在对AttributedString/Substring类型进行扩展时，不能将其下标设置为可写的。可以使用方法或者计算属性来代替下标进行操作。

例如，将下标操作改写为方法：

extension AttributedString {
    func setAttribute(_ attribute: NSAttributedString.Key, value: Any, range: NSRange) -> AttributedString {
        let mutableAttributedString = NSMutableAttributedString(attributedString: self)
        mutableAttributedString.addAttribute(attribute, value: value, range: range)
        return AttributedString(mutableAttributedString)
    }
}

let attributedString = AttributedString("Hello, world!")
let range = NSRange(location: 0, length: 5)
let coloredString = attributedString.setAttribute(.foregroundColor, value: UIColor.red, range: range)

或者使用计算属性：

extension AttributedString {
    var foregroundColor: UIColor? {
        get {
            return attribute(.foregroundColor, at: 0, effectiveRange: nil) as? UIColor
        }
        set {
            if let color = newValue {
                addAttribute(.foregroundColor, value: color, range: NSRange(location: 0, length: length))
            } else {
                removeAttribute(.foregroundColor, range: NSRange(location: 0, length: length))
            }
        }
    }
}

let attributedString = AttributedString("Hello, world!")
attributedString.foregroundColor = UIColor.red