Use PyBorrowFlagLayout to ensure the baseclass has a borrow flag

pyo3-derive-backend/src/pyclass.rs

@@ -362,7 +362,7 @@ fn impl_class(
         quote! { 0 }
     };
     let base_layout = if attr.has_extends {
-        quote! { <Self::BaseType as pyo3::derive_utils::PyBaseTypeUtils>::Layout }
+        quote! { <Self::BaseType as pyo3::derive_utils::PyBaseTypeUtils>::LayoutAsBase }
     } else {
         quote! { pyo3::pycell::PyCellBase<pyo3::types::PyAny> }
     };

src/derive_utils.rs

@@ -223,13 +223,13 @@ impl GetPropertyValue for PyObject {
 pub trait PyBaseTypeUtils {
     type Dict;
     type WeakRef;
-    type Layout;
+    type LayoutAsBase;
     type BaseNativeType;
 }
 
 impl<T: PyClass> PyBaseTypeUtils for T {
     type Dict = T::Dict;
     type WeakRef = T::WeakRef;
-    type Layout = crate::pycell::PyCellInner<T>;
+    type LayoutAsBase = crate::pycell::PyCellInner<T>;
     type BaseNativeType = T::BaseNativeType;
 }

src/freelist.rs

@@ -4,7 +4,7 @@
 
 use crate::ffi;
 use crate::pyclass::{tp_free_fallback, PyClassAlloc};
-use crate::type_object::{PyObjectLayout, PyTypeInfo};
+use crate::type_object::{PyLayout, PyTypeInfo};
 use crate::Python;
 use std::mem;
 use std::os::raw::c_void;

src/instance.rs

@@ -3,7 +3,7 @@ use crate::err::{PyErr, PyResult};
 use crate::gil;
 use crate::object::PyObject;
 use crate::objectprotocol::ObjectProtocol;
-use crate::type_object::PyDowncastImpl;
+use crate::type_object::{PyBorrowFlagLayout, PyDowncastImpl};
 use crate::types::PyAny;
 use crate::{
     ffi, AsPyPointer, FromPyObject, IntoPy, IntoPyPointer, PyCell, PyClass, PyClassInitializer,
@@ -39,7 +39,7 @@ impl<T> Py<T> {
     pub fn new(py: Python, value: impl Into<PyClassInitializer<T>>) -> PyResult<Py<T>>
     where
         T: PyClass,
-        T::BaseLayout: crate::type_object::PyObjectSizedLayout<T::BaseType>,
+        T::BaseLayout: PyBorrowFlagLayout<T::BaseType>,
     {
         let initializer = value.into();
         let obj = unsafe { initializer.create_cell(py)? };

src/pycell.rs

@@ -2,7 +2,7 @@
 use crate::conversion::{AsPyPointer, FromPyPointer, ToPyObject};
 use crate::pyclass_init::PyClassInitializer;
 use crate::pyclass_slots::{PyClassDict, PyClassWeakRef};
-use crate::type_object::{PyDowncastImpl, PyObjectLayout, PyObjectSizedLayout, PyTypeInfo};
+use crate::type_object::{PyBorrowFlagLayout, PyDowncastImpl, PyLayout, PySizedLayout, PyTypeInfo};
 use crate::types::PyAny;
 use crate::{ffi, FromPy, PyClass, PyErr, PyNativeType, PyObject, PyResult, Python};
 use std::cell::{Cell, UnsafeCell};
@@ -17,10 +17,10 @@ pub struct PyCellBase<T: PyTypeInfo> {
     borrow_flag: Cell<BorrowFlag>,
 }
 
-unsafe impl<T> PyObjectLayout<T> for PyCellBase<T>
+unsafe impl<T> PyLayout<T> for PyCellBase<T>
 where
     T: PyTypeInfo + PyNativeType,
-    T::Layout: PyObjectSizedLayout<T>,
+    T::Layout: PySizedLayout<T>,
 {
     const IS_NATIVE_TYPE: bool = true;
     unsafe fn unchecked_ref(&self) -> &T {
@@ -31,11 +31,18 @@ where
     }
 }
 
