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Make validate_downcastable_from unsafe #6851

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Merged
youknowone merged 1 commit into from
Jan 26, 2026
Merged

Make validate_downcastable_from unsafe #6851

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2 changes: 1 addition & 1 deletion crates/derive-impl/src/pyclass.rs
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Expand Up @@ -667,7 +667,7 @@ pub(crate) fn impl_pyclass(attr: PunctuatedNestedMeta, item: Item) -> Result<Tok
const PAYLOAD_TYPE_ID: ::core::any::TypeId = <#base_type as ::rustpython_vm::PyPayload>::PAYLOAD_TYPE_ID;

#[inline]
fn validate_downcastable_from(obj: &::rustpython_vm::PyObject) -> bool {
unsafe fn validate_downcastable_from(obj: &::rustpython_vm::PyObject) -> bool {
<Self as ::rustpython_vm::class::PyClassDef>::BASICSIZE <= obj.class().slots.basicsize && obj.class().fast_issubclass(<Self as ::rustpython_vm::class::StaticType>::static_type())
}

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2 changes: 1 addition & 1 deletion crates/derive-impl/src/pystructseq.rs
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Expand Up @@ -595,7 +595,7 @@ pub(crate) fn impl_pystruct_sequence(
const PAYLOAD_TYPE_ID: ::core::any::TypeId = <::rustpython_vm::builtins::PyTuple as ::rustpython_vm::PyPayload>::PAYLOAD_TYPE_ID;

#[inline]
fn validate_downcastable_from(obj: &::rustpython_vm::PyObject) -> bool {
unsafe fn validate_downcastable_from(obj: &::rustpython_vm::PyObject) -> bool {
obj.class().fast_issubclass(<Self as ::rustpython_vm::class::StaticType>::static_type())
}

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2 changes: 1 addition & 1 deletion crates/vm/src/builtins/str.rs
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Expand Up @@ -1944,7 +1944,7 @@ impl PyPayload for PyUtf8Str {

const PAYLOAD_TYPE_ID: core::any::TypeId = core::any::TypeId::of::<PyStr>();

fn validate_downcastable_from(obj: &PyObject) -> bool {
unsafe fn validate_downcastable_from(obj: &PyObject) -> bool {
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@youknowone youknowone Jan 24, 2026

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huh, every other parts are safe but this one was unsafe

// SAFETY: we know the object is a PyStr in this context
let wtf8 = unsafe { obj.downcast_unchecked_ref::<PyStr>() };
wtf8.is_utf8()
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2 changes: 1 addition & 1 deletion crates/vm/src/object/core.rs
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Expand Up @@ -753,7 +753,7 @@ impl PyObject {
/// Check if this object can be downcast to T.
#[inline(always)]
pub fn downcastable<T: PyPayload>(&self) -> bool {
T::downcastable_from(self)
self.typeid() == T::PAYLOAD_TYPE_ID && unsafe { T::validate_downcastable_from(self) }
}

/// Attempt to downcast this reference to a subclass.
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12 changes: 4 additions & 8 deletions crates/vm/src/object/payload.rs
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Expand Up @@ -28,19 +28,15 @@ pub(crate) fn cold_downcast_type_error(
pub trait PyPayload: MaybeTraverse + PyThreadingConstraint + Sized + 'static {
const PAYLOAD_TYPE_ID: core::any::TypeId = core::any::TypeId::of::<Self>();

/// # Safety: this function should only be called if `payload_type_id` matches the type of `obj`.
/// # 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 && Self::validate_downcastable_from(obj)
}

#[inline]
fn validate_downcastable_from(_obj: &PyObject) -> bool {
unsafe fn validate_downcastable_from(_obj: &PyObject) -> bool {
true
}

fn try_downcast_from(obj: &PyObject, vm: &VirtualMachine) -> PyResult<()> {
if Self::downcastable_from(obj) {
if obj.downcastable::<Self>() {
return Ok(());
}

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