New compile option LUA_COMPAT_LOOPVAR

When on, this option makes for-loop control variables not read only.

lparser.c

@@ -1682,13 +1682,22 @@ static void forbody (LexState *ls, int base, int line, int nvars, int isgen) {
 }
 
 
+/*
+** Control whether for-loop control variables are read-only
+*/
+#if defined(LUA_COMPAT_LOOPVAR)
+#define LOOPVARKIND	VDKREG
+#else  /* by default, these variables are read only */
+#define LOOPVARKIND	RDKCONST
+#endif
+
 static void fornum (LexState *ls, TString *varname, int line) {
   /* fornum -> NAME = exp,exp[,exp] forbody */
   FuncState *fs = ls->fs;
   int base = fs->freereg;
   new_localvarliteral(ls, "(for state)");
   new_localvarliteral(ls, "(for state)");
-  new_varkind(ls, varname, RDKCONST);  /* control variable */
+  new_varkind(ls, varname, LOOPVARKIND);  /* control variable */
   checknext(ls, '=');
   exp1(ls);  /* initial value */
   checknext(ls, ',');
@@ -1715,7 +1724,7 @@ static void forlist (LexState *ls, TString *indexname) {
   new_localvarliteral(ls, "(for state)");  /* iterator function */
   new_localvarliteral(ls, "(for state)");  /* state */
   new_localvarliteral(ls, "(for state)");  /* closing var. (after swap) */
-  new_varkind(ls, indexname, RDKCONST);  /* control variable */
+  new_varkind(ls, indexname, LOOPVARKIND);  /* control variable */
   /* other declared variables */
   while (testnext(ls, ',')) {
     new_localvar(ls, str_checkname(ls));

luaconf.h

@@ -342,6 +342,13 @@
 #define LUA_COMPAT_GLOBAL
 
 
+/*
+@@ LUA_COMPAT_LOOPVAR makes for-loop control variables not read-only,
+** as they were in previous versions.
+*/
+/* #define LUA_COMPAT_LOOPVAR */
+
+
 /*
 @@ LUA_COMPAT_MATHLIB controls the presence of several deprecated
 ** functions in the mathematical library.