Revert "Make wrapped C++ functions pickleable (#5580)"

CMakeLists.txt

@@ -186,7 +186,6 @@ set(PYBIND11_HEADERS
     include/pybind11/detail/descr.h
     include/pybind11/detail/dynamic_raw_ptr_cast_if_possible.h
     include/pybind11/detail/exception_translation.h
-    include/pybind11/detail/function_record_pyobject.h
     include/pybind11/detail/init.h
     include/pybind11/detail/internals.h
     include/pybind11/detail/native_enum_data.h

include/pybind11/attr.h

@@ -193,7 +193,6 @@ struct argument_record {
 
 /// Internal data structure which holds metadata about a bound function (signature, overloads,
 /// etc.)
-#define PYBIND11_DETAIL_FUNCTION_RECORD_ABI_ID "v1" // PLEASE UPDATE if the struct is changed.
 struct function_record {
     function_record()
         : is_constructor(false), is_new_style_constructor(false), is_stateless(false),

include/pybind11/detail/internals.h

@@ -37,11 +37,11 @@
 /// further ABI-incompatible changes may be made before the ABI is officially
 /// changed to the new version.
 #ifndef PYBIND11_INTERNALS_VERSION
-#    define PYBIND11_INTERNALS_VERSION 11
+#    define PYBIND11_INTERNALS_VERSION 12
 #endif
 
-#if PYBIND11_INTERNALS_VERSION < 11
-#    error "PYBIND11_INTERNALS_VERSION 11 is the minimum for all platforms for pybind11v3."
+#if PYBIND11_INTERNALS_VERSION < 12
+#    error "PYBIND11_INTERNALS_VERSION 12 is the minimum for all platforms for pybind11v3."
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
@@ -254,6 +254,10 @@ struct internals {
     // Unused if PYBIND11_SIMPLE_GIL_MANAGEMENT is defined:
     PyInterpreterState *istate = nullptr;
 
+    // Note that we have to use a std::string to allocate memory to ensure a unique address
+    // We want unique addresses since we use pointer equality to compare function records
+    std::string function_record_capsule_name = internals_function_record_capsule_name;
+
     type_map<PyObject *> native_enum_type_map;
 
     internals()
@@ -747,6 +751,26 @@ const char *c_str(Args &&...args) {
     return strings.front().c_str();
 }
 
+inline const char *get_function_record_capsule_name() {
+    // On GraalPy, pointer equality of the names is currently not guaranteed
+#if !defined(GRAALVM_PYTHON)
+    return get_internals().function_record_capsule_name.c_str();
+#else
+    return nullptr;
+#endif
+}
+
+// Determine whether or not the following capsule contains a pybind11 function record.
+// Note that we use `internals` to make sure that only ABI compatible records are touched.
+//
+// This check is currently used in two places:
+// - An important optimization in functional.h to avoid overhead in C++ -> Python -> C++
+// - The sibling feature of cpp_function to allow overloads
+inline bool is_function_record_capsule(const capsule &cap) {
+    // Pointer equality as we rely on internals() to ensure unique pointers
+    return cap.name() == get_function_record_capsule_name();
+}
+
 PYBIND11_NAMESPACE_END(detail)
 
 /// Returns a named pointer that is shared among all extension modules (using the same

include/pybind11/functional.h

@@ -91,8 +91,15 @@ struct type_caster<std::function<Return(Args...)>> {
             auto *cfunc_self = PyCFunction_GET_SELF(cfunc.ptr());
             if (cfunc_self == nullptr) {
                 PyErr_Clear();
-            } else {
-                function_record *rec = function_record_ptr_from_PyObject(cfunc_self);
+            } else if (isinstance<capsule>(cfunc_self)) {
+                auto c = reinterpret_borrow<capsule>(cfunc_self);
+
+                function_record *rec = nullptr;
+                // Check that we can safely reinterpret the capsule into a function_record
+                if (detail::is_function_record_capsule(c)) {
+                    rec = c.get_pointer<function_record>();
+                }
+
                 while (rec != nullptr) {
                     if (rec->is_stateless
                         && same_type(typeid(function_type),