Add extra types to `VaArgSafe` by clarfonthey · Pull Request #162858 · rust-lang/rust

clarfonthey · 2026-09-16T17:09:27Z

Option 3 as proposed by @RalfJung here: #162478 (comment)

This adds the following types to VaArgSafe:

Reference equivalents to the same pointers (it's still unsafe to get next_arg)
NonNull<T> (for similar reasons)
Option<NonNull<T>> (since it's ABI-compatible)
NonZero<T> (since it's ABI-compatible)
Option<NonZero<T>> (since it's ABI-compatible)

Additionally, as part of implementing this in the compiler, NonZero is promoted from a diagnostic item to a lang item. I'm under the impression this is fine since we're already doing a lang FCP (and none of the lang folks have chimed in yet) but it's worth mentioning.

Also have a Zulip thread for the compiler impl: #t-compiler/help > `VaArgSafe` change + random `Ty` questions @ 💬

clarfonthey · 2026-09-16T17:15:28Z

@rfcbot merge libs,lang

rust-rfcbot · 2026-09-16T17:15:32Z

rustbot · 2026-09-16T17:15:38Z

r? @Darksonn

rustbot has assigned @Darksonn.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

Owners of files modified in this PR: libs
libs expanded to 12 candidates
Random selection from Darksonn, JohnTitor, Mark-Simulacrum, aapoalas, jhpratt

nia-e · 2026-09-16T17:21:37Z

The docs for VaArgSafe::next_arg currently say

Currently, all types implementing VaArgSafe also implement Copy, but this may change in the future.

this should also be adjusted, but otherwise yeah this makes sense.

@rfcbot reviewed

clarfonthey · 2026-09-16T17:24:10Z

Updated docs to remove note on everything being Copy, also added NonZero and NonNull per discussion in lang meeting.

steffahn · 2026-09-16T17:29:38Z

The safety comments of next_arg would also need to be updated to cover all the new types.

folkertdev · 2026-09-16T17:32:45Z

Can you add tests for these in tests/ui/c-variadic/roundtrip.rs?

View changes since the review

The result is very cursed but hopefully still achieves the desired result. Would appreciate an explicit review on those.

RalfJung · 2026-09-16T17:53:13Z

+#[stable(feature = "c_variadic_nonzero", since = "CURRENT_RUSTC_VERSION")]
+unsafe impl<T: ZeroablePrimitive + VaArgSafe> VaArgSafe for NonZero<T> {}
+#[stable(feature = "c_variadic_nonzero", since = "CURRENT_RUSTC_VERSION")]
+unsafe impl<T: ZeroablePrimitive + VaArgSafe> VaArgSafe for Option<NonZero<T>> {}
+
+#[stable(feature = "c_variadic_nonnull", since = "1.99.0")]
+unsafe impl<T> VaArgSafe for NonNull<T> {}
+#[stable(feature = "c_variadic_nonnull", since = "1.99.0")]
+unsafe impl<T> VaArgSafe for Option<NonNull<T>> {}
 #[stable(feature = "c_variadic", since = "1.99.0")]
 unsafe impl<T> VaArgSafe for *mut T {}
 #[stable(feature = "c_variadic", since = "1.99.0")]
 unsafe impl<T> VaArgSafe for *const T {}
+#[stable(feature = "c_variadic_refs", since = "CURRENT_RUSTC_VERSION")]
+unsafe impl<T> VaArgSafe for &mut T {}
+#[stable(feature = "c_variadic_refs", since = "CURRENT_RUSTC_VERSION")]
+unsafe impl<T> VaArgSafe for &T {}


I assume the intent is that e.g. NonNull<T> is compatible with *mut T. That will need adjustments in the docs and corresponding adjustments in the const-eval / Miri checking logic.

View changes since the review

I thought that we already had this guarantee, but if we don't, I would rather not overcomplicate this PR and can remove it. (Note: this only needs to hold for sized pointees.)

Rust guarantees they have the same layout, so it's just the docs of next_arg that should document that this equivalence applies here.

Same layout != same ABI.
But we also guarantee that they have the same ABI.

