WIP [esnext] implement async iteration proposal

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index ecde4f28831eced822e4e9d7a25b9a0293407fef..1fce16a6112e62dd385f0df04422a4c523eb897f 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -554,6 +554,7 @@ Parser::Parser(ParseInfo* info)
   set_allow_harmony_trailing_commas(FLAG_harmony_trailing_commas);
   set_allow_harmony_class_fields(FLAG_harmony_class_fields);
   set_allow_harmony_object_spread(FLAG_harmony_object_spread);
+  set_allow_harmony_async_iteration(FLAG_harmony_async_iteration);
   for (int feature = 0; feature < v8::Isolate::kUseCounterFeatureCount;
        ++feature) {
     use_counts_[feature] = 0;
@@ -1805,6 +1806,45 @@ Expression* Parser::BuildIteratorNextResult(Expression* iterator,
       throw_call, pos);
 }
 
+Expression* Parser::AwaitIteratorNextResult(Expression* iterator,
+                                            Variable* result, int pos) {
+  Expression* next_literal = factory()->NewStringLiteral(
+      ast_value_factory()->next_string(), kNoSourcePosition);
+  Expression* next_property =
+      factory()->NewProperty(iterator, next_literal, kNoSourcePosition);
+  ZoneList<Expression*>* next_arguments =
+      new (zone()) ZoneList<Expression*>(0, zone());
+  Expression* next_call =
+      factory()->NewCall(next_property, next_arguments, pos);
+
+  // Await(iterator.next())
+  Expression* await_next_call = RewriteAwaitExpression(next_call, pos);
+
+  Expression* result_proxy = factory()->NewVariableProxy(result);
+  Expression* left = factory()->NewAssignment(Token::ASSIGN, result_proxy,
+                                              await_next_call, pos);
+
+  // %_IsJSReceiver(...)
+  ZoneList<Expression*>* is_spec_object_args =
+      new (zone()) ZoneList<Expression*>(1, zone());
+  is_spec_object_args->Add(left, zone());
+  Expression* is_spec_object_call = factory()->NewCallRuntime(
+      Runtime::kInlineIsJSReceiver, is_spec_object_args, pos);
+
+  // %ThrowIteratorResultNotAnObject(result)
+  Expression* result_proxy_again = factory()->NewVariableProxy(result);
+  ZoneList<Expression*>* throw_arguments =
+      new (zone()) ZoneList<Expression*>(1, zone());
+  throw_arguments->Add(result_proxy_again, zone());
+  Expression* throw_call = factory()->NewCallRuntime(
+      Runtime::kThrowIteratorResultNotAnObject, throw_arguments, pos);
+
+  return factory()->NewBinaryOperation(
+      Token::AND,
+      factory()->NewUnaryOperation(Token::NOT, is_spec_object_call, pos),
+      throw_call, pos);
+}
+
 Statement* Parser::InitializeForEachStatement(ForEachStatement* stmt,
                                               Expression* each,
                                               Expression* subject,
@@ -2092,6 +2132,132 @@ Statement* Parser::InitializeForOfStatement(ForOfStatement* for_of,
   return finalize ? FinalizeForOfStatement(for_of, completion, nopos) : for_of;
 }
 
