Statically check for assignments to const in harmony mode.

src/parser.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 1245 matching lines @@
   // parsed into an empty statement.
 
   // Keep the source position of the statement
   int statement_pos = scanner().peek_location().beg_pos;
   Statement* stmt = NULL;
   switch (peek()) {
     case Token::LBRACE:
       return ParseBlock(labels, ok);
 
     case Token::CONST:  // fall through
+    case Token::LET:
     case Token::VAR:
       stmt = ParseVariableStatement(kStatement, ok);
       break;
 
     case Token::SEMICOLON:
       Next();
       return EmptyStatement();
 
     case Token::IF:
       stmt = ParseIfStatement(labels, ok);
@@ skipping 421 matching lines @@
   // True if the binding needs initialization. 'let' and 'const' declared
   // bindings are created uninitialized by their declaration nodes and
   // need initialization. 'var' declared bindings are always initialized
   // immediately by their declaration nodes.
   bool needs_init = false;
   bool is_const = false;
   Token::Value init_op = Token::INIT_VAR;
   if (peek() == Token::VAR) {
     Consume(Token::VAR);
   } else if (peek() == Token::CONST) {
+    // TODO(ES6): The ES6 Draft Rev4 section 12.2.2 reads:
+    //
+    // ConstDeclaration : const ConstBinding (',' ConstBinding)* ';'
+    //
+    // * It is a Syntax Error if the code that matches this production is not
+    //   contained in extended code.
+    //
+    // However disallowing const in classic mode will break compatibility with
+    // existing pages. Therefore we keep allowing const with the old
+    // non-harmony semantics in classic mode.
     Consume(Token::CONST);
     switch (top_scope_->language_mode()) {
       case CLASSIC_MODE:
         mode = CONST;
         init_op = Token::INIT_CONST;
         break;
       case STRICT_MODE:
         ReportMessage("strict_const", Vector<const char*>::empty());
         *ok = false;
         return NULL;
       case EXTENDED_MODE:
         if (var_context != kSourceElement &&
             var_context != kForStatement) {
           // In extended mode 'const' declarations are only allowed in source
           // element positions.
           ReportMessage("unprotected_const", Vector<const char*>::empty());
           *ok = false;
           return NULL;
         }
         mode = CONST_HARMONY;
         init_op = Token::INIT_CONST_HARMONY;
     }
     is_const = true;
     needs_init = true;
   } else if (peek() == Token::LET) {
-    ASSERT(top_scope_->is_extended_mode());
+    // ES6 Draft Rev4 section 12.2.1:
+    //
+    // LetDeclaration : let LetBindingList ;
+    //
+    // * It is a Syntax Error if the code that matches this production is not
+    //   contained in extended code.
+    if (!is_extended_mode()) {
+      ReportMessage("illegal_let", Vector<const char*>::empty());
+      *ok = false;
+      return NULL;
+    }
     Consume(Token::LET);
     if (var_context != kSourceElement &&
         var_context != kForStatement) {
       // Let declarations are only allowed in source element positions.
       ASSERT(var_context == kStatement);
       ReportMessage("unprotected_let", Vector<const char*>::empty());
       *ok = false;
       return NULL;
     }
     mode = LET;
@@ skipping 918 matching lines @@
   if (expression == NULL || !expression->IsValidLeftHandSide()) {
     Handle<String> type =
         isolate()->factory()->invalid_lhs_in_assignment_symbol();
     expression = NewThrowReferenceError(type);
   }
 
   if (!top_scope_->is_classic_mode()) {
     // Assignment to eval or arguments is disallowed in strict mode.
     CheckStrictModeLValue(expression, "strict_lhs_assignment", CHECK_OK);
   }
+  MarkAsLValue(expression);
 
   Token::Value op = Next();  // Get assignment operator.
   int pos = scanner().location().beg_pos;
   Expression* right = ParseAssignmentExpression(accept_IN, CHECK_OK);
 
   // TODO(1231235): We try to estimate the set of properties set by
   // constructors. We define a new property whenever there is an
   // assignment to a property of 'this'. We should probably only add
   // properties if we haven't seen them before. Otherwise we'll
   // probably overestimate the number of properties.
@@ skipping 213 matching lines @@
     if (expression == NULL || !expression->IsValidLeftHandSide()) {
       Handle<String> type =
           isolate()->factory()->invalid_lhs_in_prefix_op_symbol();
       expression = NewThrowReferenceError(type);
     }
 
     if (!top_scope_->is_classic_mode()) {
       // Prefix expression operand in strict mode may not be eval or arguments.
       CheckStrictModeLValue(expression, "strict_lhs_prefix", CHECK_OK);
     }
+    MarkAsLValue(expression);
 
     int position = scanner().location().beg_pos;
     return new(zone()) CountOperation(isolate(),
                                       op,
                                       true /* prefix */,
                                       expression,
                                       position);
 
   } else {
     return ParsePostfixExpression(ok);
@@ skipping 15 matching lines @@
     if (expression == NULL || !expression->IsValidLeftHandSide()) {
       Handle<String> type =
           isolate()->factory()->invalid_lhs_in_postfix_op_symbol();
       expression = NewThrowReferenceError(type);
     }
 
     if (!top_scope_->is_classic_mode()) {
       // Postfix expression operand in strict mode may not be eval or arguments.
       CheckStrictModeLValue(expression, "strict_lhs_prefix", CHECK_OK);
     }
+    MarkAsLValue(expression);
 
     Token::Value next = Next();
     int position = scanner().location().beg_pos;
     expression =
         new(zone()) CountOperation(isolate(),
                                    next,
                                    false /* postfix */,
                                    expression,
                                    position);
   }
@@ skipping 1338 matching lines @@
       next != Token::FUTURE_STRICT_RESERVED_WORD &&
       !Token::IsKeyword(next)) {
     ReportUnexpectedToken(next);
     *ok = false;
     return Handle<String>();
   }
   return GetSymbol(ok);
 }
 
 
+void Parser::MarkAsLValue(Expression* expression) {
+  VariableProxy* proxy = expression != NULL
+      ? expression->AsVariableProxy()
+      : NULL;
+
+  if (proxy != NULL) proxy->MarkAsLValue();
+}
+
+
 // Checks LHS expression for assignment and prefix/postfix increment/decrement
 // in strict mode.
 void Parser::CheckStrictModeLValue(Expression* expression,
                                    const char* error,
                                    bool* ok) {
   ASSERT(!top_scope_->is_classic_mode());
   VariableProxy* lhs = expression != NULL
       ? expression->AsVariableProxy()
       : NULL;
 
@@ skipping 1152 matching lines @@
       ASSERT(info->isolate()->has_pending_exception());
     } else {
       result = parser.ParseProgram(info);
     }
   }
   info->SetFunction(result);
   return (result != NULL);
 }
 
 } }  // namespace v8::internal