Rollup merge of #150601 - folkertdev:c-variadic-const-eval, r=RalfJung

support c-variadic functions in `rustc_const_eval`

tracking issue: https://github.com/rust-lang/rust/issues/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

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -698,13 +698,11 @@ impl<'a> AstValidator<'a> {
             unreachable!("C variable argument list cannot be used in closures")
         };
 
-        // C-variadics are not yet implemented in const evaluation.
-        if let Const::Yes(const_span) = sig.header.constness {
-            self.dcx().emit_err(errors::ConstAndCVariadic {
-                spans: vec![const_span, variadic_param.span],
-                const_span,
-                variadic_span: variadic_param.span,
-            });
+        if let Const::Yes(_) = sig.header.constness
+            && !self.features.enabled(sym::const_c_variadic)
+        {
+            let msg = format!("c-variadic const function definitions are unstable");
+            feature_err(&self.sess, sym::const_c_variadic, sig.span, msg).emit();
         }
 
         if let Some(coroutine_kind) = sig.header.coroutine_kind {

compiler/rustc_ast_passes/src/errors.rs

@@ -823,17 +823,6 @@ pub(crate) struct ConstAndCoroutine {
     pub coroutine_kind: &'static str,
 }
 
-#[derive(Diagnostic)]
-#[diag("functions cannot be both `const` and C-variadic")]
-pub(crate) struct ConstAndCVariadic {
-    #[primary_span]
-    pub spans: Vec<Span>,
-    #[label("`const` because of this")]
-    pub const_span: Span,
-    #[label("C-variadic because of this")]
-    pub variadic_span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag("functions cannot be both `{$coroutine_kind}` and C-variadic")]
 pub(crate) struct CoroutineAndCVariadic {

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -815,6 +815,10 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     });
                 }
 
+                if self.tcx.fn_sig(callee).skip_binder().c_variadic() {
+                    self.check_op(ops::FnCallCVariadic)
+                }
+
                 // At this point, we are calling a function, `callee`, whose `DefId` is known...
 
                 // `begin_panic` and `panic_display` functions accept generic

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -75,6 +75,27 @@ impl<'tcx> NonConstOp<'tcx> for FnCallIndirect {
     }
 }
 
+/// A c-variadic function call.
+#[derive(Debug)]
+pub(crate) struct FnCallCVariadic;
+impl<'tcx> NonConstOp<'tcx> for FnCallCVariadic {
+    fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
+        Status::Unstable {
+            gate: sym::const_c_variadic,
+            gate_already_checked: false,
+            safe_to_expose_on_stable: false,
+            is_function_call: true,
+        }
+    }
+
+    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
+        ccx.tcx.sess.create_feature_err(
+            errors::NonConstCVariadicCall { span, kind: ccx.const_kind() },
+            sym::const_c_variadic,
+        )
+    }
+}
+
 /// A call to a function that is in a trait, or has trait bounds that make it conditionally-const.
 #[derive(Debug)]
 pub(crate) struct ConditionallyConstCall<'tcx> {

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -9,7 +9,7 @@ use rustc_errors::msg;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{self as hir, CRATE_HIR_ID, LangItem};
 use rustc_middle::mir::AssertMessage;
-use rustc_middle::mir::interpret::{Pointer, ReportedErrorInfo};
+use rustc_middle::mir::interpret::ReportedErrorInfo;
 use rustc_middle::query::TyCtxtAt;
 use rustc_middle::ty::layout::{HasTypingEnv, TyAndLayout, ValidityRequirement};
 use rustc_middle::ty::{self, Ty, TyCtxt};
@@ -22,7 +22,7 @@ use super::error::*;
 use crate::errors::{LongRunning, LongRunningWarn};
 use crate::interpret::{
     self, AllocId, AllocInit, AllocRange, ConstAllocation, CtfeProvenance, FnArg, Frame,
-    GlobalAlloc, ImmTy, InterpCx, InterpResult, OpTy, PlaceTy, RangeSet, Scalar,
+    GlobalAlloc, ImmTy, InterpCx, InterpResult, OpTy, PlaceTy, Pointer, RangeSet, Scalar,
     compile_time_machine, err_inval, interp_ok, throw_exhaust, throw_inval, throw_ub,
     throw_ub_custom, throw_unsup, throw_unsup_format,
 };

compiler/rustc_const_eval/src/errors.rs

@@ -531,6 +531,19 @@ pub struct NonConstClosure {
     pub non_or_conditionally: &'static str,
 }
 
+#[derive(Diagnostic)]
+#[diag(r#"calling const c-variadic functions is unstable in {$kind ->
+    [const] constant
+    [static] static
+    [const_fn] constant function
+    *[other] {""}
+}s"#, code = E0015)]
+pub struct NonConstCVariadicCall {
+    #[primary_span]
+    pub span: Span,
+    pub kind: ConstContext,
+}
+
 #[derive(Subdiagnostic)]
 pub enum NonConstClosureNote {
     #[note("function defined here, but it is not `const`")]
@@ -757,11 +770,13 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             WriteToReadOnly(_) => msg!("writing to {$allocation} which is read-only"),
             DerefFunctionPointer(_) => msg!("accessing {$allocation} which contains a function"),
             DerefVTablePointer(_) => msg!("accessing {$allocation} which contains a vtable"),
+            DerefVaListPointer(_) => msg!("accessing {$allocation} which contains a variable argument list"),
             DerefTypeIdPointer(_) => msg!("accessing {$allocation} which contains a `TypeId`"),
             InvalidBool(_) => msg!("interpreting an invalid 8-bit value as a bool: 0x{$value}"),
             InvalidChar(_) => msg!("interpreting an invalid 32-bit value as a char: 0x{$value}"),
             InvalidTag(_) => msg!("enum value has invalid tag: {$tag}"),
             InvalidFunctionPointer(_) => msg!("using {$pointer} as function pointer but it does not point to a function"),
+            InvalidVaListPointer(_) => msg!("using {$pointer} as variable argument list pointer but it does not point to a variable argument list"),
             InvalidVTablePointer(_) => msg!("using {$pointer} as vtable pointer but it does not point to a vtable"),
             InvalidVTableTrait { .. } => msg!("using vtable for `{$vtable_dyn_type}` but `{$expected_dyn_type}` was expected"),
             InvalidStr(_) => msg!("this string is not valid UTF-8: {$err}"),
@@ -776,6 +791,9 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             }
             AbiMismatchArgument { .. } => msg!("calling a function whose parameter #{$arg_idx} has type {$callee_ty} passing argument of type {$caller_ty}"),
             AbiMismatchReturn { .. } => msg!("calling a function with return type {$callee_ty} passing return place of type {$caller_ty}"),
+            VaArgOutOfBounds => "more C-variadic arguments read than were passed".into(),
+            CVariadicMismatch { ..} => "calling a function where the caller and callee disagree on whether the function is C-variadic".into(),
+            CVariadicFixedCountMismatch { .. } => msg!("calling a C-variadic function with {$caller} fixed arguments, but the function expects {$callee}"),
         }
     }
 
@@ -800,6 +818,7 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             | InvalidMeta(InvalidMetaKind::TooBig)
             | InvalidUninitBytes(None)
             | DeadLocal
+            | VaArgOutOfBounds
             | UninhabitedEnumVariantWritten(_)
             | UninhabitedEnumVariantRead(_) => {}
 
@@ -820,7 +839,10 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
                 diag.arg("len", len);
                 diag.arg("index", index);
             }
-            UnterminatedCString(ptr) | InvalidFunctionPointer(ptr) | InvalidVTablePointer(ptr) => {
+            UnterminatedCString(ptr)
+            | InvalidFunctionPointer(ptr)
+            | InvalidVaListPointer(ptr)
+            | InvalidVTablePointer(ptr) => {
                 diag.arg("pointer", ptr);
             }
             InvalidVTableTrait { expected_dyn_type, vtable_dyn_type } => {
@@ -874,6 +896,7 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
             WriteToReadOnly(alloc)
             | DerefFunctionPointer(alloc)
             | DerefVTablePointer(alloc)
+            | DerefVaListPointer(alloc)
             | DerefTypeIdPointer(alloc) => {
                 diag.arg("allocation", alloc);
             }
@@ -910,6 +933,14 @@ impl<'a> ReportErrorExt for UndefinedBehaviorInfo<'a> {
                 diag.arg("caller_ty", caller_ty);
                 diag.arg("callee_ty", callee_ty);
             }
+            CVariadicMismatch { caller_is_c_variadic, callee_is_c_variadic } => {
+                diag.arg("caller_is_c_variadic", caller_is_c_variadic);
+                diag.arg("callee_is_c_variadic", callee_is_c_variadic);
+            }
+            CVariadicFixedCountMismatch { caller, callee } => {
+                diag.arg("caller", caller);
+                diag.arg("callee", callee);
+            }
         }
     }
 }

compiler/rustc_const_eval/src/interpret/call.rs

@@ -19,7 +19,7 @@ use tracing::{info, instrument, trace};
 use super::{
     CtfeProvenance, FnVal, ImmTy, InterpCx, InterpResult, MPlaceTy, Machine, OpTy, PlaceTy,
     Projectable, Provenance, ReturnAction, ReturnContinuation, Scalar, StackPopInfo, interp_ok,
-    throw_ub, throw_ub_custom, throw_unsup_format,
+    throw_ub, throw_ub_custom,
 };
 use crate::enter_trace_span;
 use crate::interpret::EnteredTraceSpan;
@@ -354,13 +354,18 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     ) -> InterpResult<'tcx> {
         let _trace = enter_trace_span!(M, step::init_stack_frame, %instance, tracing_separate_thread = Empty);
 
