Firstly, sincerely thanks for telling me how to add Clippy, that would have been a good idea in this context and now I'll remember it's there.

Secondly, yes, I think should be uplifted. I'd even argue for default deny because I can't think of a reason you would want this behaviour and yet not find this more explicit phrasing more maintainable:

  let guard = m.lock();
  Mutex::unlock(guard); // We don't hold the guard, just touch and go

[I'm assuming anybody who actually wants this would prefer Mutex::unlock() over drop() because it says what's going on, and if they care that they can't write this in stable their effort would be appreciated pushing Mutex::unlock stabilisation rather than blocking a default-deny]