Disable segfaulty subclassing by default

See https://github.com/PyO3/pyo3/issues/220 and https://github.com/PyO3/pyo3/issues/407 for details on those segfaults

CHANGELOG.md

@@ -14,6 +14,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
  * Use existing fields and methods before calling custom __getattr__. [#505](https://github.com/PyO3/pyo3/pull/512)
  * `PyBytes` can now be indexed just like `Vec<u8>`
 
+## Removed 
+
+ * `#[pyclass(subclass)]` is hidden a `unsound-subclass` feature because it's causing segmentation faults.
+
 ### Fixed
 
  * More readable error message for generics in pyclass [#503](https://github.com/PyO3/pyo3/pull/503)

Cargo.toml

@@ -55,6 +55,9 @@ extension-module = []
 # are welcome.
 # abi3 = []
 
+# Activate subclassing support
+unsound-subclass = ["pyo3cls/unsound-subclass"]
+
 [workspace]
 members = [
     "pyo3cls",

examples/rustapi_module/Cargo.toml

@@ -9,7 +9,7 @@ edition = "2018"
 
 [dependencies.pyo3]
 path = "../../"
-features = ["extension-module"]
+features = ["extension-module", "unsound-subclass"]
 
 [lib]
 name = "rustapi_module"

guide/src/class.md

@@ -102,10 +102,10 @@ so that they can benefit from a freelist. `XXX` is a number of items for the fre
 If a custom class contains references to other Python objects that can be collected, the `PyGCProtocol` trait has to be implemented.
 * `weakref` - Adds support for Python weak references.
 * `extends=BaseType` - Use a custom base class. The base `BaseType` must implement `PyTypeInfo`.
-* `subclass` - Allows Python classes to inherit from this class.
 * `dict` - Adds `__dict__` support, so that the instances of this type have a dictionary containing arbitrary instance variables.
 * `module="XXX"` - Set the name of the module the class will be shown as defined in. If not given, the class
   will be a virtual member of the `builtins` module.
+* `subclass` - Allows Python classes to inherit from this class. This feature is hidden behind a `unsound-subclass` feature because it is currently causing segmentation faults
 
 ## Constructor
 

pyo3-derive-backend/Cargo.toml

@@ -14,3 +14,6 @@ edition = "2018"
 quote = "0.6.12"
 proc-macro2 = "0.4.30"
 syn = { version = "0.15.34", features = ["full", "extra-traits"] }
+
+[features]
+unsound-subclass = []

pyo3-derive-backend/src/pyclass.rs

@@ -132,6 +132,12 @@ impl PyClassArgs {
                 parse_quote! {pyo3::type_object::PY_TYPE_FLAG_WEAKREF}
             }
             "subclass" => {
+                if cfg!(not(feature = "unsound-subclass")) {
+                    return Err(syn::Error::new_spanned(
+                        exp.path.clone(),
+                        "You need to activate the `unsound-subclass` feature if you want to use subclassing",
+                    ));
+                }
                 parse_quote! {pyo3::type_object::PY_TYPE_FLAG_BASETYPE}
             }
             "dict" => {

pyo3cls/Cargo.toml

@@ -18,3 +18,6 @@ quote= "0.6.12"
 proc-macro2 = "0.4.30"
 syn = { version = "0.15.34", features = ["full", "extra-traits"] }
 pyo3-derive-backend = { path = "../pyo3-derive-backend", version = "=0.7.0" }
+
+[features]
+unsound-subclass = ["pyo3-derive-backend/unsound-subclass"]

tests/test_inheritance.rs

@@ -1,5 +1,6 @@
 use pyo3::prelude::*;
 use pyo3::py_run;
+#[cfg(feature = "unsound-subclass")]
 use pyo3::types::IntoPyDict;
 use std::isize;
 
@@ -11,9 +12,11 @@ struct BaseClass {
     val1: usize,
 }
 
+#[cfg(feature = "unsound-subclass")]
 #[pyclass(subclass)]
 struct SubclassAble {}
 
+#[cfg(feature = "unsound-subclass")]
 #[test]
 fn subclass() {
     let gil = Python::acquire_gil();