-        // Compute callee information.
-        // FIXME: for variadic support, do we have to somehow determine callee's extra_args?
-        let callee_fn_abi = self.fn_abi_of_instance(instance, ty::List::empty())?;
-
-        if callee_fn_abi.c_variadic || caller_fn_abi.c_variadic {
-            throw_unsup_format!("calling a c-variadic function is not supported");
-        }
+        // The first order of business is to figure out the callee signature.
+        // However, that requires the list of variadic arguments.
+        // We use the *caller* information to determine where to split the list of arguments,
+        // and then later check that the callee indeed has the same number of fixed arguments.
+        let extra_tys = if caller_fn_abi.c_variadic {
+            let fixed_count = usize::try_from(caller_fn_abi.fixed_count).unwrap();
+            let extra_tys = args[fixed_count..].iter().map(|arg| arg.layout().ty);
+            self.tcx.mk_type_list_from_iter(extra_tys)
+        } else {
+            ty::List::empty()
+        };
+        let callee_fn_abi = self.fn_abi_of_instance(instance, extra_tys)?;
 
         if caller_fn_abi.conv != callee_fn_abi.conv {
             throw_ub_custom!(
@@ -372,6 +377,19 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             )
         }
 
+        if caller_fn_abi.c_variadic != callee_fn_abi.c_variadic {
+            throw_ub!(CVariadicMismatch {
+                caller_is_c_variadic: caller_fn_abi.c_variadic,
+                callee_is_c_variadic: callee_fn_abi.c_variadic,
+            });
+        }
+        if caller_fn_abi.c_variadic && caller_fn_abi.fixed_count != callee_fn_abi.fixed_count {
+            throw_ub!(CVariadicFixedCountMismatch {
+                caller: caller_fn_abi.fixed_count,
+                callee: callee_fn_abi.fixed_count,
+            });
+        }
+
         // Check that all target features required by the callee (i.e., from
         // the attribute `#[target_feature(enable = ...)]`) are enabled at
         // compile time.
@@ -444,6 +462,10 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             // `pass_argument` would be the loop body. It takes care to
             // not advance `caller_iter` for ignored arguments.
             let mut callee_args_abis = callee_fn_abi.args.iter().enumerate();
+            // Determine whether there is a special VaList argument. This is always the
+            // last argument, and since arguments start at index 1 that's `arg_count`.
+            let va_list_arg =
+                callee_fn_abi.c_variadic.then(|| mir::Local::from_usize(body.arg_count));
             for local in body.args_iter() {
                 // Construct the destination place for this argument. At this point all
                 // locals are still dead, so we cannot construct a `PlaceTy`.
@@ -452,7 +474,31 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 // type, but the result gets cached so this avoids calling the instantiation
                 // query *again* the next time this local is accessed.
                 let ty = self.layout_of_local(self.frame(), local, None)?.ty;
-                if Some(local) == body.spread_arg {
+                if Some(local) == va_list_arg {
+                    // This is the last callee-side argument of a variadic function.
+                    // This argument is a VaList holding the remaining caller-side arguments.
+                    self.storage_live(local)?;
+
+                    let place = self.eval_place(dest)?;
+                    let mplace = self.force_allocation(&place)?;
+
+                    // Consume the remaining arguments by putting them into the variable argument
+                    // list.
+                    let varargs = self.allocate_varargs(&mut caller_args, &mut callee_args_abis)?;
+                    // When the frame is dropped, these variable arguments are deallocated.
+                    self.frame_mut().va_list = varargs.clone();
+                    let key = self.va_list_ptr(varargs.into());
+
+                    // Zero the VaList, so it is fully initialized.
+                    self.write_bytes_ptr(
+                        mplace.ptr(),
+                        (0..mplace.layout.size.bytes()).map(|_| 0u8),
+                    )?;
+
+                    // Store the "key" pointer in the right field.
+                    let key_mplace = self.va_list_key_field(&mplace)?;
+                    self.write_pointer(key, &key_mplace)?;
+                } else if Some(local) == body.spread_arg {
                     // Make the local live once, then fill in the value field by field.
                     self.storage_live(local)?;
                     // Must be a tuple
@@ -491,7 +537,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             if instance.def.requires_caller_location(*self.tcx) {
                 callee_args_abis.next().unwrap();
             }
-            // Now we should have no more caller args or callee arg ABIs
+            // Now we should have no more caller args or callee arg ABIs.
             assert!(
                 callee_args_abis.next().is_none(),
                 "mismatch between callee ABI and callee body arguments"

compiler/rustc_const_eval/src/interpret/intrinsics.rs

@@ -23,8 +23,8 @@ use super::memory::MemoryKind;
 use super::util::ensure_monomorphic_enough;
 use super::{
     AllocId, CheckInAllocMsg, ImmTy, InterpCx, InterpResult, Machine, OpTy, PlaceTy, Pointer,
-    PointerArithmetic, Provenance, Scalar, err_ub_custom, err_unsup_format, interp_ok, throw_inval,
-    throw_ub_custom, throw_ub_format,
+    PointerArithmetic, Projectable, Provenance, Scalar, err_ub_custom, err_unsup_format, interp_ok,
+    throw_inval, throw_ub, throw_ub_custom, throw_ub_format, throw_unsup_format,
 };
 use crate::interpret::Writeable;
 
@@ -750,6 +750,57 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 self.float_muladd_intrinsic::<Quad>(args, dest, MulAddType::Nondeterministic)?
             }
 
+            sym::va_copy => {
+                let va_list = self.deref_pointer(&args[0])?;
+                let key_mplace = self.va_list_key_field(&va_list)?;
+                let key = self.read_pointer(&key_mplace)?;
+
+                let varargs = self.get_ptr_va_list(key)?;
+                let copy_key = self.va_list_ptr(varargs.clone());
+
+                let copy_key_mplace = self.va_list_key_field(dest)?;
+                self.write_pointer(copy_key, &copy_key_mplace)?;
+            }
+
+            sym::va_end => {
+                let va_list = self.deref_pointer(&args[0])?;
+                let key_mplace = self.va_list_key_field(&va_list)?;
+                let key = self.read_pointer(&key_mplace)?;
+
+                self.deallocate_va_list(key)?;
+            }
+
+            sym::va_arg => {
+                let va_list = self.deref_pointer(&args[0])?;
+                let key_mplace = self.va_list_key_field(&va_list)?;
+                let key = self.read_pointer(&key_mplace)?;
+
+                // Invalidate the old list and get its content. We'll recreate the
+                // new list (one element shorter) below.
+                let mut varargs = self.deallocate_va_list(key)?;
+
+                let Some(arg_mplace) = varargs.pop_front() else {
+                    throw_ub!(VaArgOutOfBounds);
+                };
+
+                // NOTE: In C some type conversions are allowed (e.g. casting between signed and
+                // unsigned integers). For now we require c-variadic arguments to be read with the
+                // exact type they were passed as.
+                if arg_mplace.layout.ty != dest.layout.ty {
+                    throw_unsup_format!(
+                        "va_arg type mismatch: requested `{}`, but next argument is `{}`",
+                        dest.layout.ty,
+                        arg_mplace.layout.ty
+                    );
+                }
+                // Copy the argument.
+                self.copy_op(&arg_mplace, dest)?;
+
+                // Update the VaList pointer.
+                let new_key = self.va_list_ptr(varargs);
+                self.write_pointer(new_key, &key_mplace)?;
+            }
+
             // Unsupported intrinsic: skip the return_to_block below.
             _ => return interp_ok(false),
         }
@@ -1230,4 +1281,26 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             interp_ok(Some(ImmTy::from_scalar(val, cast_to)))
         }
     }
+
+    /// Get the MPlace of the key from the place storing the VaList.
+    pub(super) fn va_list_key_field<P: Projectable<'tcx, M::Provenance>>(
+        &self,
+        va_list: &P,
+    ) -> InterpResult<'tcx, P> {
+        // The struct wrapped by VaList.
+        let va_list_inner = self.project_field(va_list, FieldIdx::ZERO)?;
+
+        // Find the first pointer field in this struct. The exact index is target-specific.
+        let ty::Adt(adt, substs) = va_list_inner.layout().ty.kind() else {
+            bug!("invalid VaListImpl layout");
+        };
+
+        for (i, field) in adt.non_enum_variant().fields.iter().enumerate() {
+            if field.ty(*self.tcx, substs).is_raw_ptr() {
+                return self.project_field(&va_list_inner, FieldIdx::from_usize(i));
+            }
+        }
+
+        bug!("no VaListImpl field is a pointer");
+    }
 }