This applies to most of your additions, e.g. NonZero<i32> vs i32.

So, looking at the docs:

Unlike *mut T, the pointer must always be non-null, even if the pointer is never dereferenced. This is so that enums may use this forbidden value as a discriminant – Option<NonNull> has the same size as *mut T. However the pointer may still dangle if it isn’t dereferenced.

That feels pretty much like an explicit guarantee that *mut T, NonNull<T>, and Option<NonNull<T>> are layout-compatible, but I guess we should probably make it more explicit in the reference.

From my understanding, variadic arguments simply require layout compatibility, since for example i32 and u32 are interchangeable in them.

If only :)

The comment on next_arg goes into detail on what it means for types to be compatible. It is much more subtle than layout compatibility. E.g. i32 and u32 are only compatible if the runtime value is representable in both types. (so like 3 is compatible but -3 is not.

Oh, yes, that is cursed.

Morally the same is true for the nonzero types and references (which cannot be NULL), but those requirements are already captured by the type itself in these new cases.

The docs very explicitly say

The actual type of the argument U is compatible with T (as defined below).

So without changing the docs, if the caller uses NonZeroI32 and the callee uses next_arg::<i32>(), we very clearly have UB. (NonZeroI32 is not an "integer type". That term refers to primitive types, iN and uN.)

Yes, I did explicitly go through and update the docs for those cases: the general rule for those is that if not all bit patterns are valid, you are guaranteeing that you have passed a valid bit pattern, at risk of UB.

theemathas · 2026-09-17T11:17:20Z

That's at https://std-dev-guide.rust-lang.org/development/stabilization.html#stabilization-report

theemathas · 2026-09-17T15:43:27Z

PR for destabilizing VaArgSafe: #162909

rustbot · 2026-09-18T14:28:44Z

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

RalfJung

CTFE code looks good, thanks. :)
I think long-term it may make sense to harmonize this with the usual ABI compatibility rules but we can take baby steps here.

View changes since this review

clarfonthey · 2026-09-20T13:36:58Z

Because I'm feeling particularly spicy, is there a reason we shouldn't support char and NonZero<char> as well?

Since we're already allowing invalid bit patterns, might as well.

steffahn · 2026-09-20T14:14:48Z

char is not even considered FFI safe to begin with

warning: `extern` fn uses type `char`, which is not FFI-safe
 --> src/lib.rs:3:20
  |
1 | extern "C" fn f(_: char) {} 
  |                    ^^^^ not FFI-safe
  |
  = help: consider using `u32` or `libc::wchar_t` instead
  = note: the `char` type has no C equivalent
  = note: `#[warn(improper_ctypes_definitions)]` on by default

clarfonthey · 2026-09-20T14:16:00Z

Never mind, then. That makes that easy.

rustbot · 2026-09-20T14:26:01Z

This PR was rebased onto a different main 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.

RalfJung

Congrats, you managed to achieve what many thought was impossible -- you made me think "that feels almost like there's too many comments here now." ;)

(Not really, it's fine, just more than I wold have written.)

Also when comments are full sentences please end them in a period. Too often have I seen people add more sentences to such comments without adding the periods which makes for extremely confusing times.

View changes since this review

RalfJung · 2026-09-20T16:48:20Z

+                }
+
+                match self.tcx.tcx.as_lang_item(adt.did()) {
+                    // `NonNull` is allowed as just a pointer with fewer an alid values


Suggested change

// `NonNull` is allowed as just a pointer with fewer an alid values

// `NonNull` is allowed as just a pointer with fewer valid values.

I assume?

RalfJung · 2026-09-20T16:49:22Z

-        if self.layout_of(caller_type)?.size != self.layout_of(callee_type)?.size {
+        // All other cases require that the layout of the types match;
+        // in practice, since the only types that could get to this point are integers and pointers,
+        // the alignment check isn't necessary, but might as well verify


Suggested change

// the alignment check isn't necessary, but might as well verify

// the alignment check isn't necessary, but might as well verify.