-// This impl ensures `PyCellBase` can be a base type.
-impl<T> PyObjectSizedLayout<T> for PyCellBase<T>
+// Thes impls ensures `PyCellBase` can be a base type.
+impl<T> PySizedLayout<T> for PyCellBase<T>
 where
     T: PyTypeInfo + PyNativeType,
-    T::Layout: PyObjectSizedLayout<T>,
+    T::Layout: PySizedLayout<T>,
+{
+}
+
+unsafe impl<T> PyBorrowFlagLayout<T> for PyCellBase<T>
+where
+    T: PyTypeInfo + PyNativeType,
+    T::Layout: PySizedLayout<T>,
 {
 }
 
@@ -56,7 +63,7 @@ impl<T: PyClass> AsPyPointer for PyCellInner<T> {
     }
 }
 
-unsafe impl<T: PyClass> PyObjectLayout<T> for PyCellInner<T> {
+unsafe impl<T: PyClass> PyLayout<T> for PyCellInner<T> {
     const IS_NATIVE_TYPE: bool = false;
     fn get_super_or(&mut self) -> Option<&mut T::BaseLayout> {
         Some(&mut self.ob_base)
@@ -76,8 +83,9 @@ unsafe impl<T: PyClass> PyObjectLayout<T> for PyCellInner<T> {
     }
 }
 
-// This impl ensures `PyCellInner` can be a base type.
-impl<T: PyClass> PyObjectSizedLayout<T> for PyCellInner<T> {}
+// Thes impls ensures `PyCellInner` can be a base type.
+impl<T: PyClass> PySizedLayout<T> for PyCellInner<T> {}
+unsafe impl<T: PyClass> PyBorrowFlagLayout<T> for PyCellInner<T> {}
 
 impl<T: PyClass> PyCellInner<T> {
     fn get_borrow_flag(&self) -> BorrowFlag {
@@ -164,7 +172,7 @@ impl<T: PyClass> PyCell<T> {
     ///
     pub fn new(py: Python, value: impl Into<PyClassInitializer<T>>) -> PyResult<&Self>
     where
-        T::BaseLayout: crate::type_object::PyObjectSizedLayout<T::BaseType>,
+        T::BaseLayout: PyBorrowFlagLayout<T::BaseType>,
     {
         unsafe {
             let initializer = value.into();
@@ -325,9 +333,12 @@ impl<T: PyClass> PyCell<T> {
         std::mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
     }
 
+    /// Allocates new PyCell without initilizing value.
+    /// Requires `T::BaseLayout: PyBorrowFlagLayout<T::BaseType>` to ensure that
+    /// this layout has a borrow flag.
     pub(crate) unsafe fn internal_new(py: Python) -> PyResult<*mut Self>
     where
-        T::BaseLayout: crate::type_object::PyObjectSizedLayout<T::BaseType>,
+        T::BaseLayout: PyBorrowFlagLayout<T::BaseType>,
     {
         let base = T::alloc(py);
         if base.is_null() {
@@ -342,7 +353,7 @@ impl<T: PyClass> PyCell<T> {
     }
 }
 
-unsafe impl<T: PyClass> PyObjectLayout<T> for PyCell<T> {
+unsafe impl<T: PyClass> PyLayout<T> for PyCell<T> {
     const IS_NATIVE_TYPE: bool = false;
     fn get_super_or(&mut self) -> Option<&mut T::BaseLayout> {
         Some(&mut self.inner.ob_base)
@@ -598,6 +609,7 @@ impl BorrowFlag {
     }
 }
 
+/// An error returned by [`PyCell::try_borrow`](struct.PyCell.html#method.try_borrow).
 pub struct PyBorrowError {
     _private: (),
 }
@@ -614,6 +626,7 @@ impl fmt::Display for PyBorrowError {
     }
 }
 
