[parser] early error when declaration Pattern missing Initializer

src/preparser.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <cmath>
 
 #include "src/allocation.h"
 #include "src/base/logging.h"
 #include "src/conversions-inl.h"
 #include "src/conversions.h"
@@ skipping 513 matching lines @@
   // The ES6 Draft Rev3 specifies the following grammar for const declarations
   //
   // ConstDeclaration ::
   //   const ConstBinding (',' ConstBinding)* ';'
   // ConstBinding ::
   //   Identifier '=' AssignmentExpression
   //
   // TODO(ES6):
   // ConstBinding ::
   //   BindingPattern '=' AssignmentExpression
   bool require_initializer = false;

[adamk, 2015/11/03 20:13:53]

Similarly (though sadly differently), this is already tracking the "initializer
required" information.

[caitp (gmail), 2015/11/03 23:55:31]

The preparser doesn't do an amazing job of tracking this info, because it
doesn't care about `for-in/of`, so I don't think adding the `Token::IN ||
CheckContextualKeyword("of")` stuff can really disappear completely. The
preparser is actually doing some weird stuff here, and I don't 100% understand
it :\ What is the "is_strict_const" stuff about?

   bool is_strict_const = false;
   bool lexical = false;
   if (peek() == Token::VAR) {
     if (is_strong(language_mode())) {
       Scanner::Location location = scanner()->peek_location();
       ReportMessageAt(location, MessageTemplate::kStrongVar);
       *ok = false;
       return Statement::Default();
     }
     Consume(Token::VAR);
@@ skipping 28 matching lines @@
   // The scope of a var/const declared variable anywhere inside a function
   // is the entire function (ECMA-262, 3rd, 10.1.3, and 12.2). The scope
   // of a let declared variable is the scope of the immediately enclosing
   // block.
   int nvars = 0;  // the number of variables declared
   int bindings_start = peek_position();
   do {
     // Parse binding pattern.
     if (nvars > 0) Consume(Token::COMMA);
     {
+      int decl_pos = peek_position();
       ExpressionClassifier pattern_classifier;
       Token::Value next = peek();
       PreParserExpression pattern =
           ParsePrimaryExpression(&pattern_classifier, CHECK_OK);
       ValidateBindingPattern(&pattern_classifier, CHECK_OK);
       if (lexical) {
         ValidateLetPattern(&pattern_classifier, CHECK_OK);
       }
 
       if (!allow_harmony_destructuring() && !pattern.IsIdentifier()) {
         ReportUnexpectedToken(next);
         *ok = false;
         return Statement::Default();
       }
+
+      bool is_for_iteration_variable =
+          var_context == kForStatement &&
+          (peek() == Token::IN || PeekContextualKeyword(CStrVector("of")));
+
+      if (!is_for_iteration_variable &&
+          (pattern.IsObjectLiteral() || pattern.IsArrayLiteral()) &&
+          peek() != Token::ASSIGN) {
+        ReportMessageAt(
+            Scanner::Location(decl_pos, scanner()->location().end_pos),
+            MessageTemplate::kInvalidDestructuringDeclaration);
+        *ok = false;
+        return Statement::Default();
+      }
     }
 
     Scanner::Location variable_loc = scanner()->location();
     nvars++;
     if (peek() == Token::ASSIGN || require_initializer ||
         // require initializers for multiple consts.
         (is_strict_const && peek() == Token::COMMA)) {
       Expect(Token::ASSIGN, CHECK_OK);
       ExpressionClassifier classifier;
       ParseAssignmentExpression(var_context != kForStatement, &classifier,
@@ skipping 647 matching lines @@
     Expect(Token::RBRACE, CHECK_OK);
     return PreParserExpression::Default();
   }
 }
 
 #undef CHECK_OK
 
 
 }  // namespace internal
 }  // namespace v8