Swift make method parameter mutable?

How can I deal with this error without creating additional variable?

func reduceToZero(x:Int) -> Int {
    while (x != 0) {
        x = x-1            // ERROR: cannot assign to 'let' value 'x'
    }
    return x
}

I don't want to create additional variable just to store the value of x. Is it even possible to do what I want?

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.

'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

For Swift 1 and 2 (for Swift 3 see answer by achi using an inout parameter): Argument of a function in Swift is let by default so change it to var if you need to alter the value i.e,

func reduceToZero(var x:Int) -> Int {
    while (x != 0) {
        x = x-1     
    }
    return x
}