Block scoped const variables.

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 1120 matching lines @@
 Statement* Parser::ParseSourceElement(ZoneStringList* labels,
                                       bool* ok) {
   // (Ecma 262 5th Edition, clause 14):
   // SourceElement:
   //    Statement
   //    FunctionDeclaration
   //
   // In harmony mode we allow additionally the following productions
   // SourceElement:
   //    LetDeclaration
+  //    ConstDeclaration
 
   if (peek() == Token::FUNCTION) {
     return ParseFunctionDeclaration(ok);
-  } else if (peek() == Token::LET) {
+  } else if (peek() == Token::LET || peek() == Token::CONST) {
     return ParseVariableStatement(kSourceElement, ok);
-  } else {
-    return ParseStatement(labels, ok);
   }
+  return ParseStatement(labels, ok);
 }
 
 
 void* Parser::ParseSourceElements(ZoneList<Statement*>* processor,
                                   int end_token,
                                   bool* ok) {
   // SourceElements ::
   //   (SourceElement)* <end_token>
 
   // Allocate a target stack to use for this set of source
@@ skipping 197 matching lines @@
 
 VariableProxy* Parser::Declare(Handle<String> name,
                                VariableMode mode,
                                FunctionLiteral* fun,
                                bool resolve,
                                bool* ok) {
   Variable* var = NULL;
   // If we are inside a function, a declaration of a var/const variable is a
   // truly local variable, and the scope of the variable is always the function
   // scope.
+  // Let/const variables in harmony mode are always added to the immediately
+  // enclosing scope.
+  Scope* declaration_scope = mode == LET || mode == CONST_HARMONY
+      ? top_scope_ : top_scope_->DeclarationScope();
 
   // If a function scope exists, then we can statically declare this
   // variable and also set its mode. In any case, a Declaration node
   // will be added to the scope so that the declaration can be added
   // to the corresponding activation frame at runtime if necessary.
   // For instance declarations inside an eval scope need to be added
   // to the calling function context.
   // Similarly, strict mode eval scope does not leak variable declarations to
   // the caller's scope so we declare all locals, too.
-
-  Scope* declaration_scope = mode == LET ? top_scope_
-      : top_scope_->DeclarationScope();
+  // Also for block scoped let/const bindings the variable can be
+  // statically declared.
   if (declaration_scope->is_function_scope() ||
       declaration_scope->is_strict_mode_eval_scope() ||
       declaration_scope->is_block_scope()) {
     // Declare the variable in the function scope.
     var = declaration_scope->LocalLookup(name);
     if (var == NULL) {
       // Declare the name.
       var = declaration_scope->DeclareLocal(name, mode);
     } else {
       // The name was declared in this scope before; check for conflicting
       // re-declarations. We have a conflict if either of the declarations is
       // not a var. There is similar code in runtime.cc in the Declare
       // functions. The function CheckNonConflictingScope checks for conflicting
       // var and let bindings from different scopes whereas this is a check for
       // conflicting declarations within the same scope. This check also covers
       //
       // function () { let x; { var x; } }
       //
       // because the var declaration is hoisted to the function scope where 'x'
       // is already bound.
       if ((mode != VAR) || (var->mode() != VAR)) {
         // We only have vars, consts and lets in declarations.
         ASSERT(var->mode() == VAR ||
                var->mode() == CONST ||
+               var->mode() == CONST_HARMONY ||
                var->mode() == LET);
         if (harmony_scoping_) {
           // In harmony mode we treat re-declarations as early errors. See
           // ES5 16 for a definition of early errors.
           SmartArrayPointer<char> c_string = name->ToCString(DISALLOW_NULLS);
           const char* elms[2] = { "Variable", *c_string };
           Vector<const char*> args(elms, 2);
           ReportMessage("redeclaration", args);
           *ok = false;
           return NULL;
         }
-        const char* type = (var->mode() == VAR) ? "var" :
-                           (var->mode() == CONST) ? "const" : "let";
+        const char* type = (var->mode() == VAR)
+            ? "var" : var->is_const_mode() ? "const" : "let";
         Handle<String> type_string =
             isolate()->factory()->NewStringFromUtf8(CStrVector(type), TENURED);
         Expression* expression =
             NewThrowTypeError(isolate()->factory()->redeclaration_symbol(),
                               type_string, name);
         declaration_scope->SetIllegalRedeclaration(expression);
       }
     }
   }
 
@@ skipping 12 matching lines @@
   // same variable if it is declared several times. This is not a
   // semantic issue as long as we keep the source order, but it may be
   // a performance issue since it may lead to repeated
   // Runtime::DeclareContextSlot() calls.
   VariableProxy* proxy = declaration_scope->NewUnresolved(
       name, scanner().location().beg_pos);
   declaration_scope->AddDeclaration(
       new(zone()) Declaration(proxy, mode, fun, top_scope_));
 