RalfJung · 2026-09-20T16:49:28Z

-        let is_c_char = |ty: Ty<'_>| matches!(ty.kind(), ty::Uint(UintTy::U8) | ty::Int(IntTy::I8));
+        // Since we've already checked for identical types and have narrowed down the layouts as
+        // being identical, we can filter out any non-integer, non-pointer types and similarly
+        // normalize integer and pointer types to make matching easier


Suggested change

// normalize integer and pointer types to make matching easier

// normalize integer and pointer types to make matching easier.

RalfJung · 2026-09-20T16:50:27Z

+                if (caller_target_ty.is_c_void(self.tcx.tcx)
+                    || callee_target_ty.is_c_void(self.tcx.tcx))
+                    && (caller_target_ty.is_byte_sized_integral()
+                        || callee_target_ty.is_byte_sized_integral())


I think the version with the && inside and the || outside would be easier to match up with the text from the C standard that we are quoting above.

Fair; I mostly just wanted to cover the case where the types were the same but technically under this condition without having to do any recursion.

RalfJung · 2026-09-20T16:50:51Z

+                // Note: we already know that the layout is identical
+                if caller_signed == callee_signed {
+                    // So, if the signedness is the same, this means that one of the types is
+                    // `usize` or `isize`, which have the same ABI as their same-layout counterparts


Suggested change

// `usize` or `isize`, which have the same ABI as their same-layout counterparts

// `usize` or `isize`, which have the same ABI as their same-layout counterparts.

RalfJung · 2026-09-20T16:50:57Z

+                    interp_ok(VarArgCompatible::Compatible)
+                } else {
+                    // And if the signedness is different, this means that we're casting by changing
+                    // the sign but not the size of the integer type, which is fine


Suggested change

// the sign but not the size of the integer type, which is fine

// the sign but not the size of the integer type, which is fine.

RalfJung · 2026-09-20T16:51:19Z

            // - "or, the type of the next argument is nullptr_t and type is a pointer type that has the same
            //   representation and alignment requirements as a pointer to a character type"
            //   This one does not have an equivalent form in Rust.
-            _ => interp_ok(VarArgCompatible::Incompatible),


The fallback arm still exists, you just reordered it wrt the comment?

Yeah, fair. I guess that my thought process was that the branch was covering more before, but, not really.

rust-log-analyzer · 2026-09-20T17:29:55Z

The job test-aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

failures:

---- [ui] tests/ui/c-variadic/roundtrip.rs stdout ----

error: test did not exit with success! code=None so test would pass with `run-crash`
status: signal: 6 (SIGABRT) (core dumped)
command: cd "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/c-variadic/roundtrip" && RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" RUST_TEST_THREADS="4" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/c-variadic/roundtrip/a"
stdout: none
--- stderr -------------------------------

thread 'main' (40062) panicked at /checkout/tests/ui/c-variadic/roundtrip.rs:33:5:
assertion failed: ap.next_arg::<u32>() == 0xAAAA_AAAA
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

thread 'main' (40062) panicked at library/core/src/panicking.rs:225:5:
panic in a function that cannot unwind
stack backtrace:
---
   4:     0xff746cd8c950 - std[ed984737de44978e]::panicking::default_hook
   5:     0xff746cd8cdc4 - std[ed984737de44978e]::panicking::panic_with_hook
   6:     0xff746cd65624 - std[ed984737de44978e]::panicking::panic_handler::{closure#0}
   7:     0xff746cd585dc - std[ed984737de44978e]::sys::backtrace::__rust_end_short_backtrace::<std[ed984737de44978e]::panicking::panic_handler::{closure#0}, !>
   8:     0xff746cd67444 - __rustc[58b66a6159f8e6fd]::rust_begin_unwind
   9:     0xff746cd48770 - core[60c3db2fddfd9e0]::panicking::panic_nounwind_fmt
  10:     0xff746cd48684 - core[60c3db2fddfd9e0]::panicking::panic_nounwind
  11:     0xff746cd48850 - core[60c3db2fddfd9e0]::panicking::panic_cannot_unwind
  12:     0xab99e1e328f4 - roundtrip[2ae0f7d716a9c1b5]::variadic::<core[60c3db2fddfd9e0]::option::Option<core[60c3db2fddfd9e0]::num::nonzero::NonZero<i32>>>
  13:     0xab99e1e332b8 - roundtrip[2ae0f7d716a9c1b5]::main
  14:     0xab99e1e33224 - std[ed984737de44978e]::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
  15:     0xab99e1e3323c - std[ed984737de44978e]::rt::lang_start::<()>::{closure#0}
  16:     0xff746cd8ae1c - std[ed984737de44978e]::rt::lang_start_internal
  17:     0xab99e1e35334 - main
  18:     0xff746cb1259c - <unknown>
---