compiler/rustc_const_eval/src/interpret/memory.rs

@@ -23,8 +23,8 @@ use tracing::{debug, instrument, trace};
 
 use super::{
     AllocBytes, AllocId, AllocInit, AllocMap, AllocRange, Allocation, CheckAlignMsg,
-    CheckInAllocMsg, CtfeProvenance, GlobalAlloc, InterpCx, InterpResult, Machine, MayLeak,
-    Misalignment, Pointer, PointerArithmetic, Provenance, Scalar, alloc_range, err_ub,
+    CheckInAllocMsg, CtfeProvenance, GlobalAlloc, InterpCx, InterpResult, MPlaceTy, Machine,
+    MayLeak, Misalignment, Pointer, PointerArithmetic, Provenance, Scalar, alloc_range, err_ub,
     err_ub_custom, interp_ok, throw_ub, throw_ub_custom, throw_unsup, throw_unsup_format,
 };
 use crate::const_eval::ConstEvalErrKind;
@@ -67,6 +67,8 @@ pub enum AllocKind {
     LiveData,
     /// A function allocation (that fn ptrs point to).
     Function,
+    /// A variable argument list allocation (used by c-variadic functions).
+    VaList,
     /// A vtable allocation.
     VTable,
     /// A TypeId allocation.
@@ -126,6 +128,9 @@ pub struct Memory<'tcx, M: Machine<'tcx>> {
     /// Map for "extra" function pointers.
     extra_fn_ptr_map: FxIndexMap<AllocId, M::ExtraFnVal>,
 
+    /// Map storing variable argument lists.
+    va_list_map: FxIndexMap<AllocId, VecDeque<MPlaceTy<'tcx, M::Provenance>>>,
+
     /// To be able to compare pointers with null, and to check alignment for accesses
     /// to ZSTs (where pointers may dangle), we keep track of the size even for allocations
     /// that do not exist any more.
@@ -161,6 +166,7 @@ impl<'tcx, M: Machine<'tcx>> Memory<'tcx, M> {
         Memory {
             alloc_map: M::MemoryMap::default(),
             extra_fn_ptr_map: FxIndexMap::default(),
+            va_list_map: FxIndexMap::default(),
             dead_alloc_map: FxIndexMap::default(),
             validation_in_progress: Cell::new(false),
         }
@@ -199,9 +205,11 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
                 return M::extern_static_pointer(self, def_id);
             }
             None => {
+                let is_fn_ptr = self.memory.extra_fn_ptr_map.contains_key(&alloc_id);
+                let is_va_list = self.memory.va_list_map.contains_key(&alloc_id);
                 assert!(
-                    self.memory.extra_fn_ptr_map.contains_key(&alloc_id),
-                    "{alloc_id:?} is neither global nor a function pointer"
+                    is_fn_ptr || is_va_list,
+                    "{alloc_id:?} is neither global, va_list nor a function pointer"
                 );
             }
             _ => {}
@@ -229,6 +237,19 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         self.global_root_pointer(Pointer::from(id)).unwrap()
     }
 
+    /// Insert a new variable argument list in the global map of variable argument lists.
+    pub fn va_list_ptr(
+        &mut self,
+        varargs: VecDeque<MPlaceTy<'tcx, M::Provenance>>,
+    ) -> Pointer<M::Provenance> {
+        let id = self.tcx.reserve_alloc_id();
+        let old = self.memory.va_list_map.insert(id, varargs);
+        assert!(old.is_none());
+        // Variable argument lists are global allocations, so make sure we get the right root
+        // pointer. We know this is not an `extern static` so this cannot fail.
+        self.global_root_pointer(Pointer::from(id)).unwrap()
+    }
+
     pub fn allocate_ptr(
         &mut self,
         size: Size,
@@ -956,6 +977,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     pub fn is_alloc_live(&self, id: AllocId) -> bool {
         self.memory.alloc_map.contains_key_ref(&id)
             || self.memory.extra_fn_ptr_map.contains_key(&id)
+            || self.memory.va_list_map.contains_key(&id)
             // We check `tcx` last as that has to acquire a lock in `many-seeds` mode.
             // This also matches the order in `get_alloc_info`.
             || self.tcx.try_get_global_alloc(id).is_some()
@@ -995,6 +1017,11 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             return AllocInfo::new(Size::ZERO, align, AllocKind::Function, Mutability::Not);
         }
 
+        // # Variable argument lists
+        if self.memory.va_list_map.contains_key(&id) {
+            return AllocInfo::new(Size::ZERO, Align::ONE, AllocKind::VaList, Mutability::Not);
+        }
+
         // # Global allocations
         if let Some(global_alloc) = self.tcx.try_get_global_alloc(id) {
             // NOTE: `static` alignment from attributes has already been applied to the allocation.
@@ -1069,6 +1096,43 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             .into()
     }
 
+    pub fn get_ptr_va_list(
+        &self,
+        ptr: Pointer<Option<M::Provenance>>,
+    ) -> InterpResult<'tcx, &VecDeque<MPlaceTy<'tcx, M::Provenance>>> {
+        trace!("get_ptr_va_list({:?})", ptr);
+        let (alloc_id, offset, _prov) = self.ptr_get_alloc_id(ptr, 0)?;
+        if offset.bytes() != 0 {
+            throw_ub!(InvalidVaListPointer(Pointer::new(alloc_id, offset)))
+        }
+
+        let Some(va_list) = self.memory.va_list_map.get(&alloc_id) else {
+            throw_ub!(InvalidVaListPointer(Pointer::new(alloc_id, offset)))
+        };
+
+        interp_ok(va_list)
+    }
+
+    /// Removes this VaList from the global map of variable argument lists. This does not deallocate
+    /// the VaList elements, that happens when the Frame is popped.
+    pub fn deallocate_va_list(
+        &mut self,
+        ptr: Pointer<Option<M::Provenance>>,
+    ) -> InterpResult<'tcx, VecDeque<MPlaceTy<'tcx, M::Provenance>>> {
+        trace!("deallocate_va_list({:?})", ptr);
+        let (alloc_id, offset, _prov) = self.ptr_get_alloc_id(ptr, 0)?;
+        if offset.bytes() != 0 {
+            throw_ub!(InvalidVaListPointer(Pointer::new(alloc_id, offset)))
+        }
+
+        let Some(va_list) = self.memory.va_list_map.swap_remove(&alloc_id) else {
+            throw_ub!(InvalidVaListPointer(Pointer::new(alloc_id, offset)))
+        };
+
+        self.memory.dead_alloc_map.insert(alloc_id, (Size::ZERO, Align::ONE));
+        interp_ok(va_list)
+    }
+
     /// Get the dynamic type of the given vtable pointer.
     /// If `expected_trait` is `Some`, it must be a vtable for the given trait.
     pub fn get_ptr_vtable_ty(

compiler/rustc_const_eval/src/interpret/stack.rs

@@ -12,13 +12,14 @@ use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_middle::{bug, mir};
 use rustc_mir_dataflow::impls::always_storage_live_locals;
 use rustc_span::Span;
+use rustc_target::callconv::ArgAbi;
 use tracing::field::Empty;
 use tracing::{info_span, instrument, trace};
 
 use super::{
-    AllocId, CtfeProvenance, Immediate, InterpCx, InterpResult, Machine, MemPlace, MemPlaceMeta,
-    MemoryKind, Operand, PlaceTy, Pointer, Provenance, ReturnAction, Scalar, from_known_layout,
-    interp_ok, throw_ub, throw_unsup,
+    AllocId, CtfeProvenance, FnArg, Immediate, InterpCx, InterpResult, MPlaceTy, Machine, MemPlace,
+    MemPlaceMeta, MemoryKind, Operand, PlaceTy, Pointer, Provenance, ReturnAction, Scalar,
+    from_known_layout, interp_ok, throw_ub, throw_unsup,
 };
 use crate::{enter_trace_span, errors};
 
@@ -91,6 +92,10 @@ pub struct Frame<'tcx, Prov: Provenance = CtfeProvenance, Extra = ()> {
     /// Do *not* access this directly; always go through the machine hook!
     pub locals: IndexVec<mir::Local, LocalState<'tcx, Prov>>,
 
