nunoplopes · Oct 7, 2026 · Oct 6, 2026 · Oct 7, 2026 · Oct 7, 2026
diff --git a/cpp2rust/converter/converter.cpp b/cpp2rust/converter/converter.cpp
@@ -92,10 +92,14 @@ void Converter::EmitGlobalInits(Model model, std::string &out) {
   out += "}\n";
 }
 
-void Converter::EmitOpaqueRecords(std::string &out) {
+void Converter::EmitOpaqueRecords(Model model, std::string &out) {
   record_decls_.ForEachUndefined([&](const std::string &name) {
-    out += "#[derive(Clone, Copy, Default, ByteRepr, VaArg, FnPtrArg)]";
-    out += "#[byte_size(1)]";
+    if (model == Model::kRefCount) {
+      out += "#[derive(Clone, Copy, Default, ByteRepr, VaArg, FnPtrArg)]";
+      out += "#[byte_size(1)]";
+    } else {
+      out += "#[derive(Clone, Copy, Default, VaArg, FnPtrArg)]";
+    }
     out += "pub struct ";
     out += name;
     out += ";\n";
@@ -4213,7 +4217,7 @@ std::string Converter::GetRecordName(const clang::NamedDecl *decl) const {
 std::vector<const char *>
 Converter::GetStructAttributes(const clang::RecordDecl *decl) {
   if (decl->isUnion()) {
-    return {"Copy", "Clone", "FnPtrArg"};
+    return {"Copy", "Clone", "FnPtrArg", "VaArg"};
   }
 
   std::vector<const char *> struct_attrs;

diff --git a/cpp2rust/converter/converter.h b/cpp2rust/converter/converter.h
@@ -52,7 +52,7 @@ class Converter : public clang::RecursiveASTVisitor<Converter> {
 
   virtual void EmitFilePreamble();
 
-  static void EmitOpaqueRecords(std::string &out);
+  static void EmitOpaqueRecords(Model model, std::string &out);
   static void EmitGlobalInits(Model model, std::string &out);
 
   static void EmitVirtualMethods(std::string &out);

diff --git a/cpp2rust/converter/models/converter_refcount.cpp b/cpp2rust/converter/models/converter_refcount.cpp
@@ -703,6 +703,11 @@ void ConverterRefCount::EmitRustUnion(clang::RecordDecl *decl) {
   RuleRegistry::SetDerives(
       ctx_, ctx_.getCanonicalTagType(decl),
       std::vector<std::string>(attrs.begin(), attrs.end()));
+  StrCat("#[derive(");
+  for (auto *attr : attrs) {
+    StrCat(attr, ',');
+  }
+  StrCat(")]");
 
   auto size = ctx_.getTypeSizeInChars(ctx_.getCanonicalTagType(decl));
   StrCat(std::format(
@@ -2470,7 +2475,7 @@ ConverterRefCount::GetStructAttributes(const clang::RecordDecl *decl) {
   std::vector<const char *> attrs;
 
   if (decl->isUnion()) {
-    return attrs;
+    return {"VaArg", "FnPtrArg"};
   }
 
   if (RecordDerivesClone(decl)) {

diff --git a/cpp2rust/cpp2rust_lib.cpp b/cpp2rust/cpp2rust_lib.cpp
@@ -31,7 +31,7 @@ std::string TranspileSrc(std::string_view cc_code, Model model,
                                        rules_dir),
       cc_code, tool_args, std::filesystem::path(filename).filename().string(),
       filename.ends_with(".c") ? CLANG_C_COMPILER : CLANG_CXX_COMPILER);
-  Converter::EmitOpaqueRecords(rs_code);
+  Converter::EmitOpaqueRecords(model, rs_code);
   Converter::EmitVirtualMethods(rs_code);
   if (model == Model::kRefCount) {
     ConverterRefCount::EmitMethodsOnPtr(rs_code);
@@ -76,7 +76,7 @@ std::string TranspileDir(std::string_view build_dir, Model model,
   std::string rs_code;
   FrontendActionFactory factory(rs_code, model, rules_dir);
   Tool.run(&factory);
-  Converter::EmitOpaqueRecords(rs_code);
+  Converter::EmitOpaqueRecords(model, rs_code);
   Converter::EmitVirtualMethods(rs_code);
   if (model == Model::kRefCount) {
     ConverterRefCount::EmitMethodsOnPtr(rs_code);