   // For global const variables we bind the proxy to a variable.
-  if (mode == CONST && declaration_scope->is_global_scope()) {
+  if ((mode == CONST || mode == CONST_HARMONY) &&
+      declaration_scope->is_global_scope()) {
     ASSERT(resolve);  // should be set by all callers
     Variable::Kind kind = Variable::NORMAL;
     var = new(zone()) Variable(declaration_scope, name, CONST, true, kind);
   }
 
   // If requested and we have a local variable, bind the proxy to the variable
   // at parse-time. This is used for functions (and consts) declared inside
   // statements: the corresponding function (or const) variable must be in the
   // function scope and not a statement-local scope, e.g. as provided with a
   // 'with' statement:
@@ skipping 189 matching lines @@
 // variable, then *var is set to that variable. In all other cases,
 // *var is untouched; in particular, it is the caller's responsibility
 // to initialize it properly. This mechanism is used for the parsing
 // of 'for-in' loops.
 Block* Parser::ParseVariableDeclarations(
     VariableDeclarationContext var_context,
     VariableDeclarationProperties* decl_props,
     Handle<String>* out,
     bool* ok) {
   // VariableDeclarations ::
-  //   ('var' | 'const') (Identifier ('=' AssignmentExpression)?)+[',']
-
+  //   ('var' | 'const' | 'let') (Identifier ('=' AssignmentExpression)?)+[',']
+  //
+  // The ES6 Draft Rev3 specifies the following grammar for const declarations
+  //
+  // ConstDeclaration ::
+  //   const ConstBinding (',' ConstBinding)* ';'
+  // ConstBinding ::
+  //   Identifier '=' AssignmentExpression
+  //
+  // TODO(ES6):
+  // ConstBinding ::
+  //   BindingPattern '=' AssignmentExpression
   VariableMode mode = VAR;
   // 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) {
     Consume(Token::CONST);
-    if (top_scope_->is_strict_mode()) {
+    if (harmony_scoping_) {
+      if (var_context != kSourceElement &&
+          var_context != kForStatement) {
+        // In harmony 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;
+    } else if (top_scope_->is_strict_mode()) {
       ReportMessage("strict_const", Vector<const char*>::empty());
       *ok = false;
       return NULL;
+    } else {
+      mode = CONST;
+      init_op = Token::INIT_CONST;
     }
-    mode = CONST;
     is_const = true;
     needs_init = true;
-    init_op = Token::INIT_CONST;
   } else if (peek() == Token::LET) {
     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;
     needs_init = true;
     init_op = Token::INIT_LET;
   } else {
     UNREACHABLE();  // by current callers
   }
 
-  Scope* declaration_scope = (mode == LET)
+  Scope* declaration_scope = (mode == LET || mode == CONST_HARMONY)
       ? top_scope_ : top_scope_->DeclarationScope();
   // 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). Thus we can
   // transform a source-level var/const declaration into a (Function)
   // Scope declaration, and rewrite the source-level initialization into an
   // assignment statement. We use a block to collect multiple assignments.
   //
   // We mark the block as initializer block because we don't want the
   // rewriter to add a '.result' assignment to such a block (to get compliant
   // behavior for code such as print(eval('var x = 7')), and for cosmetic
@@ skipping 24 matching lines @@
     // assignment for variables and constants because the value must be assigned
     // when the variable is encountered in the source. But the variable/constant
     // is declared (and set to 'undefined') upon entering the function within
     // which the variable or constant is declared. Only function variables have
     // an initial value in the declaration (because they are initialized upon
     // entering the function).
     //
     // If we have a const declaration, in an inner scope, the proxy is always
     // bound to the declared variable (independent of possibly surrounding with
     // statements).
-    Declare(name, mode, NULL, is_const /* always bound for CONST! */,
-            CHECK_OK);
+    // For let/const declarations in harmony mode, we can also immediately
+    // pre-resolve the proxy because it resides in the same scope as the
+    // declaration.
+    Declare(name, mode, NULL, mode != VAR, CHECK_OK);
     nvars++;
     if (declaration_scope->num_var_or_const() > kMaxNumFunctionLocals) {
       ReportMessageAt(scanner().location(), "too_many_variables",
                       Vector<const char*>::empty());
       *ok = false;
       return NULL;
     }
 
     // Parse initialization expression if present and/or needed. A
     // declaration of the form:
@@ skipping 18 matching lines @@
     //
     //   const c; c = x;
     //
     // The "variable" c initialized to x is the same as the declared
     // one - there is no re-lookup (see the last parameter of the
     // Declare() call above).
 
