Throw errors for nested targets in GN.

tools/gn/function_template.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/functions.h"
 
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/template.h"
 #include "tools/gn/value.h"
@@ skipping 145 matching lines @@
     "    # inside the template (since it has this name), which will in turn\n"
     "    # depend on the code gen action.\n"
     "    deps = [ \":foo_idl_files\" ]\n"
     "  }\n";
 
 Value RunTemplate(Scope* scope,
                   const FunctionCallNode* function,
                   const std::vector<Value>& args,
                   BlockNode* block,
                   Err* err) {
+  // Of course you can have configs and targets in a template. But here, we're
+  // not actually executing the block, only declaring it. Marking the template
+  // declaration as non-nestable means that you can't put it inside a target,
+  // for example.
+  NonNestableBlock non_nestable(scope, function, "template");
+  if (!non_nestable.Enter(err))
+    return Value();
+
   // TODO(brettw) determine if the function is built-in and throw an error if
   // it is.
   if (args.size() != 1) {
     *err = Err(function->function(),
                "Need exactly one string arg to template.");
     return Value();
   }
   if (!args[0].VerifyTypeIs(Value::STRING, err))
     return Value();
   std::string template_name = args[0].string_value();
 
   const Template* existing_template = scope->GetTemplate(template_name);
   if (existing_template) {
     *err = Err(function, "Duplicate template definition.",
                "A template with this name was already defined.");
     err->AppendSubErr(Err(existing_template->GetDefinitionRange(),
                           "Previous definition."));
     return Value();
   }
 
   scope->AddTemplate(template_name, new Template(scope, function));
   return Value();
 }
 
 }  // namespace functions