diff --git a/tests/multi-file/opaque_forward_decl/out/unsafe/opaque_forward_decl.rs b/tests/multi-file/opaque_forward_decl/out/unsafe/opaque_forward_decl.rs
@@ -31,7 +31,6 @@ unsafe fn main_0() -> i32 {
 pub unsafe fn touch_0(mut c: *mut container) {
     &((*c).p);
 }
-#[derive(Clone, Copy, Default, ByteRepr, VaArg, FnPtrArg)]
-#[byte_size(1)]
+#[derive(Clone, Copy, Default, VaArg, FnPtrArg)]
 pub struct opaque;
 pub unsafe fn __cpp2rust_init_globals() {}
diff --git a/tests/unit/out/refcount/tag_vs_identifier_collision.rs b/tests/unit/out/refcount/tag_vs_identifier_collision.rs
@@ -26,7 +26,7 @@ pub struct point_struct {
     #[offset(4)]
     pub y: i32,
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(4)]
 pub struct point {
     #[offset(0)]
@@ -48,7 +48,7 @@ impl Default for point {
         }
     }
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(4)]
 pub struct slot_union {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_addrof_external.rs b/tests/unit/out/refcount/union_addrof_external.rs
@@ -29,7 +29,7 @@ impl Default for record {
         }
     }
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(128)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_basic.rs b/tests/unit/out/refcount/union_basic.rs
@@ -6,7 +6,7 @@ use std::io::prelude::*;
 use std::io::{Read, Seek, Write};
 use std::os::fd::AsFd;
 use std::rc::{Rc, Weak};
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(4)]
 pub struct basic {
     #[offset(0)]
@@ -28,7 +28,7 @@ impl Default for basic {
         }
     }
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(1)]
 pub struct empty {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_cross_arm_cast.rs b/tests/unit/out/refcount/union_cross_arm_cast.rs
@@ -49,7 +49,7 @@ impl Default for shape_b {
         }
     }
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(64)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_field_alignment.rs b/tests/unit/out/refcount/union_field_alignment.rs
@@ -6,7 +6,7 @@ use std::io::prelude::*;
 use std::io::{Read, Seek, Write};
 use std::os::fd::AsFd;
 use std::rc::{Rc, Weak};
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_flex_array_member.rs b/tests/unit/out/refcount/union_flex_array_member.rs
@@ -6,7 +6,7 @@ use std::io::prelude::*;
 use std::io::{Read, Seek, Write};
 use std::os::fd::AsFd;
 use std::rc::{Rc, Weak};
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_memset_memcpy.rs b/tests/unit/out/refcount/union_memset_memcpy.rs
@@ -46,7 +46,7 @@ impl Default for shape_b {
         }
     }
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(256)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_nested.rs b/tests/unit/out/refcount/union_nested.rs
@@ -23,7 +23,7 @@ impl Default for record {
         }
     }
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(128)]
 pub struct anon_0 {
     #[offset(0)]
@@ -52,7 +52,7 @@ pub struct inner {
     #[byte_size(128)]
     pub view: anon_0,
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(128)]
 pub struct anon_1 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_pointer_pun_address.rs b/tests/unit/out/refcount/union_pointer_pun_address.rs
@@ -34,7 +34,7 @@ fn main_0() -> i32 {
     assert!(((({ (out).to_any() } == { (a.as_pointer()).to_any() }) as i32) != 0));
     return 0;
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_pointer_pun_writethrough.rs b/tests/unit/out/refcount/union_pointer_pun_writethrough.rs
@@ -18,7 +18,7 @@ fn main_0() -> i32 {
     assert!(((((*x.borrow()) == 42_i64) as i32) != 0));
     return 0;
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_struct_dual_use.rs b/tests/unit/out/refcount/union_struct_dual_use.rs
@@ -17,7 +17,7 @@ pub struct Inner {
 pub fn sum_inner_0(mut i: Ptr<Inner>) -> i32 {
     return ({ i.with(|__s| __s.a) } + { i.with(|__s| __s.b) });
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(16)]
 pub struct anon_1 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_tagged_many_arms.rs b/tests/unit/out/refcount/union_tagged_many_arms.rs
@@ -12,7 +12,7 @@ pub const Tag_enum_T_NUM_U: Tag_enum = 1;
 pub const Tag_enum_T_TEXT: Tag_enum = 2;
 pub const Tag_enum_T_FLOAT: Tag_enum = 3;
 pub const Tag_enum_T_REF: Tag_enum = 4;
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_tagged_simple.rs b/tests/unit/out/refcount/union_tagged_simple.rs
@@ -9,7 +9,7 @@ use std::rc::{Rc, Weak};
 pub type Kind_enum = u32;
 pub const Kind_enum_KIND_NONE: Kind_enum = 0;
 pub const Kind_enum_KIND_DONE: Kind_enum = 1;
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_tagged_struct_arms.rs b/tests/unit/out/refcount/union_tagged_struct_arms.rs
@@ -47,7 +47,7 @@ pub struct anon_3 {
     #[offset(32)]
     pub width: i32,
 }
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(40)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/union_void_ptr_sized_deref.rs b/tests/unit/out/refcount/union_void_ptr_sized_deref.rs
@@ -10,7 +10,7 @@ pub type Width_enum = u32;
 pub const Width_enum_W_64: Width_enum = 0;
 pub const Width_enum_W_32: Width_enum = 1;
 pub const Width_enum_W_16: Width_enum = 2;
-#[derive(ByteRepr, DeepClone)]
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
 #[byte_size(8)]
 pub struct anon_0 {
     #[offset(0)]

