Composing flat properties onto existing Codable values

Example.swift

struct WithSecret<T> {
    let secret: String
    let value: T
}

extension WithSecret: Decodable where T: Decodable {
    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: AnyCodingKey.self)
        
        self.secret = try container.decode(String.self, forKey: "secret")
        self.value = try T.init(from: decoder)
    }
}
extension WithSecret: Encodable where T: Encodable {
    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: AnyCodingKey.self)
        
        try container.encode(secret, forKey: "secret")
        try value.encode(to: encoder)
    }
}


struct User: Codable {
    let name: String
    let age: Int
}

let userWithSecret = WithSecret(secret: "foobar", value: User(name: "Ian", age: 36))

//{"name":"Ian","age":36,"secret":"foobar"}
print(String(data: try JSONEncoder().encode(userWithSecret), encoding: .utf8)!)

AnyCodingKey: https://gist.github.com/IanKeen/3d226854c8c59a17e151a0022b71f6bb