     Scope* initialization_scope = is_const ? declaration_scope : top_scope_;
     Expression* value = NULL;
     int position = -1;
-    if (peek() == Token::ASSIGN) {
+    // Harmony consts have non-optional initializers.
+    if (peek() == Token::ASSIGN || mode == CONST_HARMONY) {
       Expect(Token::ASSIGN, CHECK_OK);
       position = scanner().location().beg_pos;
       value = ParseAssignmentExpression(var_context != kForStatement, CHECK_OK);
       // Don't infer if it is "a = function(){...}();"-like expression.
       if (fni_ != NULL &&
           value->AsCall() == NULL &&
           value->AsCallNew() == NULL) {
         fni_->Infer();
       } else {
         fni_->RemoveLastFunction();
@@ skipping 18 matching lines @@
     // executed.
     //
     // Executing the variable declaration statement will always
     // guarantee to give the global object a "local" variable; a
     // variable defined in the global object and not in any
     // prototype. This way, global variable declarations can shadow
     // properties in the prototype chain, but only after the variable
     // declaration statement has been executed. This is important in
     // browsers where the global object (window) has lots of
     // properties defined in prototype objects.
-
     if (initialization_scope->is_global_scope()) {
       // Compute the arguments for the runtime call.
       ZoneList<Expression*>* arguments = new(zone()) ZoneList<Expression*>(3);
       // We have at least 1 parameter.
       arguments->Add(NewLiteral(name));
       CallRuntime* initialize;
 
       if (is_const) {
         arguments->Add(value);
         value = NULL;  // zap the value to avoid the unnecessary assignment
@@ skipping 43 matching lines @@
     // Add an assignment node to the initialization statement block if we still
     // have a pending initialization value. We must distinguish between
     // different kinds of declarations: 'var' initializations are simply
     // assignments (with all the consequences if they are inside a 'with'
     // statement - they may change a 'with' object property). Constant
     // initializations always assign to the declared constant which is
     // always at the function scope level. This is only relevant for
     // dynamically looked-up variables and constants (the start context
     // for constant lookups is always the function context, while it is
     // the top context for var declared variables). Sigh...
-    // For 'let' declared variables the initialization is in the same scope
-    // as the declaration. Thus dynamic lookups are unnecessary even if the
-    // block scope is inside a with.
+    // For 'let' and 'const' declared variables in harmony mode the
+    // initialization is in the same scope as the declaration. Thus dynamic
+    // lookups are unnecessary even if the block scope is inside a with.
     if (value != NULL) {
       VariableProxy* proxy = initialization_scope->NewUnresolved(name);
       Assignment* assignment =
           new(zone()) Assignment(isolate(), init_op, proxy, value, position);
       block->AddStatement(new(zone()) ExpressionStatement(assignment));
     }
 
     if (fni_ != NULL) fni_->Leave();
   } while (peek() == Token::COMMA);
 
@@ skipping 2034 matching lines @@
 
     Expect(Token::LBRACE, CHECK_OK);
 
     // If we have a named function expression, we add a local variable
     // declaration to the body of the function with the name of the
     // function and let it refer to the function itself (closure).
     // NOTE: We create a proxy and resolve it here so that in the
     // future we can change the AST to only refer to VariableProxies
     // instead of Variables and Proxis as is the case now.
     if (type == FunctionLiteral::NAMED_EXPRESSION) {
-      Variable* fvar = top_scope_->DeclareFunctionVar(function_name);
+      VariableMode fvar_mode;
+      Token::Value fvar_init_op;
+      if (harmony_scoping_) {
+        fvar_mode = CONST_HARMONY;
+        fvar_init_op = Token::INIT_CONST_HARMONY;
+      } else {
+        fvar_mode = CONST;
+        fvar_init_op = Token::INIT_CONST;
+      }
+      Variable* fvar = top_scope_->DeclareFunctionVar(function_name, fvar_mode);
       VariableProxy* fproxy = top_scope_->NewUnresolved(function_name);
       fproxy->BindTo(fvar);
       body->Add(new(zone()) ExpressionStatement(
           new(zone()) Assignment(isolate(),
-                                 Token::INIT_CONST,
+                                 fvar_init_op,
                                  fproxy,
                                  new(zone()) ThisFunction(isolate()),
                                  RelocInfo::kNoPosition)));
     }
 
     // Determine if the function will be lazily compiled. The mode can only
     // be PARSE_LAZILY if the --lazy flag is true.  We will not lazily
     // compile if we do not have preparser data for the function.
     bool is_lazily_compiled = (mode() == PARSE_LAZILY &&
                                top_scope_->outer_scope()->is_global_scope() &&
@@ skipping 1434 matching lines @@
       result = parser.ParseProgram(source,
                                    info->is_global(),
                                    info->StrictMode());
     }
   }
   info->SetFunction(result);
   return (result != NULL);
 }
 
 } }  // namespace v8::internal