+    /// The complete variable argument list of this frame. Its elements must be dropped when the
+    /// frame is popped.
+    pub(super) va_list: Vec<MPlaceTy<'tcx, Prov>>,
+
     /// The span of the `tracing` crate is stored here.
     /// When the guard is dropped, the span is exited. This gives us
     /// a full stack trace on all tracing statements.
@@ -259,6 +264,7 @@ impl<'tcx, Prov: Provenance> Frame<'tcx, Prov> {
             return_cont: self.return_cont,
             return_place: self.return_place,
             locals: self.locals,
+            va_list: self.va_list,
             loc: self.loc,
             extra,
             tracing_span: self.tracing_span,
@@ -377,6 +383,7 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             return_cont,
             return_place: return_place.clone(),
             locals,
+            va_list: vec![],
             instance,
             tracing_span: SpanGuard::new(),
             extra: (),
@@ -449,11 +456,13 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
         };
 
         let return_action = if cleanup {
-            // We need to take the locals out, since we need to mutate while iterating.
             for local in &frame.locals {
                 self.deallocate_local(local.value)?;
             }
 
+            // Deallocate any c-variadic arguments.
+            self.deallocate_varargs(&frame.va_list)?;
+
             // Call the machine hook, which determines the next steps.
             let return_action = M::after_stack_pop(self, frame, unwinding)?;
             assert_ne!(return_action, ReturnAction::NoCleanup);
@@ -626,6 +635,58 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
     }
 }
 
+impl<'a, 'tcx: 'a, M: Machine<'tcx>> InterpCx<'tcx, M> {
+    /// Consume the arguments provided by the iterator and store them as a list
+    /// of variadic arguments. Return a list of the places that hold those arguments.
+    pub(crate) fn allocate_varargs<I, J>(
+        &mut self,
+        caller_args: &mut I,
+        callee_abis: &mut J,
+    ) -> InterpResult<'tcx, Vec<MPlaceTy<'tcx, M::Provenance>>>
+    where
+        I: Iterator<Item = (&'a FnArg<'tcx, M::Provenance>, &'a ArgAbi<'tcx, Ty<'tcx>>)>,
+        J: Iterator<Item = (usize, &'a ArgAbi<'tcx, Ty<'tcx>>)>,
+    {
+        // Consume the remaining arguments and store them in fresh allocations.
+        let mut varargs = Vec::new();
+        for (fn_arg, caller_abi) in caller_args {
+            // The callee ABI is entirely computed based on which arguments the caller has
+            // provided so it should not be possible to get a mismatch here.
+            let (_idx, callee_abi) = callee_abis.next().unwrap();
+            assert!(self.check_argument_compat(caller_abi, callee_abi)?);
+            // FIXME: do we have to worry about in-place argument passing?
+            let op = self.copy_fn_arg(fn_arg);
+            let mplace = self.allocate(op.layout, MemoryKind::Stack)?;
+            self.copy_op(&op, &mplace)?;
+
+            varargs.push(mplace);
+        }
+        assert!(callee_abis.next().is_none());
+
+        interp_ok(varargs)
+    }
+
+    /// Deallocate the variadic arguments in the list (that must have been created with `allocate_varargs`).
+    fn deallocate_varargs(
+        &mut self,
+        varargs: &[MPlaceTy<'tcx, M::Provenance>],
+    ) -> InterpResult<'tcx> {
+        for vararg in varargs {
+            let ptr = vararg.ptr();
+
+            trace!(
+                "deallocating vararg {:?}: {:?}",
+                vararg,
+                // Locals always have a `alloc_id` (they are never the result of a int2ptr).
+                self.dump_alloc(ptr.provenance.unwrap().get_alloc_id().unwrap())
+            );
+            self.deallocate_ptr(ptr, None, MemoryKind::Stack)?;
+        }
+
+        interp_ok(())
+    }
+}
+
 impl<'tcx, Prov: Provenance> LocalState<'tcx, Prov> {
     pub(super) fn print(
         &self,

compiler/rustc_feature/src/unstable.rs

@@ -420,6 +420,8 @@ declare_features! (
     (unstable, const_async_blocks, "1.53.0", Some(85368)),
     /// Allows `const { ... }` as a shorthand for `const _: () = const { ... };` for module items.
     (unstable, const_block_items, "CURRENT_RUSTC_VERSION", Some(149226)),
+    /// Allows defining and calling c-variadic functions in const contexts.
+    (unstable, const_c_variadic, "CURRENT_RUSTC_VERSION", Some(151787)),
     /// Allows `const || {}` closures in const contexts.
     (incomplete, const_closures, "1.68.0", Some(106003)),
     /// Allows using `[const] Destruct` bounds and calling drop impls in const contexts.

compiler/rustc_middle/src/mir/interpret/error.rs

@@ -392,6 +392,8 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     DerefFunctionPointer(AllocId),
     /// Trying to access the data behind a vtable pointer.
     DerefVTablePointer(AllocId),
+    /// Trying to access the data behind a va_list pointer.
+    DerefVaListPointer(AllocId),
     /// Trying to access the actual type id.
     DerefTypeIdPointer(AllocId),
     /// Using a non-boolean `u8` as bool.
@@ -402,6 +404,8 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     InvalidTag(Scalar<AllocId>),
     /// Using a pointer-not-to-a-function as function pointer.
     InvalidFunctionPointer(Pointer<AllocId>),
+    /// Using a pointer-not-to-a-va-list as variable argument list pointer.
+    InvalidVaListPointer(Pointer<AllocId>),
     /// Using a pointer-not-to-a-vtable as vtable pointer.
     InvalidVTablePointer(Pointer<AllocId>),
     /// Using a vtable for the wrong trait.
@@ -434,6 +438,12 @@ pub enum UndefinedBehaviorInfo<'tcx> {
     },
     /// ABI-incompatible return types.
     AbiMismatchReturn { caller_ty: Ty<'tcx>, callee_ty: Ty<'tcx> },
+    /// `va_arg` was called on an exhausted `VaList`.
+    VaArgOutOfBounds,
+    /// The caller and callee disagree on whether they are c-variadic or not.
+    CVariadicMismatch { caller_is_c_variadic: bool, callee_is_c_variadic: bool },
+    /// The caller and callee disagree on the number of fixed (i.e. non-c-variadic) arguments.
+    CVariadicFixedCountMismatch { caller: u32, callee: u32 },
 }
 
 #[derive(Debug, Clone, Copy)]

compiler/rustc_middle/src/mir/interpret/mod.rs

@@ -396,7 +396,7 @@ impl<'tcx> GlobalAlloc<'tcx> {
                 // No data to be accessed here. But vtables are pointer-aligned.
                 (Size::ZERO, tcx.data_layout.pointer_align().abi)
             }
-            // Fake allocation, there's nothing to access here
+            // Fake allocation, there's nothing to access here.
             GlobalAlloc::TypeId { .. } => (Size::ZERO, Align::ONE),
         }
     }

compiler/rustc_span/src/symbol.rs

@@ -742,6 +742,7 @@ symbols! {
         const_allocate,
         const_async_blocks,
         const_block_items,
+        const_c_variadic,
         const_closures,
         const_compare_raw_pointers,
         const_constructor,

library/core/src/ffi/va_list.rs

@@ -200,12 +200,13 @@ impl fmt::Debug for VaList<'_> {
 
 impl VaList<'_> {
     // Helper used in the implementation of the `va_copy` intrinsic.
-    pub(crate) fn duplicate(&self) -> Self {
-        Self { inner: self.inner.clone(), _marker: self._marker }
+    pub(crate) const fn duplicate(&self) -> Self {
+        Self { inner: self.inner, _marker: self._marker }
     }
 }
 
-impl Clone for VaList<'_> {
+#[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
+impl<'f> const Clone for VaList<'f> {
     #[inline]
     fn clone(&self) -> Self {
         // We only implement Clone and not Copy because some future target might not be able to
@@ -216,7 +217,8 @@ impl Clone for VaList<'_> {
     }
 }
 
-impl<'f> Drop for VaList<'f> {
+#[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
+impl<'f> const Drop for VaList<'f> {
     fn drop(&mut self) {
         // SAFETY: this variable argument list is being dropped, so won't be read from again.
         unsafe { va_end(self) }
@@ -291,7 +293,8 @@ impl<'f> VaList<'f> {
     ///
     /// [valid]: https://doc.rust-lang.org/nightly/nomicon/what-unsafe-does.html
     #[inline]
-    pub unsafe fn arg<T: VaArgSafe>(&mut self) -> T {
+    #[rustc_const_unstable(feature = "const_c_variadic", issue = "151787")]
+    pub const unsafe fn arg<T: VaArgSafe>(&mut self) -> T {
         // SAFETY: the caller must uphold the safety contract for `va_arg`.
         unsafe { va_arg(self) }
     }

library/core/src/intrinsics/mod.rs

@@ -3485,7 +3485,7 @@ pub(crate) const fn miri_promise_symbolic_alignment(ptr: *const (), align: usize
 ///
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
+pub const unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
 
