From 7520b49ac1517755e2f54c6fc88ad9e8fc6543cd Mon Sep 17 00:00:00 2001
From: mejrs <brunokolenbrander@hotmail.com>
Date: Mon, 22 Nov 2021 09:26:34 +0100
Subject: [PATCH] Implement opt-in immutable pyclasses

---
 guide/src/class.md                            |  49 ++--
 pyo3-macros-backend/src/pyclass.rs            |  62 ++++-
 src/class/basic.rs                            |  12 +-
 src/class/buffer.rs                           |   4 +-
 src/class/gc.rs                               |   4 +-
 src/class/mapping.rs                          |   6 +-
 src/class/number.rs                           |  30 +--
 src/class/sequence.rs                         |  10 +-
 src/conversion.rs                             |   5 +-
 src/impl_/pyclass.rs                          |  21 +-
 src/instance.rs                               |  16 +-
 src/pycell.rs                                 | 230 +++++++++++++-----
 src/pyclass.rs                                |   8 +-
 src/pyclass_init.rs                           |  17 +-
 src/types/mod.rs                              |   4 +-
 tests/test_compile_error.rs                   |   4 +
 tests/ui/invalid_immutable_pyclass_borrow.rs  |  13 +
 .../invalid_immutable_pyclass_borrow.stderr   |   5 +
 tests/ui/pyclass_send.stderr                  |  12 +
 19 files changed, 364 insertions(+), 148 deletions(-)
 create mode 100644 tests/ui/invalid_immutable_pyclass_borrow.rs
 create mode 100644 tests/ui/invalid_immutable_pyclass_borrow.stderr

diff --git a/guide/src/class.md b/guide/src/class.md
index fdc5fc26..1156ae1c 100644
--- a/guide/src/class.md
+++ b/guide/src/class.md
@@ -932,36 +932,41 @@ The `#[pyclass]` macro expands to roughly the code seen below. The `PyClassImplC
 # #[cfg(not(feature = "multiple-pymethods"))] {
 # use pyo3::prelude::*;
 // Note: the implementation differs slightly with the `multiple-pymethods` feature enabled.
-
-/// Class for demonstration
 struct MyClass {
     # #[allow(dead_code)]
     num: i32,
 }
-
-unsafe impl pyo3::PyTypeInfo for MyClass {
-    type AsRefTarget = PyCell<Self>;
-
+unsafe impl ::pyo3::type_object::PyTypeInfo for MyClass {
+    type AsRefTarget = ::pyo3::PyCell<Self>;
     const NAME: &'static str = "MyClass";
-    const MODULE: Option<&'static str> = None;
-
+    const MODULE: ::std::option::Option<&'static str> = ::std::option::Option::None;
     #[inline]
-    fn type_object_raw(py: pyo3::Python) -> *mut pyo3::ffi::PyTypeObject {
-        use pyo3::type_object::LazyStaticType;
+    fn type_object_raw(py: ::pyo3::Python<'_>) -> *mut ::pyo3::ffi::PyTypeObject {
+        use ::pyo3::type_object::LazyStaticType;
         static TYPE_OBJECT: LazyStaticType = LazyStaticType::new();
         TYPE_OBJECT.get_or_init::<Self>(py)
     }
 }
 
-impl pyo3::pyclass::PyClass for MyClass {
-    type Dict = pyo3::impl_::pyclass::PyClassDummySlot;
-    type WeakRef = pyo3::impl_::pyclass::PyClassDummySlot;
-    type BaseNativeType = PyAny;
+impl ::pyo3::PyClass for MyClass {
+    type Dict = ::pyo3::impl_::pyclass::PyClassDummySlot;
+    type WeakRef = ::pyo3::impl_::pyclass::PyClassDummySlot;
+    type BaseNativeType = ::pyo3::PyAny;
 }
 
-impl pyo3::IntoPy<PyObject> for MyClass {
-    fn into_py(self, py: pyo3::Python) -> pyo3::PyObject {
-        pyo3::IntoPy::into_py(pyo3::Py::new(py, self).unwrap(), py)
+unsafe impl ::pyo3::pyclass::MutablePyClass for MyClass {}
+
+impl<'a> ::pyo3::derive_utils::ExtractExt<'a> for &'a mut MyClass {
+    type Target = ::pyo3::PyRefMut<'a, MyClass>;
+}
+
+impl<'a> ::pyo3::derive_utils::ExtractExt<'a> for &'a MyClass {
+    type Target = ::pyo3::PyRef<'a, MyClass>;
+}
+
+impl ::pyo3::IntoPy<::pyo3::PyObject> for MyClass {
+    fn into_py(self, py: ::pyo3::Python) -> ::pyo3::PyObject {
+        ::pyo3::IntoPy::into_py(::pyo3::Py::new(py, self).unwrap(), py)
     }
 }
 
@@ -973,6 +978,7 @@ impl pyo3::impl_::pyclass::PyClassImpl for MyClass {
     type Layout = PyCell<MyClass>;
     type BaseType = PyAny;
     type ThreadChecker = pyo3::impl_::pyclass::ThreadCheckerStub<MyClass>;
+    type Mutabilty = pyo3::pyclass::Mutable;
 
     fn for_all_items(visitor: &mut dyn FnMut(&pyo3::impl_::pyclass::PyClassItems)) {
         use pyo3::impl_::pyclass::*;
@@ -997,6 +1003,15 @@ impl pyo3::impl_::pyclass::PyClassImpl for MyClass {
         collector.free_impl()
     }
 }
+
+impl ::pyo3::impl_::pyclass::PyClassDescriptors<MyClass>
+    for ::pyo3::impl_::pyclass::PyClassImplCollector<MyClass>
+{
+    fn py_class_descriptors(self) -> &'static [::pyo3::class::methods::PyMethodDefType] {
+        static METHODS: &[::pyo3::class::methods::PyMethodDefType] = &[];
+        METHODS
+    }
+}
 # Python::with_gil(|py| {
 #     let cls = py.get_type::<MyClass>();
 #     pyo3::py_run!(py, cls, "assert cls.__name__ == 'MyClass'")
diff --git a/pyo3-macros-backend/src/pyclass.rs b/pyo3-macros-backend/src/pyclass.rs
index 18c29957..c233900e 100644
--- a/pyo3-macros-backend/src/pyclass.rs
+++ b/pyo3-macros-backend/src/pyclass.rs
@@ -35,6 +35,7 @@ pub struct PyClassArgs {
     pub has_unsendable: bool,
     pub module: Option<syn::LitStr>,
     pub class_kind: PyClassKind,
+    pub is_immutable: bool,
 }
 
 impl PyClassArgs {
@@ -67,6 +68,7 @@ impl PyClassArgs {
             is_basetype: false,
             has_extends: false,
             has_unsendable: false,
+            is_immutable: false,
             class_kind,
         }
     }
@@ -176,6 +178,9 @@ impl PyClassArgs {
             "unsendable" => {
                 self.has_unsendable = true;
             }
+            "immutable" => {
+                self.is_immutable = true;
+            }
             _ => bail_spanned!(
                 exp.path.span() => "expected one of gc/weakref/subclass/dict/unsendable"
             ),
@@ -587,12 +592,29 @@ fn impl_enum_class(
     let default_items =
         gen_default_items(cls, vec![default_repr_impl, default_richcmp, default_int]);
 
