diff --git a/Python/initconfig.c b/Python/initconfig.c
index 7886d09..27ae48d 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -94,6 +94,7 @@ static const char usage_3[] = "\
              otherwise activate automatically)\n\
          -X pycache_prefix=PATH: enable writing .pyc files to a parallel tree rooted at the\n\
              given directory instead of to the code tree\n\
+         -X warn_default_encoding: enable opt-in EncodingWarning for 'encoding=None'\n\
 \n\
 --check-hash-based-pycs always|default|never:\n\
     control how Python invalidates hash-based .pyc files\n\
@@ -129,7 +130,8 @@ static const char usage_6[] =
 "PYTHONBREAKPOINT: if this variable is set to 0, it disables the default\n"
 "   debugger. It can be set to the callable of your debugger of choice.\n"
 "PYTHONDEVMODE: enable the development mode.\n"
-"PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.\n";
+"PYTHONPYCACHEPREFIX: root directory for bytecode cache (pyc) files.\n"
+"PYTHONWARNDEFAULTENCODING: enable opt-in EncodingWarning for 'encoding=None'.\n";
 
 #if defined(MS_WINDOWS)
 #  define PYTHONHOMEHELP "<prefix>\\python{major}{minor}"
@@ -600,6 +602,7 @@ config_check_consistency(const PyConfig *config)
     assert(config->malloc_stats >= 0);
     assert(config->site_import >= 0);
     assert(config->bytes_warning >= 0);
+    assert(config->warn_default_encoding >= 0);
     assert(config->inspect >= 0);
     assert(config->interactive >= 0);
     assert(config->optimization_level >= 0);
@@ -698,6 +701,7 @@ _PyConfig_InitCompatConfig(PyConfig *config)
     config->parse_argv = 0;
     config->site_import = -1;
     config->bytes_warning = -1;
+    config->warn_default_encoding = 0;
     config->inspect = -1;
     config->interactive = -1;
     config->optimization_level = -1;
@@ -906,6 +910,7 @@ _PyConfig_Copy(PyConfig *config, const PyConfig *config2)
 
     COPY_ATTR(site_import);
     COPY_ATTR(bytes_warning);
+    COPY_ATTR(warn_default_encoding);
     COPY_ATTR(inspect);
     COPY_ATTR(interactive);
     COPY_ATTR(optimization_level);
@@ -1007,6 +1012,7 @@ _PyConfig_AsDict(const PyConfig *config)
     SET_ITEM_WSTR(platlibdir);
     SET_ITEM_INT(site_import);
     SET_ITEM_INT(bytes_warning);
+    SET_ITEM_INT(warn_default_encoding);
     SET_ITEM_INT(inspect);
     SET_ITEM_INT(interactive);
     SET_ITEM_INT(optimization_level);
@@ -1271,6 +1277,7 @@ _PyConfig_FromDict(PyConfig *config, PyObject *dict)
     GET_WSTRLIST(warnoptions);
     GET_UINT(site_import);
     GET_UINT(bytes_warning);
+    GET_UINT(warn_default_encoding);
     GET_UINT(inspect);
     GET_UINT(interactive);
     GET_UINT(optimization_level);