On ~line 622 of CRM/Core/BAO/Navigation.php, something that looks like:
((module_exists('toolbar') && user_access('access toolbar')) ||
module_exists('admin_menu') && user_access('access administration menu')
This is obviously bad for lots of reasons. CiviCRM should basically never call into Drupal. In my case, it's attempting to call functions that no longer exist in Drupal 8.