Your example has an undefined 'source' variable - you likely meant 'curr' - but more importantly, this pattern can be solved with indices + interior mutability or split_at_mut() rather than unsafe, giving you safe simultaneous mutable access to different nodes.
That requires you use a Vec or similar data structure which may not be appropriate/forces you to reinvent garbage collection or deal with tombstoning/generations/etc for unused slots.
And I pointed out you can use interior mutability. Still sucks because the code is guaranteed sound, the compiler just can't prove it. IMO the correct choice is UnsafeCell and unsafe {}.