+    let mutability = if args.is_immutable {
+        quote! {
+            unsafe impl _pyo3::pyclass::ImmutablePyClass for #cls {}
+        }
+    } else {
+        quote! {
+            unsafe impl _pyo3::pyclass::MutablePyClass for #cls {}
+
+            impl<'a> _pyo3::derive_utils::ExtractExt<'a> for &'a mut #cls
+            {
+                type Target = _pyo3::PyRefMut<'a, #cls>;
+            }
+        }
+    };
+
     Ok(quote! {
         const _: () = {
             use #krate as _pyo3;
 
             #pytypeinfo
 
+            #mutability
+
             #pyclass_impls
 
             #default_items
@@ -788,20 +810,30 @@ impl<'a> PyClassImplsBuilder<'a> {
                 type Dict = #dict;
                 type WeakRef = #weakref;
                 type BaseNativeType = #base_nativetype;
+                type Mutability = _pyo3::pycell::Immutable;
             }
         }
     }
     fn impl_extractext(&self) -> TokenStream {
         let cls = self.cls;
-        quote! {
-            impl<'a> _pyo3::derive_utils::ExtractExt<'a> for &'a #cls
-            {
-                type Target = _pyo3::PyRef<'a, #cls>;
+        if self.attr.is_immutable {
+            quote! {
+                impl<'a> _pyo3::derive_utils::ExtractExt<'a> for &'a #cls
+                {
+                    type Target = _pyo3::PyRef<'a, #cls>;
+                }
             }
+        } else {
+            quote! {
+                impl<'a> _pyo3::derive_utils::ExtractExt<'a> for &'a #cls
+                {
+                    type Target = _pyo3::PyRef<'a, #cls>;
+                }
 
-            impl<'a> _pyo3::derive_utils::ExtractExt<'a> for &'a mut #cls
-            {
-                type Target = _pyo3::PyRefMut<'a, #cls>;
+                impl<'a> _pyo3::derive_utils::ExtractExt<'a> for &'a mut #cls
+                {
+                    type Target = _pyo3::PyRefMut<'a, #cls>;
+                }
             }
         }
     }
@@ -901,8 +933,18 @@ impl<'a> PyClassImplsBuilder<'a> {
 
         let default_items = &self.default_items;
 
+        let mutability = if self.attr.is_immutable {
+            quote! {
+                 _pyo3::pycell::Immutable
+            }
+        } else {
+            quote! {
+                _pyo3::pycell::Mutable
+            }
+        };
+
         quote! {
-            impl _pyo3::impl_::pyclass::PyClassImpl for #cls {
+            impl _pyo3::impl_::pyclass::PyClassImpl<#mutability> for #cls {
                 const DOC: &'static str = #doc;
                 const IS_GC: bool = #is_gc;
                 const IS_BASETYPE: bool = #is_basetype;
@@ -943,9 +985,9 @@ impl<'a> PyClassImplsBuilder<'a> {
                 #dict_offset
 
                 #weaklist_offset
-            }
 
-            #inventory_class
+                #inventory_class
+            }
         }
     }
 
diff --git a/src/class/basic.rs b/src/class/basic.rs
index b2d0e9ab..90f941f2 100644
--- a/src/class/basic.rs
+++ b/src/class/basic.rs
@@ -9,7 +9,9 @@
 //! [typeobj docs](https://docs.python.org/3/c-api/typeobj.html)
 
 use crate::callback::{HashCallbackOutput, IntoPyCallbackOutput};
-use crate::{exceptions, ffi, FromPyObject, PyAny, PyCell, PyClass, PyObject};
+use crate::{
+    exceptions, ffi, pyclass::MutablePyClass, FromPyObject, PyAny, PyCell, PyClass, PyObject,
+};
 use std::os::raw::c_int;
 
 /// Basic Python class customization
@@ -24,14 +26,14 @@ pub trait PyObjectProtocol<'p>: PyClass {
 
     fn __setattr__(&'p mut self, name: Self::Name, value: Self::Value) -> Self::Result
     where
-        Self: PyObjectSetAttrProtocol<'p>,
+        Self: PyObjectSetAttrProtocol<'p> + MutablePyClass,
     {
         unimplemented!()
     }
 
     fn __delattr__(&'p mut self, name: Self::Name) -> Self::Result
     where
-        Self: PyObjectDelAttrProtocol<'p>,
+        Self: PyObjectDelAttrProtocol<'p> + MutablePyClass,
     {
         unimplemented!()
     }
@@ -75,12 +77,12 @@ pub trait PyObjectGetAttrProtocol<'p>: PyObjectProtocol<'p> {
     type Name: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<PyObject>;
 }
-pub trait PyObjectSetAttrProtocol<'p>: PyObjectProtocol<'p> {
+pub trait PyObjectSetAttrProtocol<'p>: PyObjectProtocol<'p> + MutablePyClass {
     type Name: FromPyObject<'p>;
     type Value: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
-pub trait PyObjectDelAttrProtocol<'p>: PyObjectProtocol<'p> {
+pub trait PyObjectDelAttrProtocol<'p>: PyObjectProtocol<'p> + MutablePyClass {
     type Name: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
diff --git a/src/class/buffer.rs b/src/class/buffer.rs
index 7b4c8acf..be47a5c2 100644
--- a/src/class/buffer.rs
+++ b/src/class/buffer.rs
@@ -5,7 +5,7 @@
 //! For more information check [buffer protocol](https://docs.python.org/3/c-api/buffer.html)
 //! c-api
 use crate::callback::IntoPyCallbackOutput;
-use crate::{ffi, PyCell, PyClass, PyRefMut};
+use crate::{ffi, pyclass::MutablePyClass, PyCell, PyRefMut};
 use std::os::raw::c_int;
 
 /// Buffer protocol interface
@@ -13,7 +13,7 @@ use std::os::raw::c_int;
 /// For more information check [buffer protocol](https://docs.python.org/3/c-api/buffer.html)
 /// c-api.
 #[allow(unused_variables)]
-pub trait PyBufferProtocol<'p>: PyClass {
+pub trait PyBufferProtocol<'p>: MutablePyClass {
     // No default implementations so that implementors of this trait provide both methods.
 
