@@ -1255,6 +1255,22 @@ class mod_gil_not_used {
12551255 bool flag_;
12561256};
12571257
1258+ PYBIND11_NAMESPACE_BEGIN (detail)
1259+
1260+ inline bool gil_not_used_option () { return false ; }
1261+ template <typename F, typename ... O>
1262+ bool gil_not_used_option (F &&, O &&...o );
1263+ template <typename ... O>
1264+ inline bool gil_not_used_option (mod_gil_not_used f, O &&...o ) {
1265+ return f.flag () || gil_not_used_option (o...);
1266+ }
1267+ template <typename F, typename ... O>
1268+ inline bool gil_not_used_option (F &&, O &&...o ) {
1269+ return gil_not_used_option (o...);
1270+ }
1271+
1272+ PYBIND11_NAMESPACE_END (detail)
1273+
12581274// / Wrapper for Python extension modules
12591275class module_ : public object {
12601276public:
@@ -1389,6 +1405,56 @@ class module_ : public object {
13891405 // For Python 2, reinterpret_borrow was correct.
13901406 return reinterpret_borrow<module_>(m);
13911407 }
1408+
1409+ /* * \rst
1410+ Initialized a module def for use with multi-phase module initialization.
1411+
1412+ ``def`` should point to a statically allocated module_def.
1413+ ``slots`` must point to an initialized array of slots with space for at
1414+ least one additional slot to be populated based on the options.
1415+ \endrst */
1416+ template <typename ... Options>
1417+ static object initialize_multiphase_module_def (const char *name,
1418+ const char *doc,
1419+ module_def *def,
1420+ std::vector<PyModuleDef_Slot> &slots,
1421+ Options &&...options) {
1422+ // remove zero end sentinel, if present
1423+ while (!slots.empty () && slots.back ().slot == 0 ) {
1424+ slots.pop_back ();
1425+ }
1426+
1427+ bool nogil PYBIND11_MAYBE_UNUSED = detail::gil_not_used_option (options...);
1428+ if (nogil) {
1429+ #if defined(Py_mod_gil) && defined(Py_GIL_DISABLED)
1430+ slots.emplace_back (PyModuleDef_Slot{Py_mod_gil, Py_MOD_GIL_NOT_USED});
1431+ #endif
1432+ }
1433+
1434+ // must have a zero end sentinel
1435+ slots.emplace_back (PyModuleDef_Slot{0 , nullptr });
1436+
1437+ // module_def is PyModuleDef
1438+ // Placement new (not an allocation).
1439+ def = new (def)
1440+ PyModuleDef{/* m_base */ PyModuleDef_HEAD_INIT,
1441+ /* m_name */ name,
1442+ /* m_doc */ options::show_user_defined_docstrings () ? doc : nullptr ,
1443+ /* m_size */ 0 ,
1444+ /* m_methods */ nullptr ,
1445+ /* m_slots */ slots.data (),
1446+ /* m_traverse */ nullptr ,
1447+ /* m_clear */ nullptr ,
1448+ /* m_free */ nullptr };
1449+ auto *m = PyModuleDef_Init (def);
1450+ if (m == nullptr ) {
1451+ if (PyErr_Occurred ()) {
1452+ throw error_already_set ();
1453+ }
1454+ pybind11_fail (" Internal error in module_::initialize_multiphase_module_def()" );
1455+ }
1456+ return reinterpret_borrow<object>(m);
1457+ }
13921458};
13931459
13941460PYBIND11_NAMESPACE_BEGIN (detail)
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