void* pointers get corrupted: D bug or misunderstanding?

[Kagamin]

struct DrillGtkContext
{
     static extern(C)
     void gtk_search_changed(GtkEditable* widget, ref 
DrillGtkContext context)
     {
         context.onSearchChanged();
     }
     void onSearchChanged()
     {
         ...
         startCrawling(drillConfig, searchString, &resultFound);
     }
     void resultFound(immutable(FileInfo) result)
     {
         ...
         queue.g_async_queue_push(f);
     }
}

...
DrillContext* startCrawling(
     in const(DrillConfig) config,
     in immutable(string) searchValue,
     scope void delegate(immutable(FileInfo)) resultCallback)
{
     ...
     new Crawler(..., c.callback);
}