@propertyWrapper
struct Truncated {

    var wrappedValue: String {
        didSet {
            wrappedValue = String(wrappedValue.prefix(maxLength))
        }
    }

    let maxLength: Int
}

struct Tester {
    @Truncated(maxLength: 3) var limitedLengthString = ""
}

var tester = Tester()
tester.limitedLengthString = "too long"
print(tester.limitedLengthString) // prints "too"