[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 539 matching lines @@
     // * It is a Syntax Error if the code that matches this production is not
     //   contained in extended code.
     //
     // However disallowing const in sloppy mode will break compatibility with
     // existing pages. Therefore we keep allowing const with the old
     // non-harmony semantics in sloppy mode.
     Consume(Token::CONST);
     if (is_strict(language_mode()) ||
         (allow_harmony_sloppy() && !allow_legacy_const())) {
       DCHECK(var_context != kStatement);
       is_strict_const = true;

[adamk, 2015/11/04 00:37:54]

This means "is not legacy const".

-      require_initializer = var_context != kForStatement;
+      require_initializer = true;

[adamk, 2015/11/04 00:37:54]

Seems like "is_const" would be a better name for this variable now.

       lexical = true;
     }
   } else if (peek() == Token::LET && allow_let()) {
     Consume(Token::LET);
     DCHECK(var_context != kStatement);
     lexical = true;
   } else {
     *ok = false;
     return Statement::Default();
   }
 
   // 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();
+    bool is_pattern = false;
     {
       ExpressionClassifier pattern_classifier;
       Token::Value next = peek();
       PreParserExpression pattern =
           ParsePrimaryExpression(&pattern_classifier, CHECK_OK);
+      is_pattern = pattern.IsObjectLiteral();  // (or pattern.IsArrayLiteral());

[adamk, 2015/11/04 00:37:54]

I'd rather you just add the bit for ArrayLiteral to PreParserExpression and make
this an actual || in the code.

[caitp (gmail), 2015/11/04 15:07:28]

Done, although I think it will mean adding an extra bit to the TypeField for the
destructuring assignment parsing CL

+
       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")));
+
     Scanner::Location variable_loc = scanner()->location();
     nvars++;
-    if (peek() == Token::ASSIGN || require_initializer ||
+    if (peek() == Token::ASSIGN ||
+        (!is_for_iteration_variable && (require_initializer || is_pattern)) ||

[adamk, 2015/11/04 00:37:54]

Maybe put this in the same order as in the parser to make it easier to read?

[rossberg, 2015/11/04 10:52:36]

Same comment as in the parser: move this to else branch.

         // require initializers for multiple consts.
         (is_strict_const && peek() == Token::COMMA)) {

[adamk, 2015/11/04 00:37:54]

I think this bit is now redundant with your is_for_iteration_variable thing. See
https://codereview.chromium.org/671913002 for its origin, I think it was just a
mistake.

[caitp (gmail), 2015/11/04 15:07:28]

Well, this was the only place the variable was used --- I've gotten rid of it
and tests still pass, so it's probably okay

-      Expect(Token::ASSIGN, CHECK_OK);
+      if (peek() != Token::ASSIGN) {

[adamk, 2015/11/04 00:37:54]

Same note here as in the parser, you can use if (!Check(...))

+        PreParserTraits::ReportMessageAt(
+            Scanner::Location(decl_pos, scanner()->location().end_pos),
+            MessageTemplate::kDeclarationMissingInitializer,
+            is_pattern ? "destructuring" : "const");
+        *ok = false;
+        return Statement::Default();
+      }
+      Consume(Token::ASSIGN);
       ExpressionClassifier classifier;
       ParseAssignmentExpression(var_context != kForStatement, &classifier,
                                 CHECK_OK);
       ValidateExpression(&classifier, CHECK_OK);
 
       variable_loc.end_pos = scanner()->location().end_pos;
       if (first_initializer_loc && !first_initializer_loc->IsValid()) {
         *first_initializer_loc = variable_loc;
       }
     }
@@ skipping 639 matching lines @@
     Expect(Token::RBRACE, CHECK_OK);
     return PreParserExpression::Default();
   }
 }
 
 #undef CHECK_OK
 
 
 }  // namespace internal
 }  // namespace v8