[esnext] prototype runtime implementation for async functions

src/parsing/parser-base.h

Index: src/parsing/parser-base.h
diff --git a/src/parsing/parser-base.h b/src/parsing/parser-base.h
index cde95076a598852d1853db5714eda550ed173b8c..3a0dbc8cf65307750c90c91f756474a8a9d56e8a 100644
--- a/src/parsing/parser-base.h
+++ b/src/parsing/parser-base.h
@@ -36,6 +36,10 @@ enum class ParseFunctionFlags {
   kIsDefault = 4
 };
 
+enum class FunctionBody { Normal, SingleExpression };
+
+enum class FunctionParsePhase { Unknown, FormalParameters, FunctionBody };
+
 static inline ParseFunctionFlags operator|(ParseFunctionFlags lhs,
                                            ParseFunctionFlags rhs) {
   typedef unsigned char T;
@@ -313,14 +317,26 @@ class ParserBase : public Traits {
 
     bool is_generator() const { return IsGeneratorFunction(kind_); }
     bool is_async_function() const { return IsAsyncFunction(kind_); }
+    bool is_resumable() const { return is_generator() || is_async_function(); }
 
     FunctionKind kind() const { return kind_; }
     FunctionState* outer() const { return outer_function_state_; }
 
+    bool is_async_function_body() const {
+      return is_async_function() &&
+             parse_phase() == FunctionParsePhase::FunctionBody;
+    }
+
+    FunctionParsePhase parse_phase() const { return parse_phase_; }
+
+    void set_parse_phase(FunctionParsePhase parse_phase) {
+      parse_phase_ = parse_phase;
+    }
+
     void set_generator_object_variable(
         typename Traits::Type::GeneratorVariable* variable) {
       DCHECK(variable != NULL);
-      DCHECK(is_generator());
+      DCHECK(is_resumable());
       generator_object_variable_ = variable;
     }
     typename Traits::Type::GeneratorVariable* generator_object_variable()
@@ -426,6 +442,8 @@ class ParserBase : public Traits {
     // to this function. Filled in by constructor.
     bool this_function_is_parenthesized_;
 
+    FunctionParsePhase parse_phase_ = FunctionParsePhase::Unknown;
+
     friend class ParserTraits;
     friend class PreParserTraits;
     friend class Checkpoint;
@@ -721,6 +739,7 @@ class ParserBase : public Traits {
   bool is_async_function() const {
     return function_state_->is_async_function();
   }
+  bool is_resumable() const { return function_state_->is_resumable(); }
 
   // Report syntax errors.
   void ReportMessage(MessageTemplate::Template message, const char* arg = NULL,
@@ -2174,8 +2193,9 @@ ParserBase<Traits>::ParseAssignmentExpression(bool accept_IN,
     ValidateFormalParameterInitializer(&arrow_formals_classifier, ok);
 
     Scanner::Location loc(lhs_beg_pos, scanner()->location().end_pos);
-    Scope* scope =
-        this->NewScope(scope_, FUNCTION_SCOPE, FunctionKind::kArrowFunction);
+    Scope* scope = this->NewScope(scope_, FUNCTION_SCOPE,
+                                  is_async ? FunctionKind::kAsyncArrowFunction
+                                           : FunctionKind::kArrowFunction);
     // Because the arrow's parameters were parsed in the outer scope, any
     // usage flags that might have been triggered there need to be copied
     // to the arrow scope.
@@ -2610,10 +2630,14 @@ ParserBase<Traits>::ParseUnaryExpression(ExpressionClassifier* classifier,
     Consume(Token::AWAIT);
 
     ExpressionT value = ParseUnaryExpression(classifier, CHECK_OK);
-    classifier->RecordFormalParameterInitializerError(
-        Scanner::Location(beg_pos, scanner()->location().end_pos),
-        MessageTemplate::kAwaitExpressionFormalParameter);
-
+    if (!function_state_->is_async_function_body()) {
+      // Not currently parsing function body --- must have occurred within
+      // formal parameter parsing.
+      ReportMessageAt(Scanner::Location(beg_pos, scanner()->location().end_pos),
+                      MessageTemplate::kAwaitExpressionFormalParameter);
+      *ok = false;
+      return this->EmptyExpression();
+    }
     return Traits::RewriteAwaitExpression(value, beg_pos);
   } else {
     return this->ParsePostfixExpression(classifier, ok);
@@ -3211,11 +3235,12 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
   int expected_property_count = -1;
   Scanner::Location super_loc;
 
