support c-variadic functions in `rustc_const_eval` #150601

folkertdev · 2026-01-02T16:02:50Z

tracking issue: #44930

The new GlobalAlloc::VaList is used to create an AllocId that represents the variable argument list of a frame. The allocation itself does not store any data, all we need is the unique identifier.

The actual variable argument list is stored in Memory, and keyed by the AllocId. The Frame also stores this AllocId, so that when a frame is popped, it can deallocate the variable arguments.

At "runtime" a VaList value stores a pointer to the global allocation in its first bytes. The provenance on this pointer can be used to retrieve its AllocId, and the offset of the pointer is used to store the index of the next argument to read from the variable argument list.

Miri does not yet support va_arg, but I think that can be done separetely?

r? @RalfJung
cc @workingjubilee

rust-bors · 2026-01-04T20:20:19Z

☔ The latest upstream changes made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2026-01-05T17:34:29Z

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

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

The Miri subtree was changed

cc @rust-lang/miri

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

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

rust-bors · 2026-01-10T19:19:17Z

☔ The latest upstream changes (presumably #146923) made this pull request unmergeable. Please resolve the merge conflicts.

RalfJung · 2026-01-22T10:44:12Z

@rustbot author
Just so it is explicit when this is ready for review again :)

(I haven't yet had a chance to look at the code in detail, but given the larger structural issues it's probably better tog et those right before I do the detailed review.)

rustbot · 2026-01-22T10:44:17Z

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

compiler/rustc_const_eval/src/interpret/call.rs

compiler/rustc_const_eval/src/interpret/intrinsics.rs

RalfJung

I think I know why the provenance is being lost...

On x86, VaListInner is declared as starting with 8 integer bytes. We are storing pointer data there. When copying the VaListInner, that will drop the provenance.

What's less clear to me is where a VaList (and therefore its inner field) is being copied. But given that we expose this type to users I guess it is expected that they are allowed to copy it...

It seems like currently, all VaListInner types have a pointer field that we could use to store the pointer that the interpreter uses to represent the actual state. Not sure if that's a safe thing to bet on though...

View changes since this review

src/tools/miri/src/alloc_addresses/mod.rs

RalfJung · 2026-01-24T14:15:46Z

src/tools/miri/src/alloc_addresses/mod.rs

-                this.dedup_diagnostic(&DEDUP, |first| {
-                    NonHaltingDiagnostic::Int2Ptr { details: first }
+                this.dedup_diagnostic(&DEDUP, |first| NonHaltingDiagnostic::Int2Ptr {
+                    details: first,


Some odd reformatting happening here?

Yes, ./x fmt is happy with it though

./x fmt skips Miri files. Miri has its own rustfmt.toml so if your editor is set up incorrectly then reformatting in the editor might end up using the rustc rustfmt.toml rather than Miri's config and thus result in spurious changes.

compiler/rustc_const_eval/src/interpret/memory.rs

RalfJung · 2026-01-24T14:20:42Z

compiler/rustc_const_eval/src/interpret/stack.rs

+        self.deallocate_ptr(ptr, None, MemoryKind::Stack)?;
+
+        interp_ok(())
+    }


The corresponding allocation code for this is super far away in a different file -- that's not great.

True, though the allocation is just self.allocate. I could extract that into a method?

compiler/rustc_const_eval/src/interpret/intrinsics.rs

compiler/rustc_const_eval/src/interpret/call.rs

folkertdev

We're getting somewhere, that deref suggestion was really helpful.

View changes since this review

folkertdev · 2026-01-24T14:59:16Z

src/tools/miri/src/alloc_addresses/mod.rs

-                this.dedup_diagnostic(&DEDUP, |first| {
-                    NonHaltingDiagnostic::Int2Ptr { details: first }
+                this.dedup_diagnostic(&DEDUP, |first| NonHaltingDiagnostic::Int2Ptr {
+                    details: first,


Yes, ./x fmt is happy with it though

folkertdev · 2026-01-24T18:49:30Z

compiler/rustc_const_eval/src/interpret/stack.rs

+        self.deallocate_ptr(ptr, None, MemoryKind::Stack)?;
+
+        interp_ok(())
+    }


True, though the allocation is just self.allocate. I could extract that into a method?

folkertdev · 2026-01-24T18:52:55Z

And I'm now looking up a pointer field in the VaList, so Miri works again. I think it's fine to assume at least one field will be a pointer, if need be we can do some cfg(miri) stuff in the definition if we do run into a really weird target at some point.

compiler/rustc_const_eval/src/interpret/memory.rs

folkertdev added A-const-eval Area: Constant evaluation, covers all const contexts (static, const fn, ...) F-c_variadic `#![feature(c_variadic)]` labels Jan 2, 2026

rustbot assigned RalfJung Jan 2, 2026

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folkertdev marked this pull request as ready for review January 5, 2026 17:34

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folkertdev mentioned this pull request Jan 8, 2026

c_variadic: impl va_copy and va_end as Rust intrinsics #150436

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folkertdev added 2 commits January 22, 2026 10:37

stop using reserve_and_set_va_list_alloc

67aabb2

make va_list_map private

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remove GlobalAlloc::VaList again

83db12a

folkertdev force-pushed the c-variadic-const-eval branch from 19347f2 to 83db12a Compare January 22, 2026 11:07

add (dead) allocation bookkeeping

902fd15

folkertdev commented Jan 22, 2026

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compiler/rustc_const_eval/src/interpret/call.rs Outdated Show resolved Hide resolved

folkertdev added 2 commits January 22, 2026 15:39

zero the VaList place so it is initialized

ab52d77

add c-variadic miri test

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folkertdev commented Jan 22, 2026

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folkertdev added 2 commits January 22, 2026 22:24

WIP

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move va_list operations into InterpCx

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RalfJung reviewed Jan 24, 2026

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folkertdev added 3 commits January 24, 2026 16:17

fn addr_from_alloc_id_uncached: make va_list emit a dummy_alloc

76b68b1

use places smarter

14fa6a8

move things around

1c86e0b

folkertdev commented Jan 24, 2026

View reviewed changes

fix formatting

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RalfJung reviewed Jan 24, 2026

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compiler/rustc_const_eval/src/interpret/memory.rs Outdated Show resolved Hide resolved

folkertdev added 2 commits January 24, 2026 22:48

rename to self.va_list_ptr

666adc8

add more miri tests

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support c-variadic functions in `rustc_const_eval` #150601

support c-variadic functions in `rustc_const_eval` #150601

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