The cost is a special variant of eval to call inside macros: http://github.com/akkartik/wart/commit/e3124e9559#diff-3

I still emit the warning, because examples like this fail:

  wart> (def helper(a b) `(cons ,a ,b))
  wart> (mac foo args (helper @args))
  wart> (foo @'(1 (2 3)))
  008eval.cc:79 calling macros with splice can have subtle effects (http://arclanguage.org/item?id=15659)
  005types.cc:263 can't coerce number 2 to function