 /// Duplicates a variable argument list. The returned list is initially at the same position as
 /// the one in `src`, but can be advanced independently.
@@ -3497,7 +3497,7 @@ pub unsafe fn va_arg<T: VaArgSafe>(ap: &mut VaList<'_>) -> T;
 /// when a variable argument list is used incorrectly.
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub fn va_copy<'f>(src: &VaList<'f>) -> VaList<'f> {
+pub const fn va_copy<'f>(src: &VaList<'f>) -> VaList<'f> {
     src.duplicate()
 }
 
@@ -3516,6 +3516,6 @@ pub fn va_copy<'f>(src: &VaList<'f>) -> VaList<'f> {
 ///
 #[rustc_intrinsic]
 #[rustc_nounwind]
-pub unsafe fn va_end(ap: &mut VaList<'_>) {
+pub const unsafe fn va_end(ap: &mut VaList<'_>) {
     /* deliberately does nothing */
 }

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

@@ -184,7 +184,7 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 }
                 #[cfg(not(all(unix, feature = "native-lib")))]
                 AllocKind::Function => dummy_alloc(params),
-                AllocKind::VTable => dummy_alloc(params),
+                AllocKind::VTable | AllocKind::VaList => dummy_alloc(params),
                 AllocKind::TypeId | AllocKind::Dead => unreachable!(),
             };
             // We don't have to expose this pointer yet, we do that in `prepare_for_native_call`.

src/tools/miri/src/borrow_tracker/stacked_borrows/mod.rs

@@ -655,7 +655,7 @@ trait EvalContextPrivExt<'tcx, 'ecx>: crate::MiriInterpCxExt<'tcx> {
                         dcx.log_protector();
                     }
                 },
-                AllocKind::Function | AllocKind::VTable | AllocKind::TypeId | AllocKind::Dead => {
+                AllocKind::Function | AllocKind::VTable | AllocKind::TypeId | AllocKind::Dead | AllocKind::VaList => {
                     // No stacked borrows on these allocations.
                 }
             }
@@ -1014,7 +1014,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 trace!("Stacked Borrows tag {tag:?} exposed in {alloc_id:?}");
                 alloc_extra.borrow_tracker_sb().borrow_mut().exposed_tags.insert(tag);
             }
-            AllocKind::Function | AllocKind::VTable | AllocKind::TypeId | AllocKind::Dead => {
+            AllocKind::Function
+            | AllocKind::VTable
+            | AllocKind::TypeId
+            | AllocKind::Dead
+            | AllocKind::VaList => {
                 // No stacked borrows on these allocations.
             }
         }

src/tools/miri/src/borrow_tracker/tree_borrows/mod.rs

@@ -577,7 +577,11 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 let protected = protected_tags.contains_key(&tag);
                 alloc_extra.borrow_tracker_tb().borrow_mut().expose_tag(tag, protected);
             }
-            AllocKind::Function | AllocKind::VTable | AllocKind::TypeId | AllocKind::Dead => {
+            AllocKind::Function
+            | AllocKind::VTable
+            | AllocKind::TypeId
+            | AllocKind::Dead
+            | AllocKind::VaList => {
                 // No tree borrows on these allocations.
             }
         }

src/tools/miri/tests/fail/c-variadic-mismatch-count.rs

@@ -0,0 +1,13 @@
+#![feature(c_variadic)]
+
+unsafe extern "C" fn helper(_: i32, _: ...) {}
+
+fn main() {
+    unsafe {
+        let f = helper as *const ();
+        let f = std::mem::transmute::<_, unsafe extern "C" fn(...)>(f);
+
+        f(1);
+        //~^ ERROR: Undefined Behavior
+    }
+}

src/tools/miri/tests/fail/c-variadic-mismatch-count.stderr

@@ -0,0 +1,13 @@
+error: Undefined Behavior: calling a C-variadic function with 0 fixed arguments, but the function expects 1
+  --> tests/fail/c-variadic-mismatch-count.rs:LL:CC
+   |
+LL |         f(1);
+   |         ^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/fail/c-variadic-mismatch.rs

@@ -0,0 +1,13 @@
+#![feature(c_variadic)]
+
+unsafe extern "C" fn helper(_: i32, _: ...) {}
+
+fn main() {
+    unsafe {
+        let f = helper as *const ();
+        let f = std::mem::transmute::<_, unsafe extern "C" fn(_: i32, _: i64)>(f);
+
+        f(1i32, 1i64);
+        //~^ ERROR: Undefined Behavior: calling a function where the caller and callee disagree on whether the function is C-variadic
+    }
+}

src/tools/miri/tests/fail/c-variadic-mismatch.stderr

@@ -0,0 +1,13 @@
+error: Undefined Behavior: calling a function where the caller and callee disagree on whether the function is C-variadic
+  --> tests/fail/c-variadic-mismatch.rs:LL:CC
+   |
+LL |         f(1i32, 1i64);
+   |         ^^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/fail/c-variadic.rs

@@ -0,0 +1,15 @@
+#![feature(c_variadic)]
+
+//@error-in-other-file: Undefined Behavior: more C-variadic arguments read than were passed
+
+fn read_too_many() {
+    unsafe extern "C" fn variadic(mut ap: ...) {
+        ap.arg::<i32>();
+    }
+
+    unsafe { variadic() };
+}
+
+fn main() {
+    read_too_many();
+}

src/tools/miri/tests/fail/c-variadic.stderr

@@ -0,0 +1,22 @@
+error: Undefined Behavior: more C-variadic arguments read than were passed
+  --> RUSTLIB/core/src/ffi/va_list.rs:LL:CC
+   |
+LL |         unsafe { va_arg(self) }
+   |                  ^^^^^^^^^^^^ Undefined Behavior occurred here
+   |
+   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
+   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
+   = note: stack backtrace:
+           0: std::ffi::VaList::<'_>::arg
+               at RUSTLIB/core/src/ffi/va_list.rs:LL:CC
+           1: read_too_many::variadic
+               at tests/fail/c-variadic.rs:LL:CC
+           2: read_too_many
+               at tests/fail/c-variadic.rs:LL:CC
+           3: main
+               at tests/fail/c-variadic.rs:LL:CC
+
+note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace
+
+error: aborting due to 1 previous error
+

src/tools/miri/tests/pass/c-variadic.rs

@@ -0,0 +1,115 @@
+#![feature(c_variadic)]
+
+use std::ffi::{CStr, VaList, c_char, c_double, c_int, c_long};
+
+fn ignores_arguments() {
+    unsafe extern "C" fn variadic(_: ...) {}
+
+    unsafe { variadic() };
+    unsafe { variadic(1, 2, 3) };
+}
+
+fn echo() {
+    unsafe extern "C" fn variadic(mut ap: ...) -> i32 {
+        ap.arg()
+    }
+
+    assert_eq!(unsafe { variadic(1) }, 1);
+    assert_eq!(unsafe { variadic(3, 2, 1) }, 3);
+}
+
+fn forward_by_val() {
+    unsafe fn helper(mut ap: VaList) -> i32 {
+        ap.arg()
+    }
+
+    unsafe extern "C" fn variadic(ap: ...) -> i32 {
+        helper(ap)
+    }
+
+    assert_eq!(unsafe { variadic(1) }, 1);
+    assert_eq!(unsafe { variadic(3, 2, 1) }, 3);
+}
+
+fn forward_by_ref() {
+    unsafe fn helper(ap: &mut VaList) -> i32 {
+        ap.arg()
+    }
+
+    unsafe extern "C" fn variadic(mut ap: ...) -> i32 {
+        helper(&mut ap)
+    }
+
+    assert_eq!(unsafe { variadic(1) }, 1);
+    assert_eq!(unsafe { variadic(3, 2, 1) }, 3);
+}
+
+#[allow(improper_ctypes_definitions)]
+fn nested() {
+    unsafe fn helper(mut ap1: VaList, mut ap2: VaList) -> (i32, i32) {
+        (ap1.arg(), ap2.arg())
+    }
+
+    unsafe extern "C" fn variadic2(ap1: VaList, ap2: ...) -> (i32, i32) {
+        helper(ap1, ap2)
+    }
+
+    unsafe extern "C" fn variadic1(ap1: ...) -> (i32, i32) {
+        variadic2(ap1, 2, 2)
+    }
+
+    assert_eq!(unsafe { variadic1(1) }, (1, 2));
+
+    let (a, b) = unsafe { variadic1(1, 1) };
+
+    assert_eq!(a, 1);
+    assert_eq!(b, 2);
+}
+
+fn various_types() {
+    unsafe extern "C" fn check_list_2(mut ap: ...) {
+        macro_rules! continue_if {
+            ($cond:expr) => {
+                if !($cond) {
+                    panic!();
+                }
+            };
+        }
+
+        unsafe fn compare_c_str(ptr: *const c_char, val: &str) -> bool {
+            match CStr::from_ptr(ptr).to_str() {
+                Ok(cstr) => cstr == val,
+                Err(_) => panic!(),
+            }
+        }
+
+        continue_if!(ap.arg::<c_double>().floor() == 3.14f64.floor());
+        continue_if!(ap.arg::<c_long>() == 12);
+        continue_if!(ap.arg::<c_int>() == 'a' as c_int);
+        continue_if!(ap.arg::<c_double>().floor() == 6.18f64.floor());
+        continue_if!(compare_c_str(ap.arg::<*const c_char>(), "Hello"));
+        continue_if!(ap.arg::<c_int>() == 42);
+        continue_if!(compare_c_str(ap.arg::<*const c_char>(), "World"));
+    }
+
+    unsafe {
+        check_list_2(
+            3.14 as c_double,
+            12 as c_long,
+            b'a' as c_int,
+            6.28 as c_double,
+            c"Hello".as_ptr(),
+            42 as c_int,
+            c"World".as_ptr(),
+        );
+    }
+}
+
+fn main() {
+    ignores_arguments();
+    echo();
+    forward_by_val();
+    forward_by_ref();
+    nested();
+    various_types();
+}

