Add configurable extra data at the end of lua_State

VM/include/luaconf.h

@@ -126,3 +126,6 @@
 #define LUA_VECTOR_SIZE 3	/* must be 3 or 4 */
 
 #define LUA_EXTRA_SIZE LUA_VECTOR_SIZE - 2
+
+/* size of optional extra data in bytes added at the end of lua_State. */
+#define LUA_STATE_EXTRA 0

VM/src/lstate.cpp

@@ -10,6 +10,8 @@
 #include "ldo.h"
 #include "ldebug.h"
 
+#include <string.h>
+
 /*
 ** Main thread combines a thread state and the global state
 */
@@ -78,6 +80,9 @@ static void preinit_state(lua_State* L, global_State* g)
     L->stackstate = 0;
     L->activememcat = 0;
     L->userdata = NULL;
+#if LUA_STATE_EXTRA > 0
+    memset(L->extra, 0, LUA_STATE_EXTRA);
+#endif
     setnilvalue(gt(L));
 }
 

VM/src/lstate.h

@@ -237,6 +237,9 @@ struct lua_State
     TString* namecall; /* when invoked from Luau using NAMECALL, what method do we need to invoke? */
 
     void* userdata;
+#if LUA_STATE_EXTRA > 0
+    char extra[LUA_STATE_EXTRA];
+#endif
 };
 // clang-format on