import Foundation

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

struct MyStruct: Encodable {
    var foo: [Key: Int]
}

let t = MyStruct(foo: [
    .a: 1,
    .b: 1,
    .c: 1,
])

let encoder = JSONEncoder()
encoder.outputFormatting = [.sortedKeys, .prettyPrinted]
let encoded = try encoder.encode(t)
print(String(data: encoded, encoding: .utf8)!)