+  FunctionKind arrow_kind = is_async ? kAsyncArrowFunction : kArrowFunction;
   {
     typename Traits::Type::Factory function_factory(ast_value_factory());
-    FunctionState function_state(
-        &function_state_, &scope_, formal_parameters.scope,
-        is_async ? kAsyncArrowFunction : kArrowFunction, &function_factory);
+    FunctionState function_state(&function_state_, &scope_,
+                                 formal_parameters.scope, arrow_kind,
+                                 &function_factory);
 
     function_state.SkipMaterializedLiterals(
         formal_parameters.materialized_literals_count);
@@ -3223,6 +3248,7 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
     this->ReindexLiterals(formal_parameters);
 
     Expect(Token::ARROW, CHECK_OK);
+    function_state.set_parse_phase(FunctionParsePhase::FunctionBody);
 
     if (peek() == Token::LBRACE) {
       // Multiple statement body
@@ -3240,7 +3266,7 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
       } else {
         body = this->ParseEagerFunctionBody(
             this->EmptyIdentifier(), RelocInfo::kNoPosition, formal_parameters,
-            kArrowFunction, FunctionLiteral::kAnonymousExpression, CHECK_OK);
+            arrow_kind, FunctionLiteral::kAnonymousExpression, CHECK_OK);
         materialized_literal_count =
             function_state.materialized_literal_count();
         expected_property_count = function_state.expected_property_count();
@@ -3253,18 +3279,25 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
              function_state_->return_expr_context());
       ReturnExprScope allow_tail_calls(
           function_state_, ReturnExprContext::kInsideValidReturnStatement);
-      ExpressionT expression =
-          ParseAssignmentExpression(accept_IN, &classifier, CHECK_OK);
-      Traits::RewriteNonPattern(&classifier, CHECK_OK);
       body = this->NewStatementList(1, zone());
-      this->AddParameterInitializationBlock(formal_parameters, body, CHECK_OK);
-      body->Add(factory()->NewReturnStatement(expression, pos), zone());
+      this->AddParameterInitializationBlock(formal_parameters, body, is_async,
+                                            CHECK_OK);
+      if (is_async) {
+        this->ParseAsyncArrowSingleExpressionBody(body, accept_IN, &classifier,
+                                                  pos, CHECK_OK);
+        Traits::RewriteNonPattern(&classifier, CHECK_OK);
+      } else {
+        ExpressionT expression =
+            ParseAssignmentExpression(accept_IN, &classifier, CHECK_OK);
+        Traits::RewriteNonPattern(&classifier, CHECK_OK);
+        body->Add(factory()->NewReturnStatement(expression, pos), zone());
+        if (allow_tailcalls() && !is_sloppy(language_mode())) {
+          // ES6 14.6.1 Static Semantics: IsInTailPosition
+          this->MarkTailPosition(expression);
+        }
+      }
       materialized_literal_count = function_state.materialized_literal_count();
       expected_property_count = function_state.expected_property_count();
-      if (allow_tailcalls() && !is_sloppy(language_mode())) {
-        // ES6 14.6.1 Static Semantics: IsInTailPosition
-        this->MarkTailPosition(expression);
-      }
       this->MarkCollectedTailCallExpressions();
     }
     super_loc = function_state.super_location();
@@ -3294,7 +3327,7 @@ ParserBase<Traits>::ParseArrowFunctionLiteral(
       materialized_literal_count, expected_property_count, num_parameters,
       FunctionLiteral::kNoDuplicateParameters,
       FunctionLiteral::kAnonymousExpression,
-      FunctionLiteral::kShouldLazyCompile, FunctionKind::kArrowFunction,
+      FunctionLiteral::kShouldLazyCompile, arrow_kind,
       formal_parameters.scope->start_position());
 
   function_literal->set_function_token_position(