Add optional 'const' keyword

src/llex.cpp

@@ -56,7 +56,7 @@ static const char *const luaX_tokens [] = {
     "pluto_use",
     "pluto_switch", "pluto_continue", "pluto_enum", "pluto_new", "pluto_class", "pluto_parent", "pluto_export",
           "switch",       "continue",       "enum",       "new",       "class",       "parent",       "export",
-    "let",
+    "let", "const",
     "pluto_suggest_0", "pluto_suggest_1",
     "return", "then", "true", "until", "while",
     "//", "..", "...", "==", ">=", "<=", "~=",

src/llex.h

@@ -42,7 +42,7 @@ enum RESERVED {
   TK_PUSE, // New compatibility keywords.
   TK_PSWITCH, TK_PCONTINUE, TK_PENUM, TK_PNEW, TK_PCLASS, TK_PPARENT, TK_PEXPORT,
   TK_SWITCH, TK_CONTINUE, TK_ENUM, TK_NEW, TK_CLASS, TK_PARENT, TK_EXPORT, // New non-compatible keywords.
-  TK_LET, // New optional keywords.
+  TK_LET, TK_CONST, // New optional keywords.
   TK_SUGGEST_0, TK_SUGGEST_1, // New special keywords.
   TK_RETURN, TK_THEN, TK_TRUE, TK_UNTIL, TK_WHILE,
   /* other terminal symbols */

src/lparser.cpp

@@ -3996,6 +3996,40 @@ static void localstat (LexState *ls) {
   checktoclose(fs, toclose);
 }
 
+static void conststat (LexState *ls) {
+  FuncState *fs = ls->fs;
+  auto line = ls->getLineNumber(); /* in case we need to emit a warning */
+  int vidx = new_localvar(ls, str_checkname(ls, N_OVERRIDABLE), line);
+  TypeHint hint = gettypehint(ls);
+  Vardesc *var = getlocalvardesc(fs, vidx);
+  var->vd.kind = RDKCONST;
+  *var->vd.hint = hint;
+
+  expdesc e;
+  if (testnext(ls, '=')) {
+    ls->pushContext(PARCTX_CREATE_VAR);
+    TypeHint t;
+    expr_propagate(ls, &e, t);
+    ls->popContext(PARCTX_CREATE_VAR);
+    if (luaK_exp2const(fs, &e, &var->k)) {  /* compile-time constant? */
+      var->vd.kind = RDKCTC;  /* variable is a compile-time constant */
+      fs->nactvar++;  /* don't adjustlocalvars, but count it */
+    }
+    else {
+      exp_propagate(ls, e, t);
+      process_assign(ls, var, t, line);
+      adjust_assign(ls, 1, 1, &e);
+      adjustlocalvars(ls, 1);
+    }
+  }
+  else {
+    e.k = VVOID;
+    process_assign(ls, var, TypeHint{ VT_NIL }, line);
+    adjust_assign(ls, 1, 0, &e);
+    adjustlocalvars(ls, 1);
+  }
+}
+
 
 static int funcname (LexState *ls, expdesc *v) {
   /* funcname -> NAME {fieldsel} [':' NAME] */
@@ -4298,6 +4332,11 @@ static void statement (LexState *ls, TypeHint *prop) {
         localstat(ls);
       break;
     }
+    case TK_CONST: {
+      luaX_next(ls);  /* skip CONST */
+      conststat(ls);
+      break;
+    }
     case TK_EXPORT:
     case TK_PEXPORT: {
       if (ls->fs->bl->previous)
@@ -4696,6 +4735,9 @@ LClosure *luaY_parser (lua_State *L, ZIO *z, Mbuffer *buff,
 #endif
 #ifndef PLUTO_USE_LET
   disablekeyword(&lexstate, TK_LET);
+#endif
+#ifndef PLUTO_USE_CONST
+  disablekeyword(&lexstate, TK_CONST);
 #endif
   mainfunc(&lexstate, &funcstate);
   lua_assert(!funcstate.prev && funcstate.nups == 1 && !lexstate.fs);

src/luaconf.h

@@ -861,6 +861,9 @@
 // If defined, Pluto will imply 'pluto_use let' at the beginning of every script.
 //#define PLUTO_USE_LET
 
+// If defined, Pluto will imply 'pluto_use const' at the beginning of every script.
+//#define PLUTO_USE_CONST
+
 /*
 ** {====================================================================
 ** Pluto configuration: Infinite Loop Prevention (ILP)