+Statement* Parser::InitializeForAwaitOfStatement(ForEachStatement* stmt,
+                                                 Expression* each,
+                                                 Expression* iterable,
+                                                 Statement* body, bool finalize,
+                                                 int next_result_pos) {
+  DCHECK(stmt->IsForOfStatement());
+  ForOfStatement* for_of = stmt->AsForOfStatement();
+
+  // Create the auxiliary expressions needed for iterating over the iterable,
+  // and initialize the given ForOfStatement with them.
+  // If finalize is true, also instrument the loop with code that performs the
+  // proper ES6 iterator finalization.  In that case, the result is not
+  // immediately a ForOfStatement.
+
+  const int nopos = kNoSourcePosition;
+  auto avfactory = ast_value_factory();
+
+  Variable* iterator = NewTemporary(ast_value_factory()->dot_iterator_string());
+  Variable* result = NewTemporary(ast_value_factory()->dot_result_string());
+  Variable* completion = NewTemporary(avfactory->empty_string());
+
+  // iterator = GetIterator(iterable, async)
+  Expression* assign_iterator;
+  {
+    assign_iterator = factory()->NewAssignment(
+        Token::ASSIGN, factory()->NewVariableProxy(iterator),
+        factory()->NewGetIterator(iterable, GetIterator::kAsync,
+                                  iterable->position()),
+        iterable->position());
+  }
+
+  Expression* next_result;
+  {
+    Expression* iterator_proxy = factory()->NewVariableProxy(iterator);
+    next_result =
+        AwaitIteratorNextResult(iterator_proxy, result, next_result_pos);
+  }
+
+  // result.done
+  Expression* result_done;
+  {
+    Expression* done_literal = factory()->NewStringLiteral(
+        ast_value_factory()->done_string(), kNoSourcePosition);
+    Expression* result_proxy = factory()->NewVariableProxy(result);
+    result_done =
+        factory()->NewProperty(result_proxy, done_literal, kNoSourcePosition);
+  }
+
+  // result.value
+  Expression* result_value;
+  {
+    Expression* value_literal =
+        factory()->NewStringLiteral(avfactory->value_string(), nopos);
+    Expression* result_proxy = factory()->NewVariableProxy(result);
+    result_value = factory()->NewProperty(result_proxy, value_literal, nopos);
+  }
+
+  // {{completion = kAbruptCompletion;}}
+  Statement* set_completion_abrupt;
+  if (finalize) {
+    Expression* proxy = factory()->NewVariableProxy(completion);
+    Expression* assignment = factory()->NewAssignment(
+        Token::ASSIGN, proxy,
+        factory()->NewSmiLiteral(Parser::kAbruptCompletion, nopos), nopos);
+
+    Block* block = factory()->NewBlock(nullptr, 1, true, nopos);
+    block->statements()->Add(
+        factory()->NewExpressionStatement(assignment, nopos), zone());
+    set_completion_abrupt = block;
+  }
+
+  // do { let tmp = #result_value; #set_completion_abrupt; tmp }
+  // Expression* result_value (gets overwritten)
+  if (finalize) {
+    Variable* var_tmp = NewTemporary(avfactory->empty_string());
+    Expression* tmp = factory()->NewVariableProxy(var_tmp);
+    Expression* assignment =
+        factory()->NewAssignment(Token::ASSIGN, tmp, result_value, nopos);
+
+    Block* block = factory()->NewBlock(nullptr, 2, false, nopos);
+    block->statements()->Add(
+        factory()->NewExpressionStatement(assignment, nopos), zone());
+    block->statements()->Add(set_completion_abrupt, zone());
+
+    result_value = factory()->NewDoExpression(block, var_tmp, nopos);
+  }
+
+  // each = #result_value;
+  Expression* assign_each;
+  {
+    assign_each =
+        factory()->NewAssignment(Token::ASSIGN, each, result_value, nopos);
+    if (each->IsArrayLiteral() || each->IsObjectLiteral()) {
+      assign_each = PatternRewriter::RewriteDestructuringAssignment(
+          this, assign_each->AsAssignment(), scope());
+    }
+  }
+
+  // {{completion = kNormalCompletion;}}
+  Statement* set_completion_normal;
+  if (finalize) {
+    Expression* proxy = factory()->NewVariableProxy(completion);
+    Expression* assignment = factory()->NewAssignment(
+        Token::ASSIGN, proxy,
+        factory()->NewSmiLiteral(Parser::kNormalCompletion, nopos), nopos);
+
+    Block* block = factory()->NewBlock(nullptr, 1, true, nopos);
+    block->statements()->Add(
+        factory()->NewExpressionStatement(assignment, nopos), zone());
+    set_completion_normal = block;
+  }
+
+  // { #loop-body; #set_completion_normal }
+  // Statement* body (gets overwritten)
+  if (finalize) {
+    Block* block = factory()->NewBlock(nullptr, 2, false, nopos);
+    block->statements()->Add(body, zone());
+    block->statements()->Add(set_completion_normal, zone());
+    body = block;
+  }
+
+  for_of->Initialize(body, iterator, assign_iterator, next_result, result_done,
+                     assign_each);
+  return finalize ? FinalizeForOfStatement(for_of, completion, nopos) : for_of;
+}
+
 Statement* Parser::DesugarLexicalBindingsInForStatement(
     ForStatement* loop, Statement* init, Expression* cond, Statement* next,
     Statement* body, Scope* inner_scope, const ForInfo& for_info, bool* ok) {
@@ -2755,6 +2921,7 @@ Parser::LazyParsingResult Parser::SkipFunction(
     SET_ALLOW(harmony_trailing_commas);
     SET_ALLOW(harmony_class_fields);
     SET_ALLOW(harmony_object_spread);
+    SET_ALLOW(harmony_async_iteration);
 #undef SET_ALLOW
   }
   // Aborting inner function preparsing would leave scopes in an inconsistent
@@ -3925,6 +4092,20 @@ Expression* Parser::RewriteAwaitExpression(Expression* value, int await_pos) {
 
   const int nopos = kNoSourcePosition;
 
+  if (is_async_generator()) {
+    ZoneList<Expression*>* args = new (zone()) ZoneList<Expression*>(2, zone());
+    Expression* generator_object =
+        factory()->NewVariableProxy(generator_object_variable);
+    args->Add(generator_object, zone());
+    args->Add(value, zone());
+
+    Expression* await = factory()->NewCallRuntime(
+        Context::ASYNC_GENERATOR_AWAIT_CAUGHT, args, nopos);
+
+    return factory()->NewYield(generator_object, await, nopos,
+                               Yield::kOnExceptionRethrow, Yield::kAwait);
+  }
+
   Block* do_block = factory()->NewBlock(nullptr, 2, false, nopos);
 
   Variable* promise = PromiseVariable();
@@ -3960,7 +4141,7 @@ Expression* Parser::RewriteAwaitExpression(Expression* value, int await_pos) {
 
   generator_object = factory()->NewVariableProxy(generator_object_variable);
   return factory()->NewYield(generator_object, do_expr, nopos,
-                             Yield::kOnExceptionRethrow);
+                             Yield::kOnExceptionRethrow, Yield::kAwait);
 }
 
 class NonPatternRewriter : public AstExpressionRewriter {