Properly handle holes following spreads in array literals

src/parsing/parser.cc

Index: src/parsing/parser.cc
diff --git a/src/parsing/parser.cc b/src/parsing/parser.cc
index 119c0164edd87b40de9b1c1742b78556d3bb8eec..e3ca7d688fbf8937d83e282f39f11a9f63abf2b5 100644
--- a/src/parsing/parser.cc
+++ b/src/parsing/parser.cc
@@ -5054,15 +5054,32 @@ Expression* Parser::RewriteSpreads(ArrayLiteral* lit) {
     if (spread == nullptr) {
       // If the element is not a spread, we're adding a single:
       // %AppendElement($R, value)
-      ZoneList<Expression*>* append_element_args = NewExpressionList(2);
-      append_element_args->Add(factory()->NewVariableProxy(result), zone());
-      append_element_args->Add(value, zone());
-      do_block->statements()->Add(
-          factory()->NewExpressionStatement(
-              factory()->NewCallRuntime(Runtime::kAppendElement,
-                                        append_element_args, kNoSourcePosition),
-              kNoSourcePosition),
-          zone());
+      // or, in case of a hole,
+      // ++($R.length)
+      if (!value->IsLiteral() ||
+          !value->AsLiteral()->raw_value()->IsTheHole()) {
+        ZoneList<Expression*>* append_element_args = NewExpressionList(2);
+        append_element_args->Add(factory()->NewVariableProxy(result), zone());
+        append_element_args->Add(value, zone());
+        do_block->statements()->Add(
+            factory()->NewExpressionStatement(
+                factory()->NewCallRuntime(Runtime::kAppendElement,
+                                          append_element_args,
+                                          kNoSourcePosition),
+                kNoSourcePosition),
+            zone());
+      } else {
+        auto length_property = factory()->NewProperty(

[Jakob Kummerow, 2016/09/08 09:58:56]

nit: I strongly prefer explicit types over "auto".

[adamk, 2016/09/08 17:43:20]

Done, but I wish we could have a style guide somewhere for things like this.

+            factory()->NewVariableProxy(result),
+            factory()->NewStringLiteral(ast_value_factory()->length_string(),
+                                        kNoSourcePosition),
+            kNoSourcePosition);
+        auto count_op = factory()->NewCountOperation(
+            Token::INC, true /* prefix */, length_property, kNoSourcePosition);
+        do_block->statements()->Add(
+            factory()->NewExpressionStatement(count_op, kNoSourcePosition),
+            zone());
+      }
     } else {
       // If it's a spread, we're adding a for/of loop iterating through it.
       Variable* each = NewTemporary(ast_value_factory()->dot_for_string());