tests/ui/consts/const-eval/c-variadic-fail.rs

@@ -0,0 +1,147 @@
+//@ build-fail
+
+#![feature(c_variadic)]
+#![feature(const_c_variadic)]
+#![feature(const_trait_impl)]
+#![feature(const_destruct)]
+#![feature(const_clone)]
+
+use std::ffi::VaList;
+use std::mem::MaybeUninit;
+
+const unsafe extern "C" fn read_n<const N: usize>(mut ap: ...) {
+    let mut i = N;
+    while i > 0 {
+        i -= 1;
+        let _ = ap.arg::<i32>();
+    }
+}
+
+unsafe fn read_too_many() {
+    // None passed, none read.
+    const { read_n::<0>() }
+
+    // One passed, none read. Ignoring arguments is fine.
+    const { read_n::<0>(1) }
+
+    // None passed, one read.
+    const { read_n::<1>() }
+    //~^ ERROR more C-variadic arguments read than were passed
+
+    // One passed, two read.
+    const { read_n::<2>(1) }
+    //~^ ERROR more C-variadic arguments read than were passed
+}
+
+const unsafe extern "C" fn read_as<T: core::ffi::VaArgSafe>(mut ap: ...) -> T {
+    ap.arg::<T>()
+}
+
+unsafe fn read_cast() {
+    const { read_as::<i32>(1i32) };
+    const { read_as::<u32>(1u32) };
+
+    const { read_as::<i32>(1i32, 2u64, 3.0f64) };
+    const { read_as::<u32>(1u32, 2u64, 3.0f64) };
+
+    const { read_as::<i64>(1i64) };
+    const { read_as::<u64>(1u64) };
+
+    const { read_as::<u32>(1i32) };
+    //~^ ERROR va_arg type mismatch: requested `u32`, but next argument is `i32`
+
+    const { read_as::<i32>(1u32) };
+    //~^ ERROR va_arg type mismatch: requested `i32`, but next argument is `u32`
+
+    const { read_as::<i32>(1u64) };
+    //~^ ERROR va_arg type mismatch: requested `i32`, but next argument is `u64`
+
+    const { read_as::<f64>(1i32) };
+    //~^ ERROR va_arg type mismatch: requested `f64`, but next argument is `i32`
+
+    const { read_as::<*const u8>(1i32) };
+    //~^ ERROR va_arg type mismatch: requested `*const u8`, but next argument is `i32`
+}
+
+fn use_after_free() {
+    const unsafe extern "C" fn helper(ap: ...) -> [u8; size_of::<VaList>()] {
+        unsafe { std::mem::transmute(ap) }
+    }
+
+    const {
+        unsafe {
+            let ap = helper(1, 2, 3);
+            let mut ap = std::mem::transmute::<_, VaList>(ap);
+            ap.arg::<i32>();
+            //~^ ERROR memory access failed: ALLOC0 has been freed, so this pointer is dangling [E0080]
+        }
+    };
+}
+
+fn manual_copy_drop() {
+    const unsafe extern "C" fn helper(ap: ...) {
+        // A copy created using Clone is valid, and can be used to read arguments.
+        let mut copy = ap.clone();
+        assert!(copy.arg::<i32>() == 1i32);
+
+        let mut copy: VaList = unsafe { std::mem::transmute_copy(&ap) };
+
+        // Using the copy is actually fine.
+        let _ = copy.arg::<i32>();
+        drop(copy);
+
+        // But then using the original is UB.
+        drop(ap);
+    }
+
+    const { unsafe { helper(1, 2, 3) } };
+    //~^ ERROR using ALLOC0 as variable argument list pointer but it does not point to a variable argument list [E0080]
+}
+
+fn manual_copy_forget() {
+    const unsafe extern "C" fn helper(ap: ...) {
+        let mut copy: VaList = unsafe { std::mem::transmute_copy(&ap) };
+
+        // Using the copy is actually fine.
+        let _ = copy.arg::<i32>();
+        std::mem::forget(copy);
+
+        // The read (via `copy`) deallocated the original allocation.
+        drop(ap);
+    }
+
+    const { unsafe { helper(1, 2, 3) } };
+    //~^ ERROR using ALLOC0 as variable argument list pointer but it does not point to a variable argument list [E0080]
+}
+
+fn manual_copy_read() {
+    const unsafe extern "C" fn helper(mut ap: ...) {
+        let mut copy: VaList = unsafe { std::mem::transmute_copy(&ap) };
+
+        // Reading from `ap` after reading from `copy` is UB.
+        let _ = copy.arg::<i32>();
+        let _ = ap.arg::<i32>();
+    }
+
+    const { unsafe { helper(1, 2, 3) } };
+    //~^ ERROR using ALLOC0 as variable argument list pointer but it does not point to a variable argument list [E0080]
+}
+
+fn drop_of_invalid() {
+    const {
+        let mut invalid: MaybeUninit<VaList> = MaybeUninit::zeroed();
+        let ap = unsafe { invalid.assume_init() };
+    }
+    //~^ ERROR pointer not dereferenceable: pointer must point to some allocation, but got null pointer [E0080]
+}
+
+fn main() {
+    unsafe {
+        read_too_many();
+        read_cast();
+        manual_copy_read();
+        manual_copy_drop();
+        manual_copy_forget();
+        drop_of_invalid();
+    }
+}

