zipper-like mutation in Swift

mutating.swift

struct Rect {
    var width = 1
    var height = 1
    
    mutating func scaleBy(value: Int) {
        width *= value
        height *= value
    }
}

var rects: [Rect] = [Rect(width: 5,height: 6), Rect(width: 2, height: 3)]

let rect = rects[1]
rects[1].scaleBy(3)

rects // [Rect(5, 6), Rect(6, 9)]
rect // still Rect(2, 3)