     fn bf_getbuffer(slf: PyRefMut<Self>, view: *mut ffi::Py_buffer, flags: c_int) -> Self::Result
diff --git a/src/class/gc.rs b/src/class/gc.rs
index 2641a2b3..2f1085d0 100644
--- a/src/class/gc.rs
+++ b/src/class/gc.rs
@@ -2,14 +2,14 @@
 
 //! Python GC support
 
-use crate::{ffi, AsPyPointer, PyCell, PyClass, Python};
+use crate::{ffi, pyclass::MutablePyClass, AsPyPointer, PyCell, Python};
 use std::os::raw::{c_int, c_void};
 
 #[repr(transparent)]
 pub struct PyTraverseError(c_int);
 
 /// GC support
-pub trait PyGCProtocol<'p>: PyClass {
+pub trait PyGCProtocol<'p>: MutablePyClass {
     fn __traverse__(&'p self, visit: PyVisit) -> Result<(), PyTraverseError>;
     fn __clear__(&'p mut self);
 }
diff --git a/src/class/mapping.rs b/src/class/mapping.rs
index b6868521..627ef997 100644
--- a/src/class/mapping.rs
+++ b/src/class/mapping.rs
@@ -4,7 +4,7 @@
 //! Trait and support implementation for implementing mapping support
 
 use crate::callback::IntoPyCallbackOutput;
-use crate::{FromPyObject, PyClass, PyObject};
+use crate::{pyclass::MutablePyClass, FromPyObject, PyClass, PyObject};
 
 /// Mapping interface
 #[allow(unused_variables)]
@@ -50,13 +50,13 @@ pub trait PyMappingGetItemProtocol<'p>: PyMappingProtocol<'p> {
     type Result: IntoPyCallbackOutput<PyObject>;
 }
 
-pub trait PyMappingSetItemProtocol<'p>: PyMappingProtocol<'p> {
+pub trait PyMappingSetItemProtocol<'p>: PyMappingProtocol<'p> + MutablePyClass {
     type Key: FromPyObject<'p>;
     type Value: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyMappingDelItemProtocol<'p>: PyMappingProtocol<'p> {
+pub trait PyMappingDelItemProtocol<'p>: PyMappingProtocol<'p> + MutablePyClass {
     type Key: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
diff --git a/src/class/number.rs b/src/class/number.rs
index 448834d2..e261db67 100644
--- a/src/class/number.rs
+++ b/src/class/number.rs
@@ -4,7 +4,7 @@
 //! Trait and support implementation for implementing number protocol
 use crate::callback::IntoPyCallbackOutput;
 use crate::err::PyErr;
-use crate::{ffi, FromPyObject, PyClass, PyObject};
+use crate::{ffi, pyclass::MutablePyClass, FromPyObject, PyClass, PyObject};
 
 /// Number interface
 #[allow(unused_variables)]
@@ -459,74 +459,74 @@ pub trait PyNumberROrProtocol<'p>: PyNumberProtocol<'p> {
     type Result: IntoPyCallbackOutput<PyObject>;
 }
 
-pub trait PyNumberIAddProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIAddProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberISubProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberISubProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIMulProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIMulProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIMatmulProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIMatmulProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberITruedivProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberITruedivProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIFloordivProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIFloordivProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIModProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIModProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIDivmodProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIDivmodProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIPowProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIPowProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
     // See https://bugs.python.org/issue36379
     type Modulo: FromPyObject<'p>;
 }
 
-pub trait PyNumberILShiftProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberILShiftProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIRShiftProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIRShiftProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIAndProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIAndProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIXorProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIXorProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PyNumberIOrProtocol<'p>: PyNumberProtocol<'p> {
+pub trait PyNumberIOrProtocol<'p>: PyNumberProtocol<'p> + MutablePyClass {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
diff --git a/src/class/sequence.rs b/src/class/sequence.rs
index 129a188a..6f453d78 100644
--- a/src/class/sequence.rs
+++ b/src/class/sequence.rs
@@ -6,7 +6,7 @@
 use crate::callback::IntoPyCallbackOutput;
 use crate::conversion::{FromPyObject, IntoPy};
 use crate::err::PyErr;
-use crate::{exceptions, ffi, PyAny, PyCell, PyClass, PyObject};
+use crate::{exceptions, ffi, pyclass::MutablePyClass, PyAny, PyCell, PyClass, PyObject};
 use std::os::raw::c_int;
 
 /// Sequence interface
@@ -88,13 +88,13 @@ pub trait PySequenceGetItemProtocol<'p>: PySequenceProtocol<'p> {
     type Result: IntoPyCallbackOutput<PyObject>;
 }
 
-pub trait PySequenceSetItemProtocol<'p>: PySequenceProtocol<'p> {
+pub trait PySequenceSetItemProtocol<'p>: PySequenceProtocol<'p> + MutablePyClass {
     type Index: FromPyObject<'p> + From<isize>;
     type Value: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<()>;
 }
 
-pub trait PySequenceDelItemProtocol<'p>: PySequenceProtocol<'p> {
+pub trait PySequenceDelItemProtocol<'p>: PySequenceProtocol<'p> + MutablePyClass {
     type Index: FromPyObject<'p> + From<isize>;
     type Result: IntoPyCallbackOutput<()>;
 }
@@ -115,14 +115,14 @@ pub trait PySequenceRepeatProtocol<'p>: PySequenceProtocol<'p> {
 }
 
 pub trait PySequenceInplaceConcatProtocol<'p>:
-    PySequenceProtocol<'p> + IntoPy<PyObject> + 'p
+    PySequenceProtocol<'p> + IntoPy<PyObject> + MutablePyClass
 {
     type Other: FromPyObject<'p>;
     type Result: IntoPyCallbackOutput<Self>;
 }
 
 pub trait PySequenceInplaceRepeatProtocol<'p>:
-    PySequenceProtocol<'p> + IntoPy<PyObject> + 'p
+    PySequenceProtocol<'p> + IntoPy<PyObject> + MutablePyClass + 'p
 {
     type Index: FromPyObject<'p> + From<isize>;
     type Result: IntoPyCallbackOutput<Self>;
diff --git a/src/conversion.rs b/src/conversion.rs
index ccd690be..f2da392d 100644
--- a/src/conversion.rs
+++ b/src/conversion.rs
@@ -5,7 +5,8 @@ use crate::err::{self, PyDowncastError, PyResult};
 use crate::type_object::PyTypeInfo;
 use crate::types::PyTuple;
 use crate::{
-    ffi, gil, Py, PyAny, PyCell, PyClass, PyNativeType, PyObject, PyRef, PyRefMut, Python,
+    ffi, gil, pyclass::MutablePyClass, Py, PyAny, PyCell, PyClass, PyNativeType, PyObject, PyRef,
+    PyRefMut, Python,
 };
 use std::ptr::NonNull;
 
