An example of how to implement Codable support for a type that conforms to LosslessStringConvertible.

LosslessStringConvertibleCodable.swift

// MARK: - Default Decodable Implementation

extension Decodable where Self: LosslessStringConvertible {
    // MARK: Public Initialization
    
    public init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        let stringValue = try container.decode(String.self)
        
        guard let value = Self(stringValue) else {
            throw DecodingError.dataCorruptedError(
                in: container,
                debugDescription: "Expected to be able to losslessly convert from String."
            )
        }
        
        self = value
    }
}

// MARK: - Default Encodable Implementation

extension Encodable where Self: LosslessStringConvertible {
    // MARK: Public Instance Interface
    
    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        try container.encode(description)
    }
}

This is done as a default implementation for all types that conform to LosslessStringConvertible. Creating such conformances on standard library types is not advised in production. If you want to ship this I would advise scoping the implementation to just your specific type(s), or defining your own LosslessStringConvertible-implementing protocol (e.g. LosslessStringCodable) and scoping the default implementation to implementers of that protocol.