+/// An error returned by [`PyCell::try_borrow_mut`](struct.PyCell.html#method.try_borrow_mut).
 pub struct PyBorrowMutError {
     _private: (),
 }

src/pyclass.rs

@@ -1,7 +1,7 @@
 //! `PyClass` trait
 use crate::class::methods::{PyMethodDefType, PyMethodsProtocol};
 use crate::pyclass_slots::{PyClassDict, PyClassWeakRef};
-use crate::type_object::{type_flags, PyObjectLayout};
+use crate::type_object::{type_flags, PyLayout};
 use crate::{class, ffi, gil, PyCell, PyErr, PyNativeType, PyResult, PyTypeInfo, Python};
 use std::ffi::CString;
 use std::os::raw::c_void;

src/pyclass_init.rs

@@ -1,5 +1,5 @@
 //! Initialization utilities for `#[pyclass]`.
-use crate::type_object::{PyObjectLayout, PyObjectSizedLayout, PyTypeInfo};
+use crate::type_object::{PyBorrowFlagLayout, PyLayout, PySizedLayout, PyTypeInfo};
 use crate::{PyCell, PyClass, PyResult, Python};
 use std::marker::PhantomData;
 
@@ -8,15 +8,15 @@ use std::marker::PhantomData;
 /// This trait is intended to use internally for distinguishing `#[pyclass]` and
 /// Python native types.
 pub trait PyObjectInit<T: PyTypeInfo>: Sized {
-    fn init_class<L: PyObjectLayout<T>>(self, layout: &mut L);
+    fn init_class<L: PyLayout<T>>(self, layout: &mut L);
     private_decl! {}
 }
 
 /// Initializer for Python native type, like `PyDict`.
 pub struct PyNativeTypeInitializer<T: PyTypeInfo>(PhantomData<T>);
 
 impl<T: PyTypeInfo> PyObjectInit<T> for PyNativeTypeInitializer<T> {
-    fn init_class<L: PyObjectLayout<T>>(self, _layout: &mut L) {}
+    fn init_class<L: PyLayout<T>>(self, _layout: &mut L) {}
     private_impl! {}
 }
 
@@ -108,17 +108,18 @@ impl<T: PyClass> PyClassInitializer<T> {
     pub fn add_subclass<S>(self, subclass_value: S) -> PyClassInitializer<S>
     where
         S: PyClass + PyTypeInfo<BaseType = T>,
-        S::BaseLayout: PyObjectSizedLayout<T>,
+        S::BaseLayout: PySizedLayout<T>,
         S::BaseType: PyTypeInfo<Initializer = Self>,
     {
         PyClassInitializer::new(subclass_value, self)
     }
 
+    // Create a new PyCell + initialize it
     #[doc(hidden)]
     pub unsafe fn create_cell(self, py: Python) -> PyResult<*mut PyCell<T>>
     where
         T: PyClass,
-        T::BaseLayout: PyObjectSizedLayout<T::BaseType>,
+        T::BaseLayout: PyBorrowFlagLayout<T::BaseType>,
     {
         let cell = PyCell::internal_new(py)?;
         self.init_class(&mut *cell);
@@ -127,7 +128,7 @@ impl<T: PyClass> PyClassInitializer<T> {
 }
 
 impl<T: PyClass> PyObjectInit<T> for PyClassInitializer<T> {
-    fn init_class<L: PyObjectLayout<T>>(self, layout: &mut L) {
+    fn init_class<L: PyLayout<T>>(self, layout: &mut L) {
         let Self { init, super_init } = self;
         unsafe {
             layout.py_init(init);
@@ -152,7 +153,7 @@ where
 impl<S, B> From<(S, B)> for PyClassInitializer<S>
 where
     S: PyClass + PyTypeInfo<BaseType = B>,
-    S::BaseLayout: PyObjectSizedLayout<B>,
+    S::BaseLayout: PySizedLayout<B>,
     B: PyClass + PyTypeInfo<Initializer = PyClassInitializer<B>>,
     B::BaseType: PyTypeInfo<Initializer = PyNativeTypeInitializer<B::BaseType>>,
 {

src/type_object.rs

@@ -9,12 +9,12 @@ use std::cell::UnsafeCell;
 use std::ptr::NonNull;
 use std::sync::atomic::{AtomicBool, Ordering};
 
-/// `T: PyObjectLayout<U>` represents that `T` is a concrete representaion of `U` in Python heap.
+/// `T: PyLayout<U>` represents that `T` is a concrete representaion of `U` in Python heap.
 /// E.g., `PyCell` is a concrete representaion of all `pyclass`es, and `ffi::PyObject`
 /// is of `PyAny`.
 ///
 /// This trait is intended to be used internally.
-pub unsafe trait PyObjectLayout<T: PyTypeInfo> {
+pub unsafe trait PyLayout<T: PyTypeInfo> {
     const IS_NATIVE_TYPE: bool = true;
     fn get_super_or(&mut self) -> Option<&mut T::BaseLayout> {
         None
@@ -26,12 +26,14 @@ pub unsafe trait PyObjectLayout<T: PyTypeInfo> {
     unsafe fn unchecked_mut(&self) -> &mut T;
 }
 