@@ -329,7 +330,7 @@ where
 
 impl<'a, T> FromPyObject<'a> for PyRefMut<'a, T>
 where
-    T: PyClass,
+    T: MutablePyClass,
 {
     fn extract(obj: &'a PyAny) -> PyResult<Self> {
         let cell: &PyCell<T> = PyTryFrom::try_from(obj)?;
diff --git a/src/impl_/pyclass.rs b/src/impl_/pyclass.rs
index 8f33b964..5ef2423d 100644
--- a/src/impl_/pyclass.rs
+++ b/src/impl_/pyclass.rs
@@ -2,7 +2,7 @@ use crate::{
     exceptions::{PyAttributeError, PyNotImplementedError},
     ffi,
     impl_::freelist::FreeList,
-    pycell::PyCellLayout,
+    pycell::{Mutability, PyCellLayout},
     pyclass_init::PyObjectInit,
     type_object::{PyLayout, PyTypeObject},
     PyCell, PyClass, PyMethodDefType, PyNativeType, PyResult, PyTypeInfo, Python,
@@ -145,7 +145,7 @@ unsafe impl Sync for PyClassItems {}
 ///
 /// Users are discouraged from implementing this trait manually; it is a PyO3 implementation detail
 /// and may be changed at any time.
-pub trait PyClassImpl: Sized {
+pub trait PyClassImpl<M: Mutability>: Sized {
     /// Class doc string
     const DOC: &'static str = "\0";
 
@@ -162,7 +162,7 @@ pub trait PyClassImpl: Sized {
     type Layout: PyLayout<Self>;
 
     /// Base class
-    type BaseType: PyTypeInfo + PyTypeObject + PyClassBaseType;
+    type BaseType: PyTypeInfo + PyTypeObject + PyClassBaseType<M>;
 
     /// This handles following two situations:
     /// 1. In case `T` is `Send`, stub `ThreadChecker` is used and does nothing.
@@ -818,9 +818,14 @@ impl<T> PyClassThreadChecker<T> for ThreadCheckerImpl<T> {
 /// Thread checker for types that have `Send` and `extends=...`.
 /// Ensures that `T: Send` and the parent is not accessed by another thread.
 #[doc(hidden)]
-pub struct ThreadCheckerInherited<T: Send, U: PyClassBaseType>(PhantomData<T>, U::ThreadChecker);
+pub struct ThreadCheckerInherited<T: Send, U: PyClassBaseType<M>, M: Mutability>(
+    PhantomData<T>,
+    U::ThreadChecker,
+);
 
-impl<T: Send, U: PyClassBaseType> PyClassThreadChecker<T> for ThreadCheckerInherited<T, U> {
+impl<T: Send, U: PyClassBaseType<M>, M: Mutability> PyClassThreadChecker<T>
+    for ThreadCheckerInherited<T, U, M>
+{
     fn ensure(&self) {
         self.1.ensure();
     }
@@ -831,15 +836,15 @@ impl<T: Send, U: PyClassBaseType> PyClassThreadChecker<T> for ThreadCheckerInher
 }
 
 /// Trait denoting that this class is suitable to be used as a base type for PyClass.
-pub trait PyClassBaseType: Sized {
-    type LayoutAsBase: PyCellLayout<Self>;
+pub trait PyClassBaseType<M: Mutability>: Sized {
+    type LayoutAsBase: PyCellLayout<Self, M>;
     type BaseNativeType;
     type ThreadChecker: PyClassThreadChecker<Self>;
     type Initializer: PyObjectInit<Self>;
 }
 
 /// All PyClasses can be used as a base type.
-impl<T: PyClass> PyClassBaseType for T {
+impl<T: PyClass> PyClassBaseType<T::Mutability> for T {
     type LayoutAsBase = crate::pycell::PyCell<T>;
     type BaseNativeType = T::BaseNativeType;
     type ThreadChecker = T::ThreadChecker;
diff --git a/src/instance.rs b/src/instance.rs
index 780d35fa..694e85f5 100644
--- a/src/instance.rs
+++ b/src/instance.rs
@@ -5,8 +5,8 @@ use crate::gil;
 use crate::pycell::{PyBorrowError, PyBorrowMutError, PyCell};
 use crate::types::{PyDict, PyTuple};
 use crate::{
-    ffi, AsPyPointer, FromPyObject, IntoPy, IntoPyPointer, PyAny, PyClass, PyClassInitializer,
-    PyRef, PyRefMut, PyTypeInfo, Python, ToPyObject,
+    ffi, pyclass::MutablePyClass, AsPyPointer, FromPyObject, IntoPy, IntoPyPointer, PyAny, PyClass,
+    PyClassInitializer, PyRef, PyRefMut, PyTypeInfo, Python, ToPyObject,
 };
 use std::marker::PhantomData;
 use std::mem;
@@ -427,7 +427,10 @@ where
     /// # Panics
     /// Panics if the value is currently mutably borrowed. For a non-panicking variant, use
     /// [`try_borrow_mut`](#method.try_borrow_mut).
-    pub fn borrow_mut<'py>(&'py self, py: Python<'py>) -> PyRefMut<'py, T> {
+    pub fn borrow_mut<'py>(&'py self, py: Python<'py>) -> PyRefMut<'py, T>
+    where
+        T: MutablePyClass,
+    {
         self.as_ref(py).borrow_mut()
     }
 
@@ -454,7 +457,10 @@ where
     pub fn try_borrow_mut<'py>(
         &'py self,
         py: Python<'py>,
-    ) -> Result<PyRefMut<'py, T>, PyBorrowMutError> {
+    ) -> Result<PyRefMut<'py, T>, PyBorrowMutError>
+    where
+        T: MutablePyClass,
+    {
         self.as_ref(py).try_borrow_mut()
     }
 }
@@ -819,7 +825,7 @@ where
 
 impl<'a, T> std::convert::From<PyRefMut<'a, T>> for Py<T>
 where
-    T: PyClass,
+    T: MutablePyClass,
 {
     fn from(pyref: PyRefMut<'a, T>) -> Self {
         unsafe { Py::from_borrowed_ptr(pyref.py(), pyref.as_ptr()) }
diff --git a/src/pycell.rs b/src/pycell.rs
index b8b2e395..4d116d45 100644
--- a/src/pycell.rs
+++ b/src/pycell.rs
@@ -176,7 +176,7 @@
 
 use crate::exceptions::PyRuntimeError;
 use crate::impl_::pyclass::{PyClassBaseType, PyClassDict, PyClassThreadChecker, PyClassWeakRef};
-use crate::pyclass::PyClass;
+use crate::pyclass::{MutablePyClass, PyClass};
 use crate::pyclass_init::PyClassInitializer;
 use crate::type_object::{PyLayout, PySizedLayout};
 use crate::types::PyAny;
@@ -189,19 +189,117 @@ use crate::{
 use crate::{ffi, IntoPy, PyErr, PyNativeType, PyObject, PyResult, Python};
 use std::cell::{Cell, UnsafeCell};
 use std::fmt;
+use std::marker::PhantomData;
 use std::mem::ManuallyDrop;
 use std::ops::{Deref, DerefMut};
 
