Swift make method parameter mutable?

'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

As stated in other answers, as of Swift 3 placing var before a variable has been deprecated. Though not stated in other answers is the ability to declare an inout parameter. Think: passing in a pointer.

func reduceToZero(_ x: inout Int) {
    while (x != 0) {
        x = x-1     
    }
}

var a = 3
reduceToZero(&a)
print(a) // will print '0'

This can be particularly useful in recursion.

Apple's inout declaration guidelines can be found here.