From e4d0a58980906e3ec3246bb759d4fd1379a89266 Mon Sep 17 00:00:00 2001
From: Jeong YunWon <jeong@youknowone.org>
Date: Wed, 10 Dec 2025 21:05:02 +0900
Subject: [PATCH] validate_downcast

---
 crates/vm/src/builtins/builtin_func.rs |  8 +-------
 crates/vm/src/builtins/str.rs          | 10 ++++------
 crates/vm/src/object/payload.rs        |  7 ++++++-
 3 files changed, 11 insertions(+), 14 deletions(-)

diff --git a/crates/vm/src/builtins/builtin_func.rs b/crates/vm/src/builtins/builtin_func.rs
index 8c160c2eb4b..d1ce107e374 100644
--- a/crates/vm/src/builtins/builtin_func.rs
+++ b/crates/vm/src/builtins/builtin_func.rs
@@ -148,7 +148,7 @@ impl Representable for PyNativeFunction {
 impl Unconstructible for PyNativeFunction {}
 
 // `PyCMethodObject` in CPython
-#[pyclass(name = "builtin_method", module = false, base = PyNativeFunction)]
+#[pyclass(name = "builtin_method", module = false, base = PyNativeFunction, ctx = "builtin_method_type")]
 pub struct PyNativeMethod {
     pub(crate) func: PyNativeFunction,
     pub(crate) class: &'static Py<PyType>, // TODO: the actual life is &'self
@@ -189,12 +189,6 @@ impl PyNativeMethod {
     }
 }
 
-impl PyPayload for PyNativeMethod {
-    fn class(ctx: &Context) -> &'static Py<PyType> {
-        ctx.types.builtin_method_type
-    }
-}
-
 impl fmt::Debug for PyNativeMethod {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         write!(
diff --git a/crates/vm/src/builtins/str.rs b/crates/vm/src/builtins/str.rs
index 80da626f318..9fa1472c26e 100644
--- a/crates/vm/src/builtins/str.rs
+++ b/crates/vm/src/builtins/str.rs
@@ -1932,12 +1932,10 @@ impl PyPayload for PyUtf8Str {
         std::any::TypeId::of::<PyStr>()
     }
 
-    fn downcastable_from(obj: &PyObject) -> bool {
-        obj.typeid() == Self::payload_type_id() && {
-            // SAFETY: we know the object is a PyStr in this context
-            let wtf8 = unsafe { obj.downcast_unchecked_ref::<PyStr>() };
-            wtf8.is_utf8()
-        }
+    fn validate_downcastable_from(obj: &PyObject) -> bool {
+        // SAFETY: we know the object is a PyStr in this context
+        let wtf8 = unsafe { obj.downcast_unchecked_ref::<PyStr>() };
+        wtf8.is_utf8()
     }
 
     fn try_downcast_from(obj: &PyObject, vm: &VirtualMachine) -> PyResult<()> {
diff --git a/crates/vm/src/object/payload.rs b/crates/vm/src/object/payload.rs
index 7fc40ddba90..b2211761492 100644
--- a/crates/vm/src/object/payload.rs
+++ b/crates/vm/src/object/payload.rs
@@ -25,7 +25,12 @@ pub trait PyPayload: MaybeTraverse + PyThreadingConstraint + Sized + 'static {
     /// # Safety: this function should only be called if `payload_type_id` matches the type of `obj`.
     #[inline]
     fn downcastable_from(obj: &PyObject) -> bool {
-        obj.typeid() == Self::payload_type_id()
+        obj.typeid() == Self::payload_type_id() && Self::validate_downcastable_from(obj)
+    }
+
+    #[inline]
+    fn validate_downcastable_from(_obj: &PyObject) -> bool {
+        true
     }
 
     fn try_downcast_from(obj: &PyObject, vm: &VirtualMachine) -> PyResult<()> {