Swift Codable init

Use a factory method that first uses init(from:) and then calls your custom initialization code

struct Foo: Decodable {
    let name: String
    let id: Int

    var x: String!

    private mutating func finalizeInit() {
        self.x = "\(name) \(id)"
    }

    static func createFromJSON(_ data: Data) -> Foo? {
        guard var f = try? JSONDecoder().decode(Foo.self, from: data) else { 
            return nil 
        }
        f.finalizeInit()
        return f
    }
}

let sampleData = """
    { "name": "foo", "id": 42 }
    """.data(using: .utf8)!
let f = Foo.createFromJSON(sampleData)

Either you get everything for free but standardized or you have to write a custom initializer like

struct MyStruct: Codable  {

    let id: Int 
    var name: String

    private enum CodingKeys : String, CodingKey { case id, name }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(Int.self, forKey: .id)
        let decodedName = try container.decode(String.self, forKey: .name)
        name = "\(id) \(decodedName)" 
    }
}

You can implement init() but this works independent of the decoding functionality and you have to assign a default value to all non-optional properties, that's what the error says.