+pub trait Mutability {
+    /// Creates a new borrow checker
+    fn new() -> Self;
+    /// Increments immutable borrow count, if possible
+    fn try_borrow(&self) -> Result<(), PyBorrowError>;
+
+    fn try_borrow_unguarded(&self) -> Result<(), PyBorrowError>;
+
+    /// Decrements immutable borrow count
+    fn release_borrow(&self);
+    /// Increments mutable borrow count, if possible
+    fn try_borrow_mut(&self) -> Result<(), PyBorrowMutError>;
+    /// Decremements mutable borrow count
+    fn release_borrow_mut(&self);
+}
+
+pub struct Mutable {
+    flag: Cell<BorrowFlag>,
+}
+impl Mutability for Mutable {
+    fn new() -> Self {
+        Self {
+            flag: Cell::new(BorrowFlag::UNUSED),
+        }
+    }
+
+    fn try_borrow(&self) -> Result<(), PyBorrowError> {
+        let flag = self.flag.get();
+        if flag != BorrowFlag::HAS_MUTABLE_BORROW {
+            self.flag.set(flag.increment());
+            Ok(())
+        } else {
+            Err(PyBorrowError { _private: () })
+        }
+    }
+
+    fn try_borrow_unguarded(&self) -> Result<(), PyBorrowError> {
+        let flag = self.flag.get();
+        if flag != BorrowFlag::HAS_MUTABLE_BORROW {
+            Ok(())
+        } else {
+            Err(PyBorrowError { _private: () })
+        }
+    }
+
+    fn release_borrow(&self) {
+        let flag = self.flag.get();
+        self.flag.set(flag.decrement())
+    }
+
+    fn try_borrow_mut(&self) -> Result<(), PyBorrowMutError> {
+        let flag = self.flag.get();
+        if flag == BorrowFlag::UNUSED {
+            self.flag.set(BorrowFlag::HAS_MUTABLE_BORROW);
+            Ok(())
+        } else {
+            Err(PyBorrowMutError { _private: () })
+        }
+    }
+
+    fn release_borrow_mut(&self) {
+        self.flag.set(BorrowFlag::UNUSED)
+    }
+}
+
+pub struct Immutable {
+    flag: PhantomData<Cell<BorrowFlag>>,
+}
+impl Mutability for Immutable {
+    fn new() -> Self {
+        Self { flag: PhantomData }
+    }
+
+    fn try_borrow(&self) -> Result<(), PyBorrowError> {
+        Ok(())
+    }
+
+    fn try_borrow_unguarded(&self) -> Result<(), PyBorrowError> {
+        Ok(())
+    }
+
+    fn release_borrow(&self) {}
+
+    fn try_borrow_mut(&self) -> Result<(), PyBorrowMutError> {
+        unreachable!()
+    }
+
+    fn release_borrow_mut(&self) {
+        unreachable!()
+    }
+}
+
 /// Base layout of PyCell.
 /// This is necessary for sharing BorrowFlag between parents and children.
 #[doc(hidden)]
 #[repr(C)]
-pub struct PyCellBase<T> {
+pub struct PyCellBase<T, M: Mutability> {
     ob_base: T,
-    borrow_flag: Cell<BorrowFlag>,
+    borrow_impl: M,
 }
 
-unsafe impl<T, U> PyLayout<T> for PyCellBase<U> where U: PySizedLayout<T> {}
+unsafe impl<T, U, M> PyLayout<T> for PyCellBase<U, M>
+where
+    U: PySizedLayout<T>,
+    M: Mutability,
+{
+}
 
 /// A container type for (mutably) accessing [`PyClass`] values
 ///
@@ -240,7 +338,7 @@ unsafe impl<T, U> PyLayout<T> for PyCellBase<U> where U: PySizedLayout<T> {}
 /// [module-level documentation](self).
 #[repr(C)]
 pub struct PyCell<T: PyClass> {
-    ob_base: <T::BaseType as PyClassBaseType>::LayoutAsBase,
+    ob_base: <T::BaseType as PyClassBaseType<T::Mutability>>::LayoutAsBase,
     contents: PyCellContents<T>,
 }
 
@@ -283,7 +381,10 @@ impl<T: PyClass> PyCell<T> {
     ///
     /// Panics if the value is currently borrowed. For a non-panicking variant, use
     /// [`try_borrow_mut`](#method.try_borrow_mut).
-    pub fn borrow_mut(&self) -> PyRefMut<'_, T> {
+    pub fn borrow_mut(&self) -> PyRefMut<'_, T>
+    where
+        T: MutablePyClass,
+    {
         self.try_borrow_mut().expect("Already borrowed")
     }
 
@@ -313,13 +414,9 @@ impl<T: PyClass> PyCell<T> {
     /// });
     /// ```
     pub fn try_borrow(&self) -> Result<PyRef<'_, T>, PyBorrowError> {
-        let flag = self.get_borrow_flag();
-        if flag == BorrowFlag::HAS_MUTABLE_BORROW {
-            Err(PyBorrowError { _private: () })
-        } else {
-            self.set_borrow_flag(flag.increment());
-            Ok(PyRef { inner: self })
-        }
+        self.borrow_checker()
+            .try_borrow()
+            .map(|_| PyRef { inner: self })
     }
 
     /// Mutably borrows the value `T`, returning an error if the value is currently borrowed.
@@ -343,13 +440,13 @@ impl<T: PyClass> PyCell<T> {
     ///     assert!(c.try_borrow_mut().is_ok());
     /// });
     /// ```
-    pub fn try_borrow_mut(&self) -> Result<PyRefMut<'_, T>, PyBorrowMutError> {
-        if self.get_borrow_flag() != BorrowFlag::UNUSED {
-            Err(PyBorrowMutError { _private: () })
-        } else {
-            self.set_borrow_flag(BorrowFlag::HAS_MUTABLE_BORROW);
-            Ok(PyRefMut { inner: self })
-        }
+    pub fn try_borrow_mut(&self) -> Result<PyRefMut<'_, T>, PyBorrowMutError>
+    where
+        T: MutablePyClass,
+    {
+        self.borrow_checker()
+            .try_borrow_mut()
+            .map(|_| PyRefMut { inner: self })
     }
 
