🐣 GN: Print the import trail when parse errors occur.

tools/gn/loader.cc

Index: tools/gn/loader.cc
diff --git a/tools/gn/loader.cc b/tools/gn/loader.cc
index 3ac868cfcd79590bdf4a084682f565d1875bdf80..3ca25ca6201c6fdda329de572c296fe61bb9ee4f 100644
--- a/tools/gn/loader.cc
+++ b/tools/gn/loader.cc
@@ -211,7 +211,7 @@ void LoaderImpl::ScheduleLoadFile(const Settings* settings,
   pending_loads_++;
   if (!AsyncLoadFile(origin, settings->build_settings(), file,
                      base::Bind(&LoaderImpl::BackgroundLoadFile, this,
-                                settings, file),
+                                settings, file, origin),
                      &err)) {
     g_scheduler->FailWithError(err);
     DecrementPendingLoads();
@@ -235,6 +235,7 @@ void LoaderImpl::ScheduleLoadBuildConfig(
 
 void LoaderImpl::BackgroundLoadFile(const Settings* settings,
                                     const SourceFile& file_name,
+                                    const LocationRange& origin,
                                     const ParseNode* root) {
   if (!root) {
     main_loop_->PostTask(FROM_HERE,
@@ -260,11 +261,14 @@ void LoaderImpl::BackgroundLoadFile(const Settings* settings,
 
   Err err;
   root->Execute(&our_scope, &err);
-  if (err.has_error())
-    g_scheduler->FailWithError(err);
+  if (!err.has_error())
+    our_scope.CheckForUnusedVars(&err);
 
-  if (!our_scope.CheckForUnusedVars(&err))
+  if (err.has_error()) {

[brettw, 2016/04/20 20:53:19]

We should check that the origin range isn't empty before appending the sub
error. For some things like the toplevel .gn file or the root BUILD file, there
isn't an origin and it passes LocationRange() as this parameter.

We may want to add an is_null() check to LocationRange and Location to make this
code more sensible.

[agrieve, 2016/04/21 00:59:35]

Done.

+    err.AppendSubErr(Err(origin, "which caused the package to be included."));

[brettw, 2016/04/20 20:53:19]

Can you replace "package" with "file"? I've not called anything in GN a package
(the google3 terminology), and the semantics are subtlety different.

[agrieve, 2016/04/21 00:59:35]

Done.

     g_scheduler->FailWithError(err);
+  }
+
 
   // Pass all of the items that were defined off to the builder.
   for (auto& item : collected_items) {