Obviously the Hint "Processing" message :)
nim c --hint[Processing]:off foo.nim works on OSX for me (with devel).
Did you probably made the same error I did once and spelled it --hints[Procressing]:off while trying?
Does nim --hints:list work?
Is there maybe a problem with the [] in a windows shell? You could try to add it to a config file. The following works for me:
# nim.cfg
hint[Processing]:off
How to do it with a config.nims I don't know. Using --/switch() does not work it seems
WatNu? ;-)