978    |
979    = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
980 
- error[E0080]: constructing invalid value of type [u8; 24]: at [8], encountered a pointer, but expected an integer
+ error[E0080]: constructing invalid value of type [u8; 32]: at [0], encountered a pointer, but expected an integer
982   --> $DIR/c-variadic-fail.rs:202:22
983    |
984 LL |             let ap = helper(1, 2, 3);


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args consts/const-eval/c-variadic-fail.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/consts/const-eval/c-variadic-fail.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/consts/const-eval/c-variadic-fail" "-Znext-solver=coherence" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Zextra-const-ub-checks"
stdout: none
--- stderr -------------------------------
error[E0080]: more C-variadic arguments read than were passed
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:30:13
   |
LL |     const { read_n::<1>() }
   |             ^^^^^^^^^^^^^ evaluation of `read_too_many::{constant#2}` failed inside this call
   |
note: inside `read_n::<1>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:18:17
   |
LL |         let _ = ap.next_arg::<i32>();
   |                 ^^^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:30:5
   |
LL |     const { read_n::<1>() }
   |     ^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:30:5
   |
LL |     const { read_n::<1>() }
   |     ^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: more C-variadic arguments read than were passed
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:34:13
   |
LL |     const { read_n::<2>(1) }
   |             ^^^^^^^^^^^^^^ evaluation of `read_too_many::{constant#3}` failed inside this call
   |
note: inside `read_n::<2>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:18:17
   |
LL |         let _ = ap.next_arg::<i32>();
   |                 ^^^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:34:5
   |
LL |     const { read_n::<2>(1) }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:34:5
   |
LL |     const { read_n::<2>(1) }
   |     ^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg value mismatch: value `-1_i32` cannot be represented by type `u32`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:74:13
   |
LL |     const { read_as::<u32>(-1i32) };
   |             ^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#12}` failed inside this call
   |
note: inside `read_as::<u32>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<u32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:74:5
   |
LL |     const { read_as::<u32>(-1i32) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:74:5
   |
LL |     const { read_as::<u32>(-1i32) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg value mismatch: value `-2147483648_i32` cannot be represented by type `u32`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:76:13
   |
LL |     const { read_as::<u32>(i32::MIN) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#13}` failed inside this call
   |
note: inside `read_as::<u32>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<u32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:76:5
   |
LL |     const { read_as::<u32>(i32::MIN) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:76:5
   |
LL |     const { read_as::<u32>(i32::MIN) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg value mismatch: value `4294967295_u32` cannot be represented by type `i32`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:78:13
   |
LL |     const { read_as::<i32>(u32::MAX) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#14}` failed inside this call
   |
note: inside `read_as::<i32>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:78:5
   |
LL |     const { read_as::<i32>(u32::MAX) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:78:5
   |
LL |     const { read_as::<i32>(u32::MAX) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg value mismatch: value `2147483648_u32` cannot be represented by type `i32`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:80:13
   |
LL |     const { read_as::<i32>(i32::MAX as u32 + 1) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#15}` failed inside this call
   |
note: inside `read_as::<i32>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:80:5
   |
LL |     const { read_as::<i32>(i32::MAX as u32 + 1) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:80:5
   |
LL |     const { read_as::<i32>(i32::MAX as u32 + 1) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg value mismatch: value `18446744073709551615_u64` cannot be represented by type `i64`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:82:13
   |
LL |     const { read_as::<i64>(u64::MAX) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#16}` failed inside this call
   |
note: inside `read_as::<i64>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i64>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:82:5
   |
LL |     const { read_as::<i64>(u64::MAX) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:82:5
   |
LL |     const { read_as::<i64>(u64::MAX) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg value mismatch: value `9223372036854775808_u64` cannot be represented by type `i64`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:84:13
   |
LL |     const { read_as::<i64>(i64::MAX as u64 + 1) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#17}` failed inside this call
   |
note: inside `read_as::<i64>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i64>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:84:5
   |
LL |     const { read_as::<i64>(i64::MAX as u64 + 1) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:84:5
   |
LL |     const { read_as::<i64>(i64::MAX as u64 + 1) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg type mismatch: requested `i32` is incompatible with next argument of type `u64`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:87:13
   |
LL |     const { read_as::<i32>(1u64) };
   |             ^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#18}` failed inside this call
