Add GN split_list function

tools/gn/function_foreach.cc

 // Copyright (c) 2013 The Chromium 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 "tools/gn/err.h"
 #include "tools/gn/functions.h"
+#include "tools/gn/parse_node_value_adapter.h"
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 
 namespace functions {
 
 const char kForEach[] = "foreach";
 const char kForEach_HelpShort[] =
     "foreach: Iterate over a list.";
 const char kForEach_Help[] =
     "foreach: Iterate over a list.\n"
@@ skipping 39 matching lines @@
 
   // Extract the loop variable.
   const IdentifierNode* identifier = args_vector[0]->AsIdentifier();
   if (!identifier) {
     *err =
         Err(args_vector[0].get(), "Expected an identifier for the loop var.");
     return Value();
   }
   base::StringPiece loop_var(identifier->value().value());
 
-  // Extract the list, avoid a copy if it's an identifier (common case).
-  Value value_storage_for_exec;  // Backing for list_value when we need to exec.
-  const Value* list_value = nullptr;
-  const IdentifierNode* list_identifier = args_vector[1]->AsIdentifier();
-  if (list_identifier) {
-    list_value = scope->GetValue(list_identifier->value().value(), true);
-    if (!list_value) {
-      *err = Err(args_vector[1].get(), "Undefined identifier.");
-      return Value();
-    }
-  } else {
-    // Not an identifier, evaluate the node to get the result.
-    Scope list_exec_scope(scope);
-    value_storage_for_exec = args_vector[1]->Execute(scope, err);
-    if (err->has_error())
-      return Value();
-    list_value = &value_storage_for_exec;
-  }
-  if (!list_value->VerifyTypeIs(Value::LIST, err))
+  // Extract the list to iterate over.
+  ParseNodeValueAdapter list_adapter;
+  if (!list_adapter.InitForType(scope, args_vector[1].get(), Value::LIST, err))
     return Value();
-  const std::vector<Value>& list = list_value->list_value();
+  const std::vector<Value>& list = list_adapter.get().list_value();
 
   // Block to execute.
   const BlockNode* block = function->block();
   if (!block) {
     *err = Err(function, "Expected { after foreach.");
     return Value();
   }
 
   // If the loop variable was previously defined in this scope, save it so we
   // can put it back after the loop is done.
@@ skipping 16 matching lines @@
     scope->SetValue(loop_var, old_loop_value, old_loop_value.origin());
   } else {
     // Loop variable was undefined before loop, delete it.
     scope->RemoveIdentifier(loop_var);
   }
 
   return Value();
 }
 
 }  // namespace functions