     /// Immutably borrows the value `T`, returning an error if the value is
@@ -382,11 +479,9 @@ impl<T: PyClass> PyCell<T> {
     /// });
     /// ```
     pub unsafe fn try_borrow_unguarded(&self) -> Result<&T, PyBorrowError> {
-        if self.get_borrow_flag() == BorrowFlag::HAS_MUTABLE_BORROW {
-            Err(PyBorrowError { _private: () })
-        } else {
-            Ok(&*self.contents.value.get())
-        }
+        self.borrow_checker()
+            .try_borrow_unguarded()
+            .map(|_: ()| &*self.contents.value.get())
     }
 
     /// Replaces the wrapped value with a new one, returning the old value.
@@ -395,7 +490,10 @@ impl<T: PyClass> PyCell<T> {
     ///
     /// Panics if the value is currently borrowed.
     #[inline]
-    pub fn replace(&self, t: T) -> T {
+    pub fn replace(&self, t: T) -> T
+    where
+        T: MutablePyClass,
+    {
         std::mem::replace(&mut *self.borrow_mut(), t)
     }
 
@@ -404,7 +502,10 @@ impl<T: PyClass> PyCell<T> {
     /// # Panics
     ///
     /// Panics if the value is currently borrowed.
-    pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T {
+    pub fn replace_with<F: FnOnce(&mut T) -> T>(&self, f: F) -> T
+    where
+        T: MutablePyClass,
+    {
         let mut_borrow = &mut *self.borrow_mut();
         let replacement = f(mut_borrow);
         std::mem::replace(mut_borrow, replacement)
@@ -416,7 +517,10 @@ impl<T: PyClass> PyCell<T> {
     ///
     /// Panics if the value in either `PyCell` is currently borrowed.
     #[inline]
-    pub fn swap(&self, other: &Self) {
+    pub fn swap(&self, other: &Self)
+    where
+        T: MutablePyClass,
+    {
         std::mem::swap(&mut *self.borrow_mut(), &mut *other.borrow_mut())
     }
 
@@ -674,8 +778,7 @@ impl<'p, T: PyClass> Deref for PyRef<'p, T> {
 
 impl<'p, T: PyClass> Drop for PyRef<'p, T> {
     fn drop(&mut self) {
-        let flag = self.inner.get_borrow_flag();
-        self.inner.set_borrow_flag(flag.decrement())
+        self.inner.borrow_checker().release_borrow()
     }
 }
 
@@ -707,11 +810,11 @@ impl<T: PyClass + fmt::Debug> fmt::Debug for PyRef<'_, T> {
 /// A wrapper type for a mutably borrowed value from a[`PyCell`]`<T>`.
 ///
 /// See the [module-level documentation](self) for more information.
-pub struct PyRefMut<'p, T: PyClass> {
+pub struct PyRefMut<'p, T: MutablePyClass> {
     inner: &'p PyCell<T>,
 }
 
-impl<'p, T: PyClass> PyRefMut<'p, T> {
+impl<'p, T: MutablePyClass> PyRefMut<'p, T> {
     /// Returns a `Python` token that is bound to the lifetime of the `PyRefMut`.
     pub fn py(&self) -> Python {
         unsafe { Python::assume_gil_acquired() }
@@ -720,8 +823,8 @@ impl<'p, T: PyClass> PyRefMut<'p, T> {
 
 impl<'p, T, U> AsRef<U> for PyRefMut<'p, T>
 where
-    T: PyClass<BaseType = U>,
-    U: PyClass,
+    T: PyClass<BaseType = U> + MutablePyClass,
+    U: MutablePyClass,
 {
     fn as_ref(&self) -> &T::BaseType {
         unsafe { &*self.inner.ob_base.get_ptr() }
@@ -730,8 +833,8 @@ where
 
 impl<'p, T, U> AsMut<U> for PyRefMut<'p, T>
 where
-    T: PyClass<BaseType = U>,
-    U: PyClass,
+    T: PyClass<BaseType = U> + MutablePyClass,
+    U: MutablePyClass,
 {
     fn as_mut(&mut self) -> &mut T::BaseType {
         unsafe { &mut *self.inner.ob_base.get_ptr() }
@@ -740,8 +843,8 @@ where
 
 impl<'p, T, U> PyRefMut<'p, T>
 where
-    T: PyClass<BaseType = U>,
-    U: PyClass,
+    T: PyClass<BaseType = U> + MutablePyClass,
+    U: MutablePyClass,
 {
     /// Gets a `PyRef<T::BaseType>`.
     ///
@@ -755,7 +858,7 @@ where
     }
 }
 
-impl<'p, T: PyClass> Deref for PyRefMut<'p, T> {
+impl<'p, T: MutablePyClass> Deref for PyRefMut<'p, T> {
     type Target = T;
 
     #[inline]
@@ -764,46 +867,46 @@ impl<'p, T: PyClass> Deref for PyRefMut<'p, T> {
     }
 }
 
-impl<'p, T: PyClass> DerefMut for PyRefMut<'p, T> {
+impl<'p, T: MutablePyClass> DerefMut for PyRefMut<'p, T> {
     #[inline]
     fn deref_mut(&mut self) -> &mut T {
         unsafe { &mut *self.inner.get_ptr() }
     }
 }
 
-impl<'p, T: PyClass> Drop for PyRefMut<'p, T> {
+impl<'p, T: MutablePyClass> Drop for PyRefMut<'p, T> {
     fn drop(&mut self) {
-        self.inner.set_borrow_flag(BorrowFlag::UNUSED)
+        self.inner.borrow_checker().release_borrow_mut()
     }
 }
 
-impl<T: PyClass> IntoPy<PyObject> for PyRefMut<'_, T> {
+impl<T: MutablePyClass> IntoPy<PyObject> for PyRefMut<'_, T> {
     fn into_py(self, py: Python) -> PyObject {
         unsafe { PyObject::from_borrowed_ptr(py, self.inner.as_ptr()) }
     }
 }
 
-impl<'a, T: PyClass> AsPyPointer for PyRefMut<'a, T> {
+impl<'a, T: MutablePyClass> AsPyPointer for PyRefMut<'a, T> {
     fn as_ptr(&self) -> *mut ffi::PyObject {
         self.inner.as_ptr()
     }
 }
 
