enum Foo: String, Codable {
    case a
    case b
    case c
}

extension RawRepresentable where RawValue == String, Self: Codable {
    init(from decoder: Decoder) throws {
        var container = try decoder.unkeyedContainer()
        let rawValue = try container.decode(String.self)
        do {
            self.init(rawValue: rawValue)!
        }
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.unkeyedContainer()
        try container.encode(rawValue)
    }
}

let encoded = try? JSONEncoder().encode(Foo.a)
let decoded = try? JSONDecoder().decode(Foo.self, from: encoded!)