Oh and obviously, it's not possible possible to define structures which *include* PyObject or PyVarObject if PyObject and PyVarObject become opaque. Example:
typedef struct {
PyObject ob_base;
Py_ssize_t ob_size; /* Number of items in variable part */
} PyVarObject;
This C code requires the PyObject structure to be fully defined (not being opaque).
A new C API and ABI where structures *don't* include PyObject or PyVarObject should be designed to allocate their members "before" the PyObject* pointer value. Something like the current PyGC_Head structure which is excluded from PyObject and stored *before* the "PyObject*" pointer.
Simplified code which allocates memory for an object implementin the GC protocol:
static PyObject *
_PyObject_GC_Malloc(size_t basicsize)
{
...
size_t size = sizeof(PyGC_Head) + basicsize;
...
PyGC_Head *g = (PyGC_Head *)PyObject_Malloc(size);
...
PyObject *op = (PyObject *)(g + 1);
return op;
}