The Optional Value Setter operator assigns a value to the receiving subject, only if the receiver is nil. Analogous to Ruby's ||= operator.

Optional+Utilities.swift

infix operator ??=

extension Swift.Optional {
    /// This is the Optional Value Setter operator. It assigns a value to the receiving subject, only if
    /// the receiver is `nil`.
    /// When the receiver is not `nil`, the value on the right hand side won't be accessed or computed (as
    /// it's an `@autoclosure`). If the receiver already has `.some` value, no value will be assigned to it,
    /// thus `willSet` and `didSet` won't be called.
    ///
    /// - SeeAlso: the rejected proposal of this feature in Swift Evolution: https://github.com/apple/swift-evolution/blob/master/proposals/0024-optional-value-setter.md
    public static func ??=(lhs: inout Self<Wrapped>, rhs: @autoclosure () -> Wrapped) {
        guard case .none = lhs else { return }
        lhs = rhs()
    }
}