RustPython · bschoenmaeckers · Jul 21, 2026
diff --git a/crates/derive-impl/src/pyclass.rs b/crates/derive-impl/src/pyclass.rs
@@ -1178,6 +1178,12 @@ where
                 quote_spanned! { span =>
                     slots.#slot_ident = Self::#ident().into();
                 }
+            } else if slot_name == "new" {
+                quote_spanned! { span =>
+                    slots.#slot_ident.store(Some(::rustpython_vm::types::NewFunc::Rust(
+                        Self::#ident as _
+                    )));
+                }
             } else {
                 quote_spanned! { span =>
                     slots.#slot_ident.store(Some(Self::#ident as _));
@@ -1896,7 +1902,9 @@ fn extract_impl_attrs(attr: PunctuatedNestedMeta, item: &Ident) -> Result<Extrac
                             }
                         } else if path.is_ident("Constructor") {
                             quote_spanned! { item_span =>
-                                slots.new.store(Some(<Self as ::rustpython_vm::types::Constructor>::slot_new as _));
+                                slots.new.store(Some(::rustpython_vm::types::NewFunc::Rust(
+                                    <Self as ::rustpython_vm::types::Constructor>::slot_new as _
+                                )));
                             }
                         } else {
                             quote_spanned! { item_span =>

diff --git a/crates/vm/src/builtins/bool.rs b/crates/vm/src/builtins/bool.rs
@@ -189,7 +189,11 @@ fn vectorcall_bool(
 ) -> PyResult {
     let zelf: &Py<PyType> = zelf_obj.downcast_ref().unwrap();
     let func_args = FuncArgs::from_vectorcall_owned(args, nargs, kwnames);
-    (zelf.slots.new.load().unwrap())(zelf.to_owned(), func_args, vm)
+    zelf.slots
+        .new
+        .load()
+        .unwrap()
+        .invoke(zelf.to_owned(), func_args, vm)
 }
 
 pub(crate) fn init(context: &'static Context) {

diff --git a/crates/vm/src/builtins/float.rs b/crates/vm/src/builtins/float.rs
@@ -552,7 +552,11 @@ fn vectorcall_float(
 ) -> PyResult {
     let zelf: &Py<PyType> = zelf_obj.downcast_ref().unwrap();
     let func_args = FuncArgs::from_vectorcall_owned(args, nargs, kwnames);
-    (zelf.slots.new.load().unwrap())(zelf.to_owned(), func_args, vm)
+    zelf.slots
+        .new
+        .load()
+        .unwrap()
+        .invoke(zelf.to_owned(), func_args, vm)
 }
 
 #[rustfmt::skip] // to avoid line splitting

diff --git a/crates/vm/src/builtins/int.rs b/crates/vm/src/builtins/int.rs
@@ -840,7 +840,11 @@ fn vectorcall_int(
 ) -> PyResult {
     let zelf: &Py<PyType> = zelf_obj.downcast_ref().unwrap();
     let func_args = FuncArgs::from_vectorcall_owned(args, nargs, kwnames);
-    (zelf.slots.new.load().unwrap())(zelf.to_owned(), func_args, vm)
+    zelf.slots
+        .new
+        .load()
+        .unwrap()
+        .invoke(zelf.to_owned(), func_args, vm)
 }
 
 pub(crate) fn init(context: &'static Context) {

diff --git a/crates/vm/src/builtins/set.rs b/crates/vm/src/builtins/set.rs
@@ -1464,7 +1464,11 @@ fn vectorcall_frozenset(
 ) -> PyResult {
     let zelf: &Py<PyType> = zelf_obj.downcast_ref().unwrap();
     let func_args = FuncArgs::from_vectorcall_owned(args, nargs, kwnames);
-    (zelf.slots.new.load().unwrap())(zelf.to_owned(), func_args, vm)
+    zelf.slots
+        .new
+        .load()
+        .unwrap()
+        .invoke(zelf.to_owned(), func_args, vm)
 }
 