tests/ui/consts/const-eval/c-variadic-fail.stderr

@@ -0,0 +1,355 @@
+error[E0080]: more C-variadic arguments read than were passed
+  --> $DIR/c-variadic-fail.rs:28:13
+   |
+LL |     const { read_n::<1>() }
+   |             ^^^^^^^^^^^^^ evaluation of `read_too_many::{constant#2}` failed inside this call
+   |
+note: inside `read_n::<1>`
+  --> $DIR/c-variadic-fail.rs:16:17
+   |
+LL |         let _ = ap.arg::<i32>();
+   |                 ^^^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<i32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:28:5
+   |
+LL |     const { read_n::<1>() }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:28:5
+   |
+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
+  --> $DIR/c-variadic-fail.rs:32:13
+   |
+LL |     const { read_n::<2>(1) }
+   |             ^^^^^^^^^^^^^^ evaluation of `read_too_many::{constant#3}` failed inside this call
+   |
+note: inside `read_n::<2>`
+  --> $DIR/c-variadic-fail.rs:16:17
+   |
+LL |         let _ = ap.arg::<i32>();
+   |                 ^^^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<i32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:32:5
+   |
+LL |     const { read_n::<2>(1) }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:32:5
+   |
+LL |     const { read_n::<2>(1) }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: va_arg type mismatch: requested `u32`, but next argument is `i32`
+  --> $DIR/c-variadic-fail.rs:50:13
+   |
+LL |     const { read_as::<u32>(1i32) };
+   |             ^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast::{constant#6}` failed inside this call
+   |
+note: inside `read_as::<u32>`
+  --> $DIR/c-variadic-fail.rs:37:5
+   |
+LL |     ap.arg::<T>()
+   |     ^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<u32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:50:5
+   |
+LL |     const { read_as::<u32>(1i32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:50:5
+   |
+LL |     const { read_as::<u32>(1i32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: va_arg type mismatch: requested `i32`, but next argument is `u32`
+  --> $DIR/c-variadic-fail.rs:53:13
+   |
+LL |     const { read_as::<i32>(1u32) };
+   |             ^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast::{constant#7}` failed inside this call
+   |
+note: inside `read_as::<i32>`
+  --> $DIR/c-variadic-fail.rs:37:5
+   |
+LL |     ap.arg::<T>()
+   |     ^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<i32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:53:5
+   |
+LL |     const { read_as::<i32>(1u32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:53:5
+   |
+LL |     const { read_as::<i32>(1u32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: va_arg type mismatch: requested `i32`, but next argument is `u64`
+  --> $DIR/c-variadic-fail.rs:56:13
+   |
+LL |     const { read_as::<i32>(1u64) };
+   |             ^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast::{constant#8}` failed inside this call
+   |
+note: inside `read_as::<i32>`
+  --> $DIR/c-variadic-fail.rs:37:5
+   |
+LL |     ap.arg::<T>()
+   |     ^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<i32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:56:5
+   |
+LL |     const { read_as::<i32>(1u64) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:56:5
+   |
+LL |     const { read_as::<i32>(1u64) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: va_arg type mismatch: requested `f64`, but next argument is `i32`
+  --> $DIR/c-variadic-fail.rs:59:13
+   |
+LL |     const { read_as::<f64>(1i32) };
+   |             ^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast::{constant#9}` failed inside this call
+   |
+note: inside `read_as::<f64>`
+  --> $DIR/c-variadic-fail.rs:37:5
+   |
+LL |     ap.arg::<T>()
+   |     ^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<f64>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:59:5
+   |
+LL |     const { read_as::<f64>(1i32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:59:5
+   |
+LL |     const { read_as::<f64>(1i32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: va_arg type mismatch: requested `*const u8`, but next argument is `i32`
+  --> $DIR/c-variadic-fail.rs:62:13
+   |
+LL |     const { read_as::<*const u8>(1i32) };
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `read_cast::{constant#10}` failed inside this call
+   |
+note: inside `read_as::<*const u8>`
+  --> $DIR/c-variadic-fail.rs:37:5
+   |
+LL |     ap.arg::<T>()
+   |     ^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<*const u8>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:62:5
+   |
+LL |     const { read_as::<*const u8>(1i32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:62:5
+   |
+LL |     const { read_as::<*const u8>(1i32) };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: memory access failed: ALLOC0 has been freed, so this pointer is dangling
+  --> $DIR/c-variadic-fail.rs:75:13
+   |
+LL |             ap.arg::<i32>();
+   |             ^^^^^^^^^^^^^^^ evaluation of `use_after_free::{constant#0}` failed inside this call
+   |
+note: inside `VaList::<'_>::arg::<i32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:71:5
+   |
+LL | /     const {
+LL | |         unsafe {
+LL | |             let ap = helper(1, 2, 3);
+LL | |             let mut ap = std::mem::transmute::<_, VaList>(ap);
+...  |
+LL | |     };
+   | |_____^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:71:5
+   |
+LL | /     const {
+LL | |         unsafe {
+LL | |             let ap = helper(1, 2, 3);
+LL | |             let mut ap = std::mem::transmute::<_, VaList>(ap);
+...  |
+LL | |     };
+   | |_____^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: using ALLOC1 as variable argument list pointer but it does not point to a variable argument list
+  --> $DIR/c-variadic-fail.rs:97:22
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |                      ^^^^^^^^^^^^^^^ evaluation of `manual_copy_drop::{constant#0}` failed inside this call
+   |
+note: inside `manual_copy_drop::helper`
+  --> $DIR/c-variadic-fail.rs:94:9
+   |
+LL |         drop(ap);
+   |         ^^^^^^^^
+note: inside `std::mem::drop::<VaList<'_>>`
+  --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
+note: inside `drop_in_place::<VaList<'_>> - shim(Some(VaList<'_>))`
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+note: inside `<VaList<'_> as Drop>::drop`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:97:5
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:97:5
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: using ALLOC2 as variable argument list pointer but it does not point to a variable argument list
+  --> $DIR/c-variadic-fail.rs:113:22
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |                      ^^^^^^^^^^^^^^^ evaluation of `manual_copy_forget::{constant#0}` failed inside this call
+   |
+note: inside `manual_copy_forget::helper`
+  --> $DIR/c-variadic-fail.rs:110:9
+   |
+LL |         drop(ap);
+   |         ^^^^^^^^
+note: inside `std::mem::drop::<VaList<'_>>`
+  --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
+note: inside `drop_in_place::<VaList<'_>> - shim(Some(VaList<'_>))`
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+note: inside `<VaList<'_> as Drop>::drop`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:113:5
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:113:5
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: using ALLOC3 as variable argument list pointer but it does not point to a variable argument list
+  --> $DIR/c-variadic-fail.rs:126:22
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |                      ^^^^^^^^^^^^^^^ evaluation of `manual_copy_read::{constant#0}` failed inside this call
+   |
+note: inside `manual_copy_read::helper`
+  --> $DIR/c-variadic-fail.rs:123:17
+   |
+LL |         let _ = ap.arg::<i32>();
+   |                 ^^^^^^^^^^^^^^^
+note: inside `VaList::<'_>::arg::<i32>`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:126:5
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:126:5
+   |
+LL |     const { unsafe { helper(1, 2, 3) } };
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error[E0080]: pointer not dereferenceable: pointer must point to some allocation, but got null pointer
+  --> $DIR/c-variadic-fail.rs:134:5
+   |
+LL |     }
+   |     ^ evaluation of `drop_of_invalid::{constant#0}` failed inside this call
+   |
+note: inside `drop_in_place::<VaList<'_>> - shim(Some(VaList<'_>))`
+  --> $SRC_DIR/core/src/ptr/mod.rs:LL:COL
+note: inside `<VaList<'_> as Drop>::drop`
+  --> $SRC_DIR/core/src/ffi/va_list.rs:LL:COL
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:131:5
+   |
+LL | /     const {
+LL | |         let mut invalid: MaybeUninit<VaList> = MaybeUninit::zeroed();
+LL | |         let ap = unsafe { invalid.assume_init() };
+LL | |     }
+   | |_____^
+
+note: erroneous constant encountered
+  --> $DIR/c-variadic-fail.rs:131:5
+   |
+LL | /     const {
+LL | |         let mut invalid: MaybeUninit<VaList> = MaybeUninit::zeroed();
+LL | |         let ap = unsafe { invalid.assume_init() };
+LL | |     }
+   | |_____^
+   |
+   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
+
+error: aborting due to 12 previous errors
+
+For more information about this error, try `rustc --explain E0080`.

tests/ui/consts/const-eval/c-variadic.rs

@@ -0,0 +1,155 @@
+//@ edition: 2021
+//@ run-pass
+//@ ignore-backends: gcc
+
+#![feature(c_variadic)]
+#![feature(const_c_variadic)]
+#![feature(const_destruct)]
+#![feature(const_cmp)]
+#![feature(const_trait_impl)]
+
+use std::ffi::*;
+
+fn ignores_arguments() {
+    const unsafe extern "C" fn variadic(_: ...) {}
+
+    const {
+        unsafe { variadic() };
+        unsafe { variadic(1, 2, 3) };
+    }
+}
+
+fn echo() {
+    const unsafe extern "C" fn variadic(mut ap: ...) -> i32 {
+        ap.arg()
+    }
+
+    assert_eq!(unsafe { variadic(1) }, 1);
+    assert_eq!(unsafe { variadic(3, 2, 1) }, 3);
+
+    const {
+        assert!(unsafe { variadic(1) } == 1);
+        assert!(unsafe { variadic(3, 2, 1) } == 3);
+    }
+}
+
+fn forward_by_val() {
+    const unsafe fn helper(mut ap: VaList) -> i32 {
+        ap.arg()
+    }
+
+    const unsafe extern "C" fn variadic(ap: ...) -> i32 {
+        helper(ap)
+    }
+
+    assert_eq!(unsafe { variadic(1) }, 1);
+    assert_eq!(unsafe { variadic(3, 2, 1) }, 3);
+
+    const {
+        assert!(unsafe { variadic(1) } == 1);
+        assert!(unsafe { variadic(3, 2, 1) } == 3);
+    }
+}
+
+fn forward_by_ref() {
+    const unsafe fn helper(ap: &mut VaList) -> i32 {
+        ap.arg()
+    }
+
+    const unsafe extern "C" fn variadic(mut ap: ...) -> i32 {
+        helper(&mut ap)
+    }
+
+    assert_eq!(unsafe { variadic(1) }, 1);
+    assert_eq!(unsafe { variadic(3, 2, 1) }, 3);
+
+    const {
+        assert!(unsafe { variadic(1) } == 1);
+        assert!(unsafe { variadic(3, 2, 1) } == 3);
+    }
+}
+
+#[allow(improper_ctypes_definitions)]
+fn nested() {
+    const unsafe fn helper(mut ap1: VaList, mut ap2: VaList) -> (i32, i32) {
+        (ap1.arg(), ap2.arg())
+    }
+
+    const unsafe extern "C" fn variadic2(ap1: VaList, ap2: ...) -> (i32, i32) {
+        helper(ap1, ap2)
+    }
+
+    const unsafe extern "C" fn variadic1(ap1: ...) -> (i32, i32) {
+        variadic2(ap1, 2, 2)
+    }
+
+    assert_eq!(unsafe { variadic1(1) }, (1, 2));
+
+    const {
+        let (a, b) = unsafe { variadic1(1, 1) };
+
+        assert!(a != 2);
+        assert!(a == 1);
+        assert!(b != 1);
+        assert!(b == 2);
+    }
+}
+
+fn various_types() {
+    const unsafe extern "C" fn check_list_2(mut ap: ...) {
+        macro_rules! continue_if {
+            ($cond:expr) => {
+                if !($cond) {
+                    panic!();
+                }
+            };
+        }
+
+        const unsafe fn compare_c_str(ptr: *const c_char, val: &str) -> bool {
+            match CStr::from_ptr(ptr).to_str() {
+                Ok(cstr) => cstr == val,
+                Err(_) => panic!(),
+            }
+        }
+
+        continue_if!(ap.arg::<c_double>().floor() == 3.14f64.floor());
+        continue_if!(ap.arg::<c_long>() == 12);
+        continue_if!(ap.arg::<c_int>() == 'a' as c_int);
+        continue_if!(ap.arg::<c_double>().floor() == 6.18f64.floor());
+        continue_if!(compare_c_str(ap.arg::<*const c_char>(), "Hello"));
+        continue_if!(ap.arg::<c_int>() == 42);
+        continue_if!(compare_c_str(ap.arg::<*const c_char>(), "World"));
+    }
+
+    unsafe {
+        check_list_2(
+            3.14 as c_double,
+            12 as c_long,
+            b'a' as c_int,
+            6.28 as c_double,
+            c"Hello".as_ptr(),
+            42 as c_int,
+            c"World".as_ptr(),
+        );
+        const {
+            check_list_2(
+                3.14 as c_double,
+                12 as c_long,
+                b'a' as c_int,
+                6.28 as c_double,
+                c"Hello".as_ptr(),
+                42 as c_int,
+                c"World".as_ptr(),
+            )
+        };
+    }
+}
+
+fn main() {
+    ignores_arguments();
+    echo();
+    forward_by_val();
+    forward_by_ref();
+    nested();
+    various_types();
+}