diff --git a/tests/unit/out/refcount/va_arg_struct_ctx.rs b/tests/unit/out/refcount/va_arg_struct_ctx.rs
@@ -23,6 +23,38 @@ pub fn set_error_0(mut ctx: Ptr<context>, fmt: Ptr<i8>, __args: &[VaArg]) {
         field!(ctx, last_error).write(__rhs);
     }
 }
+#[derive(VaArg, FnPtrArg, ByteRepr, DeepClone)]
+#[byte_size(8)]
+pub struct value {
+    #[offset(0)]
+    #[byte_size(8)]
+    __bytes: Value<Box<[u8]>>,
+}
+impl value {
+    pub fn i(&self) -> Ptr<i32> {
+        (self.__bytes.as_pointer() as Ptr<u8>).reinterpret_cast()
+    }
+    pub fn l(&self) -> Ptr<i64> {
+        (self.__bytes.as_pointer() as Ptr<u8>).reinterpret_cast()
+    }
+}
+impl Default for value {
+    fn default() -> Self {
+        value {
+            __bytes: Rc::new(RefCell::new(Box::from([0u8; 8]))),
+        }
+    }
+}
+pub fn pick_1(use_long: i32, __args: &[VaArg]) -> i64 {
+    let use_long: Value<i32> = Rc::new(RefCell::new(use_long));
+    let ap: Value<VaList> = Rc::new(RefCell::new(VaList::default()));
+    (*ap.borrow_mut()) = VaList::new(__args);
+    let v: Value<value> = Rc::new(RefCell::new((*ap.borrow_mut()).arg::<value>()));
+    if ((*use_long.borrow()) != 0) {
+        return ((*v.borrow()).l().read());
+    }
+    return (((*v.borrow()).i().read()) as i64);
+}
 pub fn main() {
     __cpp2rust_init_globals();
     std::process::exit(main_0());
@@ -48,6 +80,11 @@ fn main_0() -> i32 {
         )
     });
     assert!(((({ (*ctx.borrow()).last_error } == 42) as i32) != 0));
+    let v: Value<value> = <Value<value>>::default();
+    (*v.borrow_mut()).l().write((1_i64 << 40));
+    assert!((((({ pick_1(1, &[((*v.borrow()).clone()).into(),]) }) == (1_i64 << 40)) as i32) != 0));
+    (*v.borrow_mut()).i().write(7);
+    assert!((((({ pick_1(0, &[((*v.borrow()).clone()).into(),]) }) == 7_i64) as i32) != 0));
     return 0;
 }
 pub fn __cpp2rust_init_globals() {}
diff --git a/tests/unit/out/unsafe/opaque_forward_decl.rs b/tests/unit/out/unsafe/opaque_forward_decl.rs
@@ -26,7 +26,6 @@ unsafe fn main_0() -> i32 {
     &(c.p);
     return ((c.x) - (42));
 }
-#[derive(Clone, Copy, Default, ByteRepr, VaArg, FnPtrArg)]
-#[byte_size(1)]
+#[derive(Clone, Copy, Default, VaArg, FnPtrArg)]
 pub struct opaque;
 pub unsafe fn __cpp2rust_init_globals() {}
diff --git a/tests/unit/out/unsafe/tag_vs_identifier_collision.rs b/tests/unit/out/unsafe/tag_vs_identifier_collision.rs
@@ -23,7 +23,7 @@ pub struct point_struct {
     pub y: i32,
 }
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union point {
     pub whole: i32,
     pub half: i16,
@@ -34,7 +34,7 @@ impl Default for point {
     }
 }
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union slot_union {
     pub i: i32,
     pub u: u32,