 pub(crate) fn init(context: &'static Context) {

diff --git a/crates/vm/src/builtins/str.rs b/crates/vm/src/builtins/str.rs
@@ -1778,7 +1778,11 @@ fn vectorcall_str(
 ) -> PyResult {
     let zelf: &Py<PyType> = zelf_obj.downcast_ref().unwrap();
     let func_args = FuncArgs::from_vectorcall_owned(args, nargs, kwnames);
-    (zelf.slots.new.load().unwrap())(zelf.to_owned(), func_args, vm)
+    zelf.slots
+        .new
+        .load()
+        .unwrap()
+        .invoke(zelf.to_owned(), func_args, vm)
 }
 
 pub(crate) fn init(ctx: &'static Context) {

diff --git a/crates/vm/src/builtins/tuple.rs b/crates/vm/src/builtins/tuple.rs
@@ -665,7 +665,11 @@ fn vectorcall_tuple(
     let func_args = FuncArgs::from_vectorcall_owned(args, nargs, kwnames);
     // Use the type's own slot_new rather than calling PyTuple::slot_new directly,
     // so Rust-level subclasses (e.g. struct sequences) get their custom slot_new called.
-    (zelf.slots.new.load().unwrap())(zelf.to_owned(), func_args, vm)
+    zelf.slots
+        .new
+        .load()
+        .unwrap()
+        .invoke(zelf.to_owned(), func_args, vm)
 }
 
 pub(crate) fn init(context: &'static Context) {

diff --git a/crates/vm/src/builtins/type.rs b/crates/vm/src/builtins/type.rs
@@ -2133,7 +2133,7 @@ impl Constructor for PyType {
             let metatype = if !winner.is(&metatype) {
                 if let Some(ref slot_new) = winner.slots.new.load() {
                     // Pass it to the winner
-                    return slot_new(winner, args, vm);
+                    return slot_new.invoke(winner, args, vm);
                 }
                 winner
             } else {
@@ -2832,14 +2832,14 @@ impl Callable for PyType {
             // path incorrectly.
             if zelf.slots.init.load().is_none()
                 && !zelf.is(vm.ctx.types.type_type)
-                && slot_new as usize != crate::types::new_wrapper as crate::types::NewFunc as usize
+                && slot_new.identity() != crate::types::new_wrapper as *const () as usize
             {
-                return slot_new(zelf.to_owned(), args, vm);
+                return slot_new.invoke(zelf.to_owned(), args, vm);
             }
             args.clone()
         };
 
-        let obj = slot_new(zelf.to_owned(), args, vm)?;
+        let obj = slot_new.invoke(zelf.to_owned(), args, vm)?;
 
         if !obj.class().fast_issubclass(zelf) {
             return Ok(obj);
@@ -3069,7 +3069,7 @@ pub(crate) fn call_slot_new(
     // Check if staticbase's tp_new differs from typ's tp_new
     let typ_new = typ.slots.new.load();
     let staticbase_new = staticbase.slots.new.load();
-    if typ_new.map(|f| f as usize) != staticbase_new.map(|f| f as usize) {
+    if typ_new.map(|f| f.identity()) != staticbase_new.map(|f| f.identity()) {
         return Err(vm.new_type_error(format!(
             "{}.__new__({}) is not safe, use {}.__new__()",
             typ.slot_name(),
@@ -3083,7 +3083,7 @@ pub(crate) fn call_slot_new(
         .new
         .load()
         .expect("Should be able to find a new slot somewhere in the mro");
-    slot_new(subtype, args, vm)
+    slot_new.invoke(subtype, args, vm)
 }
 
 pub(crate) fn or_(zelf: PyObjectRef, other: PyObjectRef, vm: &VirtualMachine) -> PyResult {

