diff -r 55beab95170f src/Makefile
--- a/src/Makefile	Mon Jul 15 19:07:40 2019 +0300
+++ b/src/Makefile	Mon Jul 15 20:37:19 2019 +0300
@@ -7,7 +7,8 @@
 PLAT= none
 
 CC= gcc -std=gnu99
-CFLAGS= -O2 -Wall -Wextra -DLUA_COMPAT_5_3 $(SYSCFLAGS) $(MYCFLAGS)
+#CFLAGS= -O2 -Wall -Wextra -DLUA_COMPAT_5_3 $(SYSCFLAGS) $(MYCFLAGS)
+CFLAGS= -O2 -Wall -Wextra $(SYSCFLAGS) $(MYCFLAGS)
 LDFLAGS= $(SYSLDFLAGS) $(MYLDFLAGS)
 LIBS= -lm $(SYSLIBS) $(MYLIBS)
 
diff -r 55beab95170f src/llex.c
--- a/src/llex.c	Mon Jul 15 19:07:40 2019 +0300
+++ b/src/llex.c	Mon Jul 15 20:37:19 2019 +0300
@@ -41,7 +41,7 @@
     "and", "break", "do", "else", "elseif",
     "end", "false", "for", "function", "goto", "if",
     "in", "local", "nil", "not", "or", "repeat",
-    "return", "then", "true", "until", "while",
+    "return", "then", "true", "until", "pure", "while",
     "//", "..", "...", "==", ">=", "<=", "~=",
     "<<", ">>", "::", "<eof>",
     "<number>", "<integer>", "<name>", "<string>"
diff -r 55beab95170f src/llex.h
--- a/src/llex.h	Mon Jul 15 19:07:40 2019 +0300
+++ b/src/llex.h	Mon Jul 15 20:37:19 2019 +0300
@@ -28,7 +28,7 @@
   TK_AND = FIRST_RESERVED, TK_BREAK,
   TK_DO, TK_ELSE, TK_ELSEIF, TK_END, TK_FALSE, TK_FOR, TK_FUNCTION,
   TK_GOTO, TK_IF, TK_IN, TK_LOCAL, TK_NIL, TK_NOT, TK_OR, TK_REPEAT,
-  TK_RETURN, TK_THEN, TK_TRUE, TK_UNTIL, TK_WHILE,
+  TK_RETURN, TK_THEN, TK_TRUE, TK_UNTIL, TK_PURE, TK_WHILE,
   /* other terminal symbols */
   TK_IDIV, TK_CONCAT, TK_DOTS, TK_EQ, TK_GE, TK_LE, TK_NE,
   TK_SHL, TK_SHR,
diff -r 55beab95170f src/lparser.c
--- a/src/lparser.c	Mon Jul 15 19:07:40 2019 +0300
+++ b/src/lparser.c	Mon Jul 15 20:37:19 2019 +0300
@@ -367,10 +367,14 @@
         markupval(fs, v);  /* local will be used as an upval */
     }
     else {  /* not found as local at current level; try upvalues */
+      if (fs->pure) {
+        var->k=VFORBIDDEN;
+        return;
+      }
       int idx = searchupvalue(fs, n);  /* try existing upvalues */
       if (idx < 0) {  /* not found? */
         singlevaraux(fs->prev, n, var, 0);  /* try upper levels */
-        if (var->k == VVOID)  /* not found? */
+        if (var->k == VVOID || var->k == VFORBIDDEN)  /* not found? */
           return;  /* it is a global */
         /* else was LOCAL or UPVAL */
         idx  = newupvalue(fs, n, var);  /* will be a new upvalue */
@@ -389,9 +393,17 @@
   TString *varname = str_checkname(ls);
   FuncState *fs = ls->fs;
   singlevaraux(fs, varname, var, 1);
+  if (var->k == VFORBIDDEN) {
+      var->k = VVOID;
+      const char *msg = "no access to '%s'";
+      msg = luaO_pushfstring(ls->L, msg, getstr(varname));
+      luaK_semerror(ls, msg);  /* error */
+      return;
+  }
   if (var->k == VVOID) {  /* global name? */
     expdesc key;
     singlevaraux(fs, ls->envn, var, 1);  /* get environment variable */
+    if (var->k == VFORBIDDEN) var->k = VVOID;
     lua_assert(var->k != VVOID);  /* this one must exist */
     codestring(ls, &key, varname);  /* key is variable name */
     luaK_indexed(fs, var, &key);  /* env[varname] */
@@ -667,6 +679,7 @@
   fs->nlocvars = 0;
   fs->nactvar = 0;
   fs->needclose = 0;
+  fs->pure = 0;
   fs->firstlocal = ls->dyd->actvar.n;
   fs->firstlabel = ls->dyd->label.n;
   fs->bl = NULL;
@@ -924,6 +937,10 @@
   }
   parlist(ls);
   checknext(ls, ')');
+  if (ls->t.token==TK_PURE) {
+    luaX_next(ls); /* skip pure */
+    new_fs.pure=1;
+  }
   statlist(ls);
   new_fs.f->lastlinedefined = ls->linenumber;
   check_match(ls, TK_END, TK_FUNCTION, line);
diff -r 55beab95170f src/lparser.h
--- a/src/lparser.h	Mon Jul 15 19:07:40 2019 +0300
+++ b/src/lparser.h	Mon Jul 15 20:37:19 2019 +0300
@@ -25,6 +25,7 @@
 typedef enum {
   VVOID,  /* when 'expdesc' describes the last expression a list,
              this kind means an empty list (so, no expression) */
+  VFORBIDDEN, /* access forbidden */
   VNIL,  /* constant nil */
   VTRUE,  /* constant true */
   VFALSE,  /* constant false */
@@ -141,6 +142,7 @@
   lu_byte freereg;  /* first free register */
   lu_byte iwthabs;  /* instructions issued since last absolute line info */
   lu_byte needclose;  /* function needs to close upvalues when returning */
+  lu_byte pure;
 } FuncState;
 
 
diff -r 55beab95170f src/lua.h
--- a/src/lua.h	Mon Jul 15 19:07:40 2019 +0300
+++ b/src/lua.h	Mon Jul 15 20:37:19 2019 +0300
@@ -18,7 +18,7 @@
 
 #define LUA_VERSION_MAJOR	"5"
 #define LUA_VERSION_MINOR	"4"
-#define LUA_VERSION_RELEASE	"0"
+#define LUA_VERSION_RELEASE	"0+pure"
 
 #define LUA_VERSION_NUM			504
 #define LUA_VERSION_RELEASE_NUM		(LUA_VERSION_NUM * 100 + 0)