[FFFXXX]

Thats awesome, now I'm going to have to find a reason to use this for something ;)

Tiny nitpick: the #![allow()] generates a 'warning: unused attribute' warning for me.

If you change it to #![allow(unused_attributes)] it disables the warning for itself.

[ComputerGuru]

Nice catch! That warning definitely wasn't emitted the last time I tried this.. I don't know about using something like your suggestion which has actual side effects, but probably something like #![cfg_attr(invalid, allow(unused_attributes))] instead would work (without side effects).