diff --git a/crates/vm/src/class.rs b/crates/vm/src/class.rs
@@ -205,7 +205,7 @@ pub trait PyClassImpl: PyClassDef {
             let object_new = ctx.types.object_type.slots.new.load();
             let is_object_itself = core::ptr::eq(class, ctx.types.object_type);
             let is_inherited_from_object = !is_object_itself
-                && object_new.is_some_and(|obj_new| slot_new as usize == obj_new as usize);
+                && object_new.is_some_and(|obj_new| slot_new.identity() == obj_new.identity());
 
             if !is_inherited_from_object {
                 let bound_new =

diff --git a/crates/vm/src/frame.rs b/crates/vm/src/frame.rs
@@ -9240,7 +9240,7 @@ impl ExecutingFrame<'_> {
                 let cls_alloc = cls.slots.alloc.load();
                 if let (Some(cls_new_fn), Some(obj_new_fn), Some(cls_alloc_fn), Some(obj_alloc_fn)) =
                     (cls_new, object_new, cls_alloc, object_alloc)
-                    && cls_new_fn as usize == obj_new_fn as usize
+                    && cls_new_fn.identity() == obj_new_fn.identity()
                     && cls_alloc_fn as usize == obj_alloc_fn as usize
                 {
                     if type_version == 0 {

diff --git a/crates/vm/src/types/slot.rs b/crates/vm/src/types/slot.rs
@@ -310,10 +310,39 @@ pub(crate) type DescrGetFunc =
 pub(crate) type DescrSetFunc =
     fn(&PyObject, PyObjectRef, PySetterValue, &VirtualMachine) -> PyResult<()>;
 pub(crate) type AllocFunc = fn(PyTypeRef, usize, &VirtualMachine) -> PyResult;
-pub(crate) type NewFunc = fn(PyTypeRef, FuncArgs, &VirtualMachine) -> PyResult;
 pub(crate) type InitFunc = fn(PyObjectRef, FuncArgs, &VirtualMachine) -> PyResult<()>;
 pub(crate) type DelFunc = fn(&PyObject, &VirtualMachine) -> PyResult<()>;
 
+#[derive(Debug, Clone, Copy)]
+pub enum NewFunc {
+    Rust(fn(PyTypeRef, FuncArgs, &VirtualMachine) -> PyResult),
+    C(unsafe extern "C" fn(*mut PyObject, *mut PyObject, *mut PyObject) -> *mut PyObject),
+}
+
+impl NewFunc {
+    #[must_use]
+    pub fn identity(self) -> usize {
+        match self {
+            Self::Rust(func) => func as usize,
+            Self::C(func) => func as usize,
+        }
+    }
+
+    pub fn invoke(self, cls: PyTypeRef, args: FuncArgs, vm: &VirtualMachine) -> PyResult {
+        match self {
+            Self::Rust(func) => func(cls, args, vm),
+            Self::C(func) => {
+                todo!(
+                    "call C new function: {:?} with args: {:?} {:?}",
+                    func,
+                    cls,
+                    args
+                )
+            }
+        }
+    }
+}
+
 // Sequence sub-slot function types
 pub(crate) type SeqLenFunc = fn(PySequence<'_>, &VirtualMachine) -> PyResult<usize>;
 pub(crate) type SeqConcatFunc = fn(PySequence<'_>, &PyObject, &VirtualMachine) -> PyResult;
@@ -877,12 +906,12 @@ impl PyType {
                         .iter()
                         .find(|cls| cls.attributes.read().contains_key(name))
                         .is_some_and(|cls| {
-                            cls.slots.new.load().map(|f| f as usize)
-                                == Some(new_wrapper as NewFunc as usize)
+                            cls.slots.new.load().map(NewFunc::identity)
+                                == Some(NewFunc::Rust(new_wrapper as _).identity())
                         })
                 };
                 if needs_wrapper {
-                    self.slots.new.store(Some(new_wrapper));
+                    self.slots.new.store(Some(NewFunc::Rust(new_wrapper as _)));
                     self.slots.vectorcall.store(None);
                 } else {
                     let inherited = self.base.deref().and_then(|base| base.slots.new.load());