the reference here (I'm not sure that is really needed, previously features have been merged with a reference PR that was "far enough along").

I've tried to address comments here as much possible now.

I'd be happy to look at this together in the reference office hours if they are at a somewhat european-friendly time.

View changes since this review

folkertdev · 2026-07-17T09:20:26Z

+> Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].
+
+r[items.fn.async.desugar-brief]
+A c-variadic function definition is roughly equivalent to a function operating on a [`VaList`].


Looking at it again, I'm not sure this desugaring answers more questions than it raises? Because really it is not a desugaring, we're hooking into a bunch of custom compiler magic to make this work. We could provide an equivalent C function perhaps to illustrate what parts the rust compiler handles automatically in comparison.

folkertdev · 2026-07-17T09:21:56Z

+- x86 and x86-64
+- ARM
+- AArch64 and Arm64EC
+- RISC-V (except when using the ilp32e ABI)
+- LoongArch
+- s390x
+- PowerPC and PowerPC64
+- AmdGpu and Nvptx64
+- wasm32 and wasm64
+- csky
+- xtensa
+- hexagon
+- sparc64
+- mips


It is based on the list of stable architectures for asm!, I've tried to correct the names.

folkertdev · 2026-07-17T09:32:27Z

+> Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].
+
+r[items.fn.async.desugar-brief]
+A c-variadic function definition is roughly equivalent to a function operating on a [`VaList`].


i.e. it is equivalent to

// Assuming C23, so `...` as the only argument is accepted. int example(...) { va_list ap; va_start(ap); int x = va_arg(ap, int); va_end(ap); return x; }

rustbot added the S-waiting-on-review Status: The marked PR is awaiting review from a maintainer label Feb 17, 2026

folkertdev commented Feb 17, 2026

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ehuss reviewed Feb 27, 2026

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folkertdev mentioned this pull request Mar 30, 2026

report the varargs_without_pattern lint in deps rust-lang/rust#154599

Merged

folkertdev mentioned this pull request Apr 23, 2026

Stabilize c-variadic function definitions rust-lang/rust#155697

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tshepang reviewed May 2, 2026

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folkertdev and others added 5 commits May 13, 2026 14:13

add c-variadic function definitions

fe7af94

add 2 more details

4d20c3b

changes after code review

3f29b09

tweak and simplify

0807e68

Update src/items/functions.md

56c3a47

Co-authored-by: Tshepang Mbambo <hopsi@tuta.io>

folkertdev force-pushed the c-variadic branch from 00dbc77 to 56c3a47 Compare May 13, 2026 12:13

add rule for stable-targets

9880b05

ehuss reviewed May 26, 2026

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tmandry reviewed Jun 2, 2026

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Comment thread src/items/functions.md

move dyn-compat information to traits.md

a42bf50

traviscross reviewed Jun 25, 2026

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folkertdev added 2 commits June 25, 2026 20:53

add VaList to special types and traits

c3bd73c

add section on compatible types

1faf023

tgross35 reviewed Jul 10, 2026

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Comment thread src/items/functions.md

folkertdev added 2 commits July 10, 2026 15:25

fix target name casing

096b9fe

fix punctuation

66cd6b0

folkertdev commented Jul 17, 2026

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tgross35 mentioned this pull request Jul 17, 2026

Tracking issue for RFC 2137: Support defining C-compatible variadic functions in Rust (c_variadic) rust-lang/rust#44930

Open

4 tasks

		> [!WARNING]
		> Passing an unexpected number of arguments or arguments of unexpected type to a variadic function may lead to [undefined behavior][undefined].

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