From 59ed277879cf203c690de4dd5f731f0cb110636e Mon Sep 17 00:00:00 2001
From: Paul Ganssle <paul@ganssle.io>
Date: Sun, 14 Jul 2019 16:35:06 +0200
Subject: [PATCH 1/3] Enforce GC contract at compile time

PyGCProtocol must be implemented for anything registered as tracked by
the garbage collector. This modifies the `pyclass` macro to enforce this
at compile time.
---
 pyo3-derive-backend/src/pyclass.rs | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/pyo3-derive-backend/src/pyclass.rs b/pyo3-derive-backend/src/pyclass.rs
index 0d9a334a574..5e7841e6045 100644
--- a/pyo3-derive-backend/src/pyclass.rs
+++ b/pyo3-derive-backend/src/pyclass.rs
@@ -296,12 +296,15 @@ fn impl_class(
     // insert space for weak ref
     let mut has_weakref = false;
     let mut has_dict = false;
+    let mut has_gc = false;
     for f in attr.flags.iter() {
         if let syn::Expr::Path(ref epath) = f {
             if epath.path == parse_quote! {pyo3::type_object::PY_TYPE_FLAG_WEAKREF} {
                 has_weakref = true;
             } else if epath.path == parse_quote! {pyo3::type_object::PY_TYPE_FLAG_DICT} {
                 has_dict = true;
+            } else if epath.path == parse_quote! {pyo3::type_object::PY_TYPE_FLAG_GC} {
+                has_gc = true;
             }
         }
     }
@@ -321,6 +324,22 @@ fn impl_class(
         quote! { None }
     };
 
+    // Enforce at compile time that PyGCProtocol is implemented
+    let gc_impl = if has_gc {
+        let closure_name = format!("__assertion_closure_{}", cls.to_string());
+        let closure_token = syn::Ident::new(&closure_name, Span::call_site());
+        quote! {
+            fn #closure_token() {
+                use pyo3::class;
+
+                fn _assert_implements_protocol<'p, T: pyo3::class::PyGCProtocol<'p>>() {}
+                _assert_implements_protocol::<#cls>();
+            }
+        }
+    } else {
+        quote! {}
+    };
+
     let inventory_impl = impl_inventory(&cls);
 
     let base = &attr.base;
@@ -365,6 +384,9 @@ fn impl_class(
         #inventory_impl
 
         #extra
+
+        #gc_impl
+
     }
 }
 

From ca07c48931d2937fddfe8d49ddb59b835dbc5eba Mon Sep 17 00:00:00 2001
From: Paul Ganssle <paul@ganssle.io>
Date: Sun, 14 Jul 2019 17:56:36 +0200
Subject: [PATCH 2/3] Add documentation for new PyGCProtocol behavior

---
 guide/src/class.md | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/guide/src/class.md b/guide/src/class.md
index 90e0a3ff822..988ff314612 100644
--- a/guide/src/class.md
+++ b/guide/src/class.md
@@ -573,9 +573,16 @@ impl PyGCProtocol for ClassWithGCSupport {
 
 Special protocol trait implementations have to be annotated with the `#[pyproto]` attribute.
 
-It is also possible to enable GC for custom class using the `gc` parameter of the `pyclass` attribute.
+It is also possible to enable GC for custom classes using the `gc` parameter of the `pyclass` attribute.
 i.e. `#[pyclass(gc)]`. In that case instances of custom class participate in Python garbage
-collection, and it is possible to track them with `gc` module methods.
+collection, and it is possible to track them with `gc` module methods. When using the `gc` parameter,
+it is *required* to implement the `PyGCProtocol` trait, failure to do so will result in an error
+at compile time:
+
+```compile_fail
+#[pyclass(gc)]
+struct GCTracked {} // Fails because it does not implement PyGCProtocol
+```
 
 ### Iterator Types
 

From e633ae25eb62893c2f4ef1e688cba3acbbd10fcb Mon Sep 17 00:00:00 2001
From: Paul Ganssle <paul@ganssle.io>
Date: Sun, 14 Jul 2019 18:00:16 +0200
Subject: [PATCH 3/3] Add changelog error for PR #532

---
 CHANGELOG.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ab58727a839..fafc604021c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Changed
 
+ * Implementing the Using the `gc` parameter for `pyclass` (e.g. `#[pyclass(gc)]`) without implementing the `class::PyGCProtocol` trait is now a compile-time error. Failing to implement this trait could lead to segfaults. [#532](https://github.com/PyO3/pyo3/pull/532)
+
 ## [0.7.0] - 2018-05-26
 
 ### Added