Move the ThreadChecker field in front of dict and weakref.

Offsets for dict and weakref are calculated from the end of the
PyCell struct. When using the non-dummy ThreadChecker, the offsets
were invalid since the `ThreadCheckerImpl` is not zero-sized.

CHANGELOG.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Fixed
 - Conversion from types with an `__index__` method to Rust BigInts. [#1027](https://github.com/PyO3/pyo3/pull/1027)
+- Fix segfault with #[pyclass(dict, unsendable)] [#1058](https://github.com/PyO3/pyo3/pull/1058)
 
 ## [0.11.1] - 2020-06-30
 ### Added

src/pycell.rs

@@ -161,9 +161,9 @@ impl<T: PyClass> PyCellInner<T> {
 #[repr(C)]
 pub struct PyCell<T: PyClass> {
     inner: PyCellInner<T>,
+    thread_checker: T::ThreadChecker,
     dict: T::Dict,
     weakref: T::WeakRef,
-    thread_checker: T::ThreadChecker,
 }
 
 unsafe impl<T: PyClass> PyNativeType for PyCell<T> {}

tests/test_unsendable_dict.rs

@@ -0,0 +1,21 @@
+use pyo3::prelude::*;
+use pyo3::py_run;
+
+#[pyclass(dict, unsendable)]
+struct UnsendableDictClass {}
+
+#[pymethods]
+impl UnsendableDictClass {
+    #[new]
+    fn new() -> Self {
+        UnsendableDictClass {}
+    }
+}
+
+#[test]
+fn test_unsendable_dict() {
+    let gil = Python::acquire_gil();
+    let py = gil.python();
+    let inst = Py::new(py, UnsendableDictClass {}).unwrap();
+    py_run!(py, inst, "assert inst.__dict__ == {}");
+}