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

tools/gn/import_manager.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/import_manager.h"
 
 #include "tools/gn/parse_tree.h"
 #include "tools/gn/scheduler.h"
 #include "tools/gn/scope_per_file_provider.h"
 
@@ skipping 12 matching lines @@
   std::unique_ptr<Scope> scope(new Scope(settings->base_config()));
   scope->set_source_dir(file.GetDir());
 
   // Don't allow ScopePerFileProvider to provide target-related variables.
   // These will be relative to the imported file, which is probably not what
   // people mean when they use these.
   ScopePerFileProvider per_file_provider(scope.get(), false);
 
   scope->SetProcessingImport();
   node->Execute(scope.get(), err);
-  if (err->has_error())
+  if (err->has_error()) {
+    // If there was an error, append the caller location so the error message
+    // displays a why the file was imported (esp. useful for failed asserts).
+    err->AppendSubErr(Err(node_for_err, "whence it was imported."));
     return nullptr;
+  }
   scope->ClearProcessingImport();
 
   return scope;
 }
 
 }  // namesapce
 
 ImportManager::ImportManager() {
 }
 
@@ skipping 35 matching lines @@
       }
     }
   }
 
   Scope::MergeOptions options;
   options.skip_private_vars = true;
   options.mark_dest_used = true;  // Don't require all imported values be used.
   return imported_scope->NonRecursiveMergeTo(scope, options, node_for_err,
                                              "import", err);
 }