-impl<'a, T: PyClass> std::convert::TryFrom<&'a PyCell<T>> for crate::PyRefMut<'a, T> {
+impl<'a, T: MutablePyClass> std::convert::TryFrom<&'a PyCell<T>> for crate::PyRefMut<'a, T> {
     type Error = PyBorrowMutError;
     fn try_from(cell: &'a crate::PyCell<T>) -> Result<Self, Self::Error> {
         cell.try_borrow_mut()
     }
 }
 
-impl<T: PyClass + fmt::Debug> fmt::Debug for PyRefMut<'_, T> {
+impl<T: MutablePyClass + fmt::Debug> fmt::Debug for PyRefMut<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         fmt::Debug::fmt(&*(self.deref()), f)
     }
 }
 
 #[doc(hidden)]
-#[derive(Copy, Clone, Eq, PartialEq)]
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
 pub struct BorrowFlag(usize);
 
 impl BorrowFlag {
@@ -868,9 +971,11 @@ impl From<PyBorrowMutError> for PyErr {
 }
 
 #[doc(hidden)]
-pub trait PyCellLayout<T>: PyLayout<T> {
-    fn get_borrow_flag(&self) -> BorrowFlag;
-    fn set_borrow_flag(&self, flag: BorrowFlag);
+pub trait PyCellLayout<T, M>: PyLayout<T>
+where
+    M: Mutability,
+{
+    fn borrow_checker(&self) -> &M;
     /// Implementation of tp_dealloc.
     /// # Safety
     /// - slf must be a valid pointer to an instance of a T or a subclass.
@@ -878,17 +983,16 @@ pub trait PyCellLayout<T>: PyLayout<T> {
     unsafe fn tp_dealloc(slf: *mut ffi::PyObject, py: Python);
 }
 
-impl<T, U> PyCellLayout<T> for PyCellBase<U>
+impl<T, U, M> PyCellLayout<T, M> for PyCellBase<U, M>
 where
     U: PySizedLayout<T>,
     T: PyTypeInfo,
+    M: Mutability,
 {
-    fn get_borrow_flag(&self) -> BorrowFlag {
-        self.borrow_flag.get()
-    }
-    fn set_borrow_flag(&self, flag: BorrowFlag) {
-        self.borrow_flag.set(flag)
+    fn borrow_checker(&self) -> &M {
+        &self.borrow_impl
     }
+
     unsafe fn tp_dealloc(slf: *mut ffi::PyObject, py: Python) {
         // For `#[pyclass]` types which inherit from PyAny, we can just call tp_free
         if T::type_object_raw(py) == &mut PyBaseObject_Type {
@@ -910,16 +1014,14 @@ where
     }
 }
 
-impl<T: PyClass> PyCellLayout<T> for PyCell<T>
+impl<T: PyClass> PyCellLayout<T, T::Mutability> for PyCell<T>
 where
-    <T::BaseType as PyClassBaseType>::LayoutAsBase: PyCellLayout<T::BaseType>,
+    <T::BaseType as PyClassBaseType<T::Mutability>>::LayoutAsBase:
+        PyCellLayout<T::BaseType, T::Mutability>,
 {
-    fn get_borrow_flag(&self) -> BorrowFlag {
+    fn borrow_checker(&self) -> &T::Mutability {
         self.contents.thread_checker.ensure();
-        self.ob_base.get_borrow_flag()
-    }
-    fn set_borrow_flag(&self, flag: BorrowFlag) {
-        self.ob_base.set_borrow_flag(flag)
+        self.ob_base.borrow_checker()
     }
     unsafe fn tp_dealloc(slf: *mut ffi::PyObject, py: Python) {
         // Safety: Python only calls tp_dealloc when no references to the object remain.
@@ -927,6 +1029,6 @@ where
         ManuallyDrop::drop(&mut cell.contents.value);
         cell.contents.dict.clear_dict(py);
         cell.contents.weakref.clear_weakrefs(slf, py);
-        <T::BaseType as PyClassBaseType>::LayoutAsBase::tp_dealloc(slf, py)
+        <T::BaseType as PyClassBaseType<T::Mutability>>::LayoutAsBase::tp_dealloc(slf, py)
     }
 }
diff --git a/src/pyclass.rs b/src/pyclass.rs
index f08669ad..ee448b32 100644
--- a/src/pyclass.rs
+++ b/src/pyclass.rs
@@ -1,4 +1,5 @@
 //! `PyClass` and related traits.
+use crate::pycell::Mutability;
 use crate::{
     callback::IntoPyCallbackOutput,
     ffi,
@@ -22,7 +23,7 @@ use std::{
 /// The `#[pyclass]` attribute automatically implements this trait for your Rust struct,
 /// so you normally don't have to use this trait directly.
 pub trait PyClass:
-    PyTypeInfo<AsRefTarget = PyCell<Self>> + PyClassImpl<Layout = PyCell<Self>>
+    PyTypeInfo<AsRefTarget = PyCell<Self>> + PyClassImpl<Self::Mutability, Layout = PyCell<Self>>
 {
     /// Specify this class has `#[pyclass(dict)]` or not.
     type Dict: PyClassDict;
@@ -31,8 +32,13 @@ pub trait PyClass:
     /// The closest native ancestor. This is `PyAny` by default, and when you declare
     /// `#[pyclass(extends=PyDict)]`, it's `PyDict`.
     type BaseNativeType: PyTypeInfo + PyNativeType;
+
+    type Mutability: Mutability;
 }
 
+pub unsafe trait MutablePyClass: PyClass {}
+pub unsafe trait ImmutablePyClass: PyClass {}
+
 fn into_raw<T>(vec: Vec<T>) -> *mut c_void {
     Box::into_raw(vec.into_boxed_slice()) as _
 }
diff --git a/src/pyclass_init.rs b/src/pyclass_init.rs
index b8b2025b..e478472b 100644
--- a/src/pyclass_init.rs
+++ b/src/pyclass_init.rs
@@ -130,14 +130,17 @@ impl<T: PyTypeInfo> PyObjectInit<T> for PyNativeTypeInitializer<T> {
 /// ```
 pub struct PyClassInitializer<T: PyClass> {
     init: T,
-    super_init: <T::BaseType as PyClassBaseType>::Initializer,
+    super_init: <T::BaseType as PyClassBaseType<T::Mutability>>::Initializer,
 }
 
 impl<T: PyClass> PyClassInitializer<T> {
     /// Constructs a new initializer from value `T` and base class' initializer.
     ///
     /// It is recommended to use `add_subclass` instead of this method for most usage.
-    pub fn new(init: T, super_init: <T::BaseType as PyClassBaseType>::Initializer) -> Self {
+    pub fn new(
+        init: T,
+        super_init: <T::BaseType as PyClassBaseType<T::Mutability>>::Initializer,
+    ) -> Self {
         Self { init, super_init }
     }
 