diff --git a/tests/unit/out/unsafe/union_addrof_external.rs b/tests/unit/out/unsafe/union_addrof_external.rs
@@ -15,7 +15,7 @@ pub struct record {
     pub pad: [libc::c_char; 8],
 }
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union anon_0 {
     pub h: record,
     pub raw_: [libc::c_char; 128],

diff --git a/tests/unit/out/unsafe/union_basic.rs b/tests/unit/out/unsafe/union_basic.rs
@@ -7,7 +7,7 @@ use std::io::{Read, Seek, Write};
 use std::os::fd::{AsFd, FromRawFd, IntoRawFd};
 use std::rc::Rc;
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union basic {
     pub i: i32,
     pub f: f32,
@@ -18,7 +18,7 @@ impl Default for basic {
     }
 }
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union empty {
     __empty: u8,
 }

diff --git a/tests/unit/out/unsafe/union_cross_arm_cast.rs b/tests/unit/out/unsafe/union_cross_arm_cast.rs
@@ -22,7 +22,7 @@ pub struct shape_b {
     pub tail: u32,
 }
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union anon_0 {
     pub a: shape_a,
     pub b: shape_b,

diff --git a/tests/unit/out/unsafe/union_field_alignment.rs b/tests/unit/out/unsafe/union_field_alignment.rs
@@ -7,7 +7,7 @@ use std::io::{Read, Seek, Write};
 use std::os::fd::{AsFd, FromRawFd, IntoRawFd};
 use std::rc::Rc;
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union anon_0 {
     pub bytes: [u8; 1],
     pub aligner: *mut ::libc::c_void,

diff --git a/tests/unit/out/unsafe/union_flex_array_member.rs b/tests/unit/out/unsafe/union_flex_array_member.rs
@@ -7,7 +7,7 @@ use std::io::{Read, Seek, Write};
 use std::os::fd::{AsFd, FromRawFd, IntoRawFd};
 use std::rc::Rc;
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union anon_0 {
     pub bytes: [u8; 1],
     pub aligner: *mut ::libc::c_void,

diff --git a/tests/unit/out/unsafe/union_memset_memcpy.rs b/tests/unit/out/unsafe/union_memset_memcpy.rs
@@ -21,7 +21,7 @@ pub struct shape_b {
     pub fill: [libc::c_char; 8],
 }
 #[repr(C)]
-#[derive(Copy, Clone, FnPtrArg)]
+#[derive(Copy, Clone, FnPtrArg, VaArg)]
 pub union anon_0 {
     pub a: shape_a,
     pub b: shape_b,