"Skipping obsolete configuration option." doesn't say which one

In Tor 0.3.5.x:

app/config/confparse.c:
log_warn(LD_CONFIG, "Skipping obsolete configuration option '%s'", c->key);

But in 0.4.2.3-alpha:

lib/confmgt/type_defs.c:
log_warn(LD_GENERAL, "Skipping obsolete configuration option.");

Resulting in log messages like:

Oct 25 16:19:28.906 [warn] Skipping obsolete configuration option.
Oct 25 16:19:28.906 [warn] Skipping obsolete configuration option.

which are not as helpful as they could be.

The bug went in with commit c60a85d2, which I think first went into 0.4.2.1-alpha.

The new file has a XXXX next to this line:

  // XXXX move this to a higher level, once such a level exists.
  log_warn(LD_GENERAL, "Skipping obsolete configuration option.");

but I see that most tickets like #29211 (moved) and #30866 (moved) are scheduled for 0.4.3.x, and this is an 0.4.2 bug so I am filing a separate ticket.

Bug reported by OFFShare in irc.