@@ -190,7 +193,7 @@ impl<T: PyClass> PyClassInitializer<T> {
     pub fn add_subclass<S>(self, subclass_value: S) -> PyClassInitializer<S>
     where
         S: PyClass<BaseType = T>,
-        S::BaseType: PyClassBaseType<Initializer = Self>,
+        S::BaseType: PyClassBaseType<T::Mutability, Initializer = Self>,
     {
         PyClassInitializer::new(subclass_value, self)
     }
@@ -231,16 +234,16 @@ impl<T: PyClass> PyObjectInit<T> for PyClassInitializer<T> {
         /// Layout of a PyCellBase after base new has been called, but the borrow flag has not
         /// yet been initialized.
         #[repr(C)]
-        struct PartiallyInitializedPyCellBase<T> {
+        struct PartiallyInitializedPyCellBase<T: PyClass> {
             _ob_base: T,
-            borrow_flag: MaybeUninit<Cell<BorrowFlag>>,
+            borrow_flag: MaybeUninit<T::Mutability>,
         }
 
         /// Layout of a PyCell after base new has been called, but the contents have not yet been
         /// written.
         #[repr(C)]
         struct PartiallyInitializedPyCell<T: PyClass> {
-            _ob_base: <T::BaseType as PyClassBaseType>::LayoutAsBase,
+            _ob_base: <T::BaseType as PyClassBaseType<T::Mutability>>::LayoutAsBase,
             contents: MaybeUninit<PyCellContents<T>>,
         }
 
@@ -274,7 +277,7 @@ impl<T: PyClass> PyObjectInit<T> for PyClassInitializer<T> {
 impl<T> From<T> for PyClassInitializer<T>
 where
     T: PyClass,
-    T::BaseType: PyClassBaseType<Initializer = PyNativeTypeInitializer<T::BaseType>>,
+    T::BaseType: PyClassBaseType<T::Mutability, Initializer = PyNativeTypeInitializer<T::BaseType>>,
 {
     #[inline]
     fn from(value: T) -> PyClassInitializer<T> {
diff --git a/src/types/mod.rs b/src/types/mod.rs
index 1058a16a..2aa889ff 100644
--- a/src/types/mod.rs
+++ b/src/types/mod.rs
@@ -196,8 +196,8 @@ macro_rules! pyobject_native_type_sized {
     ($name:ty, $layout:path $(;$generics:ident)*) => {
         unsafe impl $crate::type_object::PyLayout<$name> for $layout {}
         impl $crate::type_object::PySizedLayout<$name> for $layout {}
-        impl<'a, $($generics,)*> $crate::impl_::pyclass::PyClassBaseType for $name {
-            type LayoutAsBase = $crate::pycell::PyCellBase<$layout>;
+        impl<'a, $($generics,)*> $crate::impl_::pyclass::PyClassBaseType<$crate::pycell::Mutable> for $name {
+            type LayoutAsBase = $crate::pycell::PyCellBase<$layout, $crate::pycell::Mutable>;
             type BaseNativeType = $name;
             type ThreadChecker = $crate::impl_::pyclass::ThreadCheckerStub<$crate::PyObject>;
             type Initializer = $crate::pyclass_init::PyNativeTypeInitializer<Self>;
diff --git a/tests/test_compile_error.rs b/tests/test_compile_error.rs
index 2fef7685..2af1c9ff 100644
--- a/tests/test_compile_error.rs
+++ b/tests/test_compile_error.rs
@@ -28,8 +28,12 @@ fn _test_compile_errors() {
     t.compile_fail("tests/ui/invalid_pymethod_names.rs");
     t.compile_fail("tests/ui/invalid_pymodule_args.rs");
     t.compile_fail("tests/ui/reject_generics.rs");
+<<<<<<< HEAD
     t.compile_fail("tests/ui/invalid_pymethod_proto_args.rs");
     t.compile_fail("tests/ui/invalid_pymethod_proto_args_py.rs");
+=======
+    t.compile_fail("tests/ui/invalid_immutable_pyclass_borrow.rs");
+>>>>>>> 0fa03a67cd (Implement opt-in immutable pyclasses)
 
     tests_rust_1_49(&t);
     tests_rust_1_56(&t);
diff --git a/tests/ui/invalid_immutable_pyclass_borrow.rs b/tests/ui/invalid_immutable_pyclass_borrow.rs
new file mode 100644
index 00000000..d020fbec
--- /dev/null
+++ b/tests/ui/invalid_immutable_pyclass_borrow.rs
@@ -0,0 +1,13 @@
+use pyo3::prelude::*;
+
+#[pyclass(immutable)]
+pub struct Foo {
+    #[pyo3(get)]
+    field: u32,
+}
+
+fn borrow_mut_fails(foo: Py<Foo>, py: Python){
+    let borrow = foo.as_ref(py).borrow_mut();
+}
+
+fn main(){}
\ No newline at end of file
diff --git a/tests/ui/invalid_immutable_pyclass_borrow.stderr b/tests/ui/invalid_immutable_pyclass_borrow.stderr
new file mode 100644
index 00000000..3574139a
--- /dev/null
+++ b/tests/ui/invalid_immutable_pyclass_borrow.stderr
@@ -0,0 +1,5 @@
+error[E0277]: the trait bound `Foo: MutablePyClass` is not satisfied
+  --> tests/ui/invalid_immutable_pyclass_borrow.rs:10:33
+   |
+10 |     let borrow = foo.as_ref(py).borrow_mut();
+   |                                 ^^^^^^^^^^ the trait `MutablePyClass` is not implemented for `Foo`
diff --git a/tests/ui/pyclass_send.stderr b/tests/ui/pyclass_send.stderr
index 287430ac..7e5addac 100644
--- a/tests/ui/pyclass_send.stderr
+++ b/tests/ui/pyclass_send.stderr
@@ -11,8 +11,20 @@ note: required because it appears within the type `NotThreadSafe`
 5   | struct NotThreadSafe {
     |        ^^^^^^^^^^^^^
 note: required by a bound in `ThreadCheckerStub`
+<<<<<<< HEAD
+<<<<<<< HEAD
    --> src/impl_/pyclass.rs
     |
     | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
+=======
+   --> src/class/impl_.rs:731:33
+    |
+731 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
+>>>>>>> 0fa03a67cd (Implement opt-in immutable pyclasses)
+=======
+   --> src/class/impl_.rs:728:33
+    |
+728 | pub struct ThreadCheckerStub<T: Send>(PhantomData<T>);
+>>>>>>> 7cded1178d (Fix formatting.)
     |                                 ^^^^ required by this bound in `ThreadCheckerStub`
     = note: this error originates in the attribute macro `pyclass` (in Nightly builds, run with -Z macro-backtrace for more info)