How can I deal with this error without creating additional variable?
func reduceToZero(x:Int) -> Int { while (x != 0) { x = x-1 //
'var' parameters are deprecated and will be removed in Swift 3. So assigning to a new parameter seems like the best way now:
func reduceToZero(x:Int) -> Int { var x = x while (x != 0) { x = x-1 } return x }
as mentioned here: 'var' parameters are deprecated and will be removed in Swift 3