   |
note: inside `read_as::<i32>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<i32>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:87:5
   |
LL |     const { read_as::<i32>(1u64) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:87:5
   |
LL |     const { read_as::<i32>(1u64) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg type mismatch: requested `f64` is incompatible with next argument of type `i32`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:90:13
   |
LL |     const { read_as::<f64>(1i32) };
   |             ^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_numeric::{constant#19}` failed inside this call
   |
note: inside `read_as::<f64>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<f64>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:90:5
   |
LL |     const { read_as::<f64>(1i32) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:90:5
   |
LL |     const { read_as::<f64>(1i32) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg type mismatch: requested `*const u16` is incompatible with next argument of type `*const c_void`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:113:13
   |
LL |     const { read_as::<*const u16>(ptr::dangling::<c_void>()) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_pointer::{constant#12}` failed inside this call
   |
note: inside `read_as::<*const u16>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<*const u16>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:113:5
   |
LL |     const { read_as::<*const u16>(ptr::dangling::<c_void>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:113:5
   |
LL |     const { read_as::<*const u16>(ptr::dangling::<c_void>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg type mismatch: requested `*const c_void` is incompatible with next argument of type `*const u16`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:115:13
   |
LL |     const { read_as::<*const c_void>(ptr::dangling::<u16>()) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_pointer::{constant#13}` failed inside this call
   |
note: inside `read_as::<*const c_void>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<*const c_void>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:115:5
   |
LL |     const { read_as::<*const c_void>(ptr::dangling::<u16>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:115:5
   |
LL |     const { read_as::<*const c_void>(ptr::dangling::<u16>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg type mismatch: requested `*const u16` is incompatible with next argument of type `*const i32`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:117:13
   |
LL |     const { read_as::<*const u16>(ptr::dangling::<i32>()) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_pointer::{constant#14}` failed inside this call
   |
note: inside `read_as::<*const u16>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<*const u16>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:117:5
   |
LL |     const { read_as::<*const u16>(ptr::dangling::<i32>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:117:5
   |
LL |     const { read_as::<*const u16>(ptr::dangling::<i32>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: va_arg type mismatch: requested `*const u8` is incompatible with next argument of type `usize`
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:120:13
   |
LL |     const { read_as::<*const u8>(1usize) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_pointer::{constant#15}` failed inside this call
   |
note: inside `read_as::<*const u8>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<*const u8>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:120:5
   |
LL |     const { read_as::<*const u8>(1usize) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:120:5
   |
LL |     const { read_as::<*const u8>(1usize) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: constructing invalid value of type NonNull<c_void>: at .pointer, encountered 0, but expected something greater or equal to 1
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:125:13
   |
LL |     const { read_as::<NonNull<c_void>>(ptr::null::<c_void>()) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_null::{constant#0}` failed inside this call
   |
note: inside `read_as::<NonNull<c_void>>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<NonNull<c_void>>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:125:5
   |
LL |     const { read_as::<NonNull<c_void>>(ptr::null::<c_void>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:125:5
   |
LL |     const { read_as::<NonNull<c_void>>(ptr::null::<c_void>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: constructing invalid value of type NonNull<c_void>: at .pointer, encountered 0, but expected something greater or equal to 1
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:127:13
   |
LL |     const { read_as::<NonNull<c_void>>(ptr::null_mut::<c_void>()) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_null::{constant#1}` failed inside this call
   |
note: inside `read_as::<NonNull<c_void>>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<NonNull<c_void>>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:127:5
   |
LL |     const { read_as::<NonNull<c_void>>(ptr::null_mut::<c_void>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:127:5
   |
LL |     const { read_as::<NonNull<c_void>>(ptr::null_mut::<c_void>()) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: constructing invalid value of type NonNull<c_void>: at .pointer, encountered 0, but expected something greater or equal to 1
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:129:13
   |
LL |     const { read_as::<NonNull<c_void>>(None::<NonNull<c_void>>) };
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast_null::{constant#2}` failed inside this call
   |
note: inside `read_as::<NonNull<c_void>>`
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:39:5
   |
LL |     ap.next_arg::<T>()
   |     ^^^^^^^^^^^^^^^^^^
note: inside `VaList::<'_>::next_arg::<NonNull<c_void>>`
  --> /rustc/FAKE_PREFIX/library/core/src/ffi/va_list.rs:544:17

