CodableMagic.swift

struct Bill: Codable {
    let b: String
    let c: String
}

struct B2: Codable {
    enum CodingKeys: String, CodingKey { case a }
    let bill: Bill
    let a: Int
    
    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.bill = try Bill(from: decoder)
        self.a = try container.decode(Int.self, forKey: .a)
    }
    
    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try bill.encode(to: encoder)
        try container.encode(a, forKey: .a)
    }
}

let json = """
{
"b": "Hello",
"c": "World",
"a": 1
}
""".data(using: .utf8)!

let dec = JSONDecoder()
var h = try! dec.decode(B2.self, from: json)
print(h)

let encoder = JSONEncoder()
let nj = try! encoder.encode(h)
print(String(data: nj, encoding: .utf8)!)