tests/ui/feature-gates/feature-gate-const-c-variadic.rs

@@ -0,0 +1,11 @@
+#![feature(c_variadic)]
+
+fn main() {
+    const unsafe extern "C" fn foo(ap: ...) {
+        //~^ ERROR c-variadic const function definitions are unstable
+        core::mem::forget(ap);
+    }
+
+    const { unsafe { foo() } }
+    //~^ ERROR calling const c-variadic functions is unstable in constants
+}

tests/ui/feature-gates/feature-gate-const-c-variadic.stderr

@@ -0,0 +1,24 @@
+error[E0658]: c-variadic const function definitions are unstable
+  --> $DIR/feature-gate-const-c-variadic.rs:4:5
+   |
+LL |     const unsafe extern "C" fn foo(ap: ...) {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0015]: calling const c-variadic functions is unstable in constants
+  --> $DIR/feature-gate-const-c-variadic.rs:9:22
+   |
+LL |     const { unsafe { foo() } }
+   |                      ^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0015, E0658.
+For more information about an error, try `rustc --explain E0015`.

tests/ui/parser/variadic-ffi-semantic-restrictions.rs

@@ -31,18 +31,18 @@ extern "C" fn f3_3(_: ..., x: isize) {}
 //~^ ERROR `...` must be the last argument of a C-variadic function
 
 const unsafe extern "C" fn f4_1(x: isize, _: ...) {}
-//~^ ERROR functions cannot be both `const` and C-variadic
-//~| ERROR destructor of `VaList<'_>` cannot be evaluated at compile-time
+//~^ ERROR destructor of `VaList<'_>` cannot be evaluated at compile-time
+//~| ERROR c-variadic const function definitions are unstable
 
 const extern "C" fn f4_2(x: isize, _: ...) {}
-//~^ ERROR functions cannot be both `const` and C-variadic
-//~| ERROR functions with a C variable argument list must be unsafe
+//~^ ERROR functions with a C variable argument list must be unsafe
 //~| ERROR destructor of `VaList<'_>` cannot be evaluated at compile-time
+//~| ERROR c-variadic const function definitions are unstable
 
 const extern "C" fn f4_3(_: ..., x: isize, _: ...) {}
-//~^ ERROR functions cannot be both `const` and C-variadic
-//~| ERROR functions with a C variable argument list must be unsafe
+//~^ ERROR functions with a C variable argument list must be unsafe
 //~| ERROR `...` must be the last argument of a C-variadic function
+//~| ERROR c-variadic const function definitions are unstable
 
 extern "C" {
     fn e_f2(..., x: isize);
@@ -64,8 +64,8 @@ impl X {
     //~| ERROR `...` must be the last argument of a C-variadic function
     const fn i_f5(x: isize, _: ...) {}
     //~^ ERROR `...` is not supported for non-extern functions
-    //~| ERROR functions cannot be both `const` and C-variadic
     //~| ERROR destructor of `VaList<'_>` cannot be evaluated at compile-time
+    //~| ERROR c-variadic const function definitions are unstable
 }
 
 trait T {

tests/ui/parser/variadic-ffi-semantic-restrictions.stderr

@@ -80,17 +80,25 @@ error: `...` must be the last argument of a C-variadic function
 LL | extern "C" fn f3_3(_: ..., x: isize) {}
    |                    ^^^^^^
 
-error: functions cannot be both `const` and C-variadic
+error[E0658]: c-variadic const function definitions are unstable
   --> $DIR/variadic-ffi-semantic-restrictions.rs:33:1
    |
 LL | const unsafe extern "C" fn f4_1(x: isize, _: ...) {}
-   | ^^^^^ `const` because of this             ^^^^^^ C-variadic because of this
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: functions cannot be both `const` and C-variadic
+error[E0658]: c-variadic const function definitions are unstable
   --> $DIR/variadic-ffi-semantic-restrictions.rs:37:1
    |
 LL | const extern "C" fn f4_2(x: isize, _: ...) {}
-   | ^^^^^ `const` because of this      ^^^^^^ C-variadic because of this
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: functions with a C variable argument list must be unsafe
   --> $DIR/variadic-ffi-semantic-restrictions.rs:37:36
@@ -109,11 +117,15 @@ error: `...` must be the last argument of a C-variadic function
 LL | const extern "C" fn f4_3(_: ..., x: isize, _: ...) {}
    |                          ^^^^^^
 
-error: functions cannot be both `const` and C-variadic
+error[E0658]: c-variadic const function definitions are unstable
   --> $DIR/variadic-ffi-semantic-restrictions.rs:42:1
    |
 LL | const extern "C" fn f4_3(_: ..., x: isize, _: ...) {}
-   | ^^^^^ `const` because of this              ^^^^^^ C-variadic because of this
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: functions with a C variable argument list must be unsafe
   --> $DIR/variadic-ffi-semantic-restrictions.rs:42:44
@@ -176,13 +188,15 @@ LL |     fn i_f4(_: ..., x: isize, _: ...) {}
    |
    = help: only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
 
-error: functions cannot be both `const` and C-variadic
+error[E0658]: c-variadic const function definitions are unstable
   --> $DIR/variadic-ffi-semantic-restrictions.rs:65:5
    |
 LL |     const fn i_f5(x: isize, _: ...) {}
-   |     ^^^^^                   ^^^^^^ C-variadic because of this
-   |     |
-   |     `const` because of this
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #151787 <https://github.com/rust-lang/rust/issues/151787> for more information
+   = help: add `#![feature(const_c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: `...` is not supported for non-extern functions
   --> $DIR/variadic-ffi-semantic-restrictions.rs:65:29
@@ -243,6 +257,10 @@ LL | const unsafe extern "C" fn f4_1(x: isize, _: ...) {}
    |                                           ^        - value is dropped here
    |                                           |
    |                                           the destructor for this type cannot be evaluated in constant functions
+   |
+   = note: see issue #133214 <https://github.com/rust-lang/rust/issues/133214> for more information
+   = help: add `#![feature(const_destruct)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0493]: destructor of `VaList<'_>` cannot be evaluated at compile-time
   --> $DIR/variadic-ffi-semantic-restrictions.rs:37:36
@@ -251,6 +269,10 @@ LL | const extern "C" fn f4_2(x: isize, _: ...) {}
    |                                    ^        - value is dropped here
    |                                    |
    |                                    the destructor for this type cannot be evaluated in constant functions
+   |
+   = note: see issue #133214 <https://github.com/rust-lang/rust/issues/133214> for more information
+   = help: add `#![feature(const_destruct)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error[E0493]: destructor of `VaList<'_>` cannot be evaluated at compile-time
   --> $DIR/variadic-ffi-semantic-restrictions.rs:65:29
@@ -259,7 +281,12 @@ LL |     const fn i_f5(x: isize, _: ...) {}
    |                             ^        - value is dropped here
    |                             |
    |                             the destructor for this type cannot be evaluated in constant functions
+   |
+   = note: see issue #133214 <https://github.com/rust-lang/rust/issues/133214> for more information
+   = help: add `#![feature(const_destruct)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
 error: aborting due to 33 previous errors
 
-For more information about this error, try `rustc --explain E0493`.
+Some errors have detailed explanations: E0493, E0658.
+For more information about an error, try `rustc --explain E0493`.