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:129:5
   |
LL |     const { read_as::<NonNull<c_void>>(None::<NonNull<c_void>>) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

note: erroneous constant encountered
  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:129:5
   |
LL |     const { read_as::<NonNull<c_void>>(None::<NonNull<c_void>>) };
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`

error[E0080]: constructing invalid value of type NonNull<c_void>: at .pointer, encountered 0, but expected something greater or equal to 1
##[error]  --> /checkout/tests/ui/consts/const-eval/c-variadic-fail.rs:131:13
   |
LL |     const { read_as::<NonNull<c_void>>(None::<NonNull<c_void>>) };
---

---- [ui] tests/ui/consts/const-eval/c-variadic-fail.rs stdout end ----
---- [ui] tests/ui/consts/const-eval/c-variadic.rs stdout ----

error: test did not exit with success! code=None so test would pass with `run-crash`
status: signal: 11 (SIGSEGV) (core dumped)
command: cd "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/consts/const-eval/c-variadic" && RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" RUST_TEST_THREADS="4" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/consts/const-eval/c-variadic/a"
stdout: none
stderr: none

---- [ui] tests/ui/consts/const-eval/c-variadic.rs stdout end ----

Mark-Simulacrum · 2026-09-20T20:22:50Z

 /// - [`c_int`], [`c_long`] and [`c_longlong`]
 /// - [`c_uint`], [`c_ulong`] and [`c_ulonglong`]
 /// - [`c_double`]
 /// - `*const T` and `*mut T`


We should probably expand this with the new types we guarantee impls for.

View changes since the review

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Sep 16, 2026

clarfonthey added the T-lang Relevant to the language team label Sep 16, 2026

rust-rfcbot added the proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. label Sep 16, 2026

clarfonthey marked this pull request as ready for review September 16, 2026 17:15

rust-rfcbot added the disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. label Sep 16, 2026

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 16, 2026

rustbot assigned Darksonn Sep 16, 2026

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 16, 2026

clarfonthey force-pushed the va-arg-refs branch from 006eb59 to 8007a9d Compare September 16, 2026 17:18

clarfonthey changed the title ~~Add thin references and pointers to VaArgSafe~~ Add extra types to VaArgSafe Sep 16, 2026

clarfonthey force-pushed the va-arg-refs branch 2 times, most recently from b020d39 to f84f398 Compare September 16, 2026 17:24

clarfonthey force-pushed the va-arg-refs branch from f84f398 to 7de1f76 Compare September 16, 2026 17:26

clarfonthey added the I-lang-nominated Nominated for discussion during a lang team meeting. label Sep 16, 2026

traviscross added P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang I-lang-radar Items that are on lang's radar and will need eventual work or consideration. labels Sep 16, 2026

folkertdev reviewed Sep 16, 2026

View reviewed changes