-/// `T: PyObjectSizedLayout<U>` represents `T` is not a instance of
+/// `T: PySizedLayout<U>` represents `T` is not a instance of
 /// [`PyVarObject`](https://docs.python.org/3.8/c-api/structures.html?highlight=pyvarobject#c.PyVarObject).
 /// , in addition that `T` is a concrete representaion of `U`.
-///
-/// `pyclass`es need this trait for their base class.
-pub trait PyObjectSizedLayout<T: PyTypeInfo>: PyObjectLayout<T> + Sized {}
+pub trait PySizedLayout<T: PyTypeInfo>: PyLayout<T> + Sized {}
+
+/// Marker type indicates that `Self` can be a baselayout of PyClass.
+/// This trait assumes a certain layout and thus is unsafe.
+pub unsafe trait PyBorrowFlagLayout<T: PyTypeInfo>: PyLayout<T> + Sized {}
 
 /// Our custom type flags
 #[doc(hidden)]
@@ -99,10 +101,10 @@ pub unsafe trait PyTypeInfo: Sized {
     type BaseType: PyTypeInfo + PyTypeObject;
 
     /// Layout
-    type Layout: PyObjectLayout<Self>;
+    type Layout: PyLayout<Self>;
 
     /// Layout of Basetype.
-    type BaseLayout: PyObjectLayout<Self::BaseType>;
+    type BaseLayout: PySizedLayout<Self::BaseType>;
 
     /// Initializer for layout
     type Initializer: PyObjectInit<Self>;

src/types/any.rs

@@ -23,15 +23,15 @@ use crate::{ffi, PyObject};
 /// ```
 #[repr(transparent)]
 pub struct PyAny(PyObject, Unsendable);
-unsafe impl crate::type_object::PyObjectLayout<PyAny> for ffi::PyObject {
+unsafe impl crate::type_object::PyLayout<PyAny> for ffi::PyObject {
     unsafe fn unchecked_ref(&self) -> &PyAny {
         &*((&self) as *const &Self as *const _)
     }
     unsafe fn unchecked_mut(&self) -> &mut PyAny {
         &mut *((&self) as *const &Self as *const _ as *mut _)
     }
 }
-impl crate::type_object::PyObjectSizedLayout<PyAny> for ffi::PyObject {}
+impl crate::type_object::PySizedLayout<PyAny> for ffi::PyObject {}
 pyobject_native_type_named!(PyAny);
 pyobject_native_type_convert!(
     PyAny,

src/types/mod.rs

@@ -58,7 +58,7 @@ macro_rules! pyobject_native_type_named (
 
 macro_rules! impl_layout {
     ($name: ty, $layout: path) => {
-        unsafe impl $crate::type_object::PyObjectLayout<$name> for $layout {
+        unsafe impl $crate::type_object::PyLayout<$name> for $layout {
             unsafe fn unchecked_ref(&self) -> &$name {
                 &*((&self) as *const &Self as *const _)
             }
@@ -73,11 +73,11 @@ macro_rules! impl_layout {
 macro_rules! pyobject_native_type {
     ($name: ty, $layout: path, $typeobject: expr, $module: expr, $checkfunction: path $(,$type_param: ident)*) => {
         impl_layout!($name, $layout);
-        impl $crate::type_object::PyObjectSizedLayout<$name> for $layout {}
+        impl $crate::type_object::PySizedLayout<$name> for $layout {}
         impl $crate::derive_utils::PyBaseTypeUtils for $name {
             type Dict = $crate::pyclass_slots::PyClassDummySlot;
             type WeakRef = $crate::pyclass_slots::PyClassDummySlot;
-            type Layout = $crate::pycell::PyCellBase<$name>;
+            type LayoutAsBase = $crate::pycell::PyCellBase<$name>;
             type BaseNativeType = $name;
         }
         pyobject_native_type_named!($name $(,$type_param)*);