为什么这些Swift静态变量有空的getter？

Question

我看了一下Objective-C选项enum NSLayoutFormatOptions的自动生成的Swift标头：

struct NSLayoutFormatOptions : RawOptionSetType {
    init(_ rawValue: UInt)
    init(rawValue: UInt)

    static var AlignAllLeft: NSLayoutFormatOptions { get }
    static var AlignAllRight: NSLayoutFormatOptions { get }
    static var AlignAllTop: NSLayoutFormatOptions { get }
    static var AlignAllBottom: NSLayoutFormatOptions { get }
    static var AlignAllLeading: NSLayoutFormatOptions { get }
    static var AlignAllTrailing: NSLayoutFormatOptions { get }
    static var AlignAllCenterX: NSLayoutFormatOptions { get }
    static var AlignAllCenterY: NSLayoutFormatOptions { get }
    static var AlignAllBaseline: NSLayoutFormatOptions { get }
    static var AlignAllLastBaseline: NSLayoutFormatOptions { get }
    static var AlignAllFirstBaseline: NSLayoutFormatOptions { get }

    static var AlignmentMask: NSLayoutFormatOptions { get }

    /* choose only one of these three
     */
    static var DirectionLeadingToTrailing: NSLayoutFormatOptions { get } // default
    static var DirectionLeftToRight: NSLayoutFormatOptions { get }
    static var DirectionRightToLeft: NSLayoutFormatOptions { get }

    static var DirectionMask: NSLayoutFormatOptions { get }
}

声明静态变量的行上的语法让我感到困惑。它看起来像协议语法，需要每个变量的getter，但这是一个struct声明。这里发生了什么？

我现在也注意到另一个结构CGSize：

struct CGSize {
    var width: CGFloat
    var height: CGFloat
}

extension CGSize {
    static var zeroSize: CGSize { get }
    init()
    init(width: Int, height: Int)
    init(width: Double, height: Double)
}

同样，这只发生在static var上。