gh-99845: Clean up _PyObject_VAR_SIZE() usage (#99847)

* code_sizeof() now uses an unsigned type (size_t) to compute the result.
* Fix _PyObject_ComputedDictPointer(): cast _PyObject_VAR_SIZE() to
  Py_ssize_t, rather than long: it's a different type on 64-bit Windows.
* Clarify that _PyObject_VAR_SIZE() uses an unsigned type (size_t).

Objects/codeobject.c

@@ -1867,15 +1867,13 @@ static PyGetSetDef code_getsetlist[] = {
 static PyObject *
 code_sizeof(PyCodeObject *co, PyObject *Py_UNUSED(args))
 {
-    Py_ssize_t res = _PyObject_VAR_SIZE(Py_TYPE(co), Py_SIZE(co));
-
+    size_t res = _PyObject_VAR_SIZE(Py_TYPE(co), Py_SIZE(co));
     _PyCodeObjectExtra *co_extra = (_PyCodeObjectExtra*) co->co_extra;
     if (co_extra != NULL) {
-        res += sizeof(_PyCodeObjectExtra) +
-               (co_extra->ce_size-1) * sizeof(co_extra->ce_extras[0]);
+        res += sizeof(_PyCodeObjectExtra);
+        res += ((size_t)co_extra->ce_size - 1) * sizeof(co_extra->ce_extras[0]);
     }
-
-    return PyLong_FromSsize_t(res);
+    return PyLong_FromSize_t(res);
 }
 
 static PyObject *

Objects/object.c

@@ -1043,22 +1043,25 @@ PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
 PyObject **
 _PyObject_ComputedDictPointer(PyObject *obj)
 {
-    Py_ssize_t dictoffset;
     PyTypeObject *tp = Py_TYPE(obj);
-
     assert((tp->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
-    dictoffset = tp->tp_dictoffset;
-    if (dictoffset == 0)
+
+    Py_ssize_t dictoffset = tp->tp_dictoffset;
+    if (dictoffset == 0) {
         return NULL;
+    }
+
     if (dictoffset < 0) {
         assert(dictoffset != -1);
+
         Py_ssize_t tsize = Py_SIZE(obj);
         if (tsize < 0) {
             tsize = -tsize;
         }
         size_t size = _PyObject_VAR_SIZE(tp, tsize);
+        assert(size <= (size_t)PY_SSIZE_T_MAX);
+        dictoffset += (Py_ssize_t)size;
 
-        dictoffset += (long)size;
         _PyObject_ASSERT(obj, dictoffset > 0);
         _PyObject_ASSERT(obj, dictoffset % SIZEOF_VOID_P == 0);
     }