What happen when casting Swift.Error or Swift.LocalizedError to NSError?

ErrorCast.swift

// Try them in Playground file.

/* Background when executing cast(swiftError as NSError)
1. NSError uses Swift.Error type name as its `domain` property.
2. NSError uses a eumeration number as its `code` property.
*/

import UIKit

enum SwiftError: Error {
    case busyNetwork
    case clientProblem
    case serverProblem
}

let busyNetworkNSError = SwiftError.busyNetwork as NSError
print("domain: \(busyNetworkNSError.domain)")
print("code: \(busyNetworkNSError.code)")
print("localizedDescription: \(busyNetworkNSError.localizedDescription)")
print("localizedFailureReason: \(busyNetworkNSError.localizedFailureReason)")
print("localizedRecoverySuggestion: \(busyNetworkNSError.localizedRecoverySuggestion)")

/* Result. NSError provides default localizedDescription "The operation couldn’t be completed \(domain) \(code)"
 domain: __lldb_expr_22.SwiftError
 code: 0
 localizedDescription: The operation couldn’t be completed. (__lldb_expr_22.SwiftError error 0.)
 localizedFailureReason: nil
 localizedRecoverySuggestion: nil
 */

enum SwiftLocalizedError: LocalizedError {
    case busyNetwork
    case clientProblem
    case serverProblem

    var failureReason: String? {
        return "The client machine is angry."
    }

    var recoverySuggestion: String? {
        return "Apologize to the machine"
    }

}

let clientProblemLocalizedNSError = SwiftLocalizedError.clientProblem as NSError
print("domain: \(clientProblemLocalizedNSError.domain)")
print("code: \(clientProblemLocalizedNSError.code)")
print("localizedDescription: \(clientProblemLocalizedNSError.localizedDescription)")
print("localizedFailureReason: \(clientProblemLocalizedNSError.localizedFailureReason)")
print("localizedRecoverySuggestion: \(clientProblemLocalizedNSError.localizedRecoverySuggestion)")

/* Result. NSError uses localizedFailureReason in localizedDescription "The operation couldn’t be completed \(localizedFailureReason))"
 domain: __lldb_expr_26.SwiftLocalizedError
 code: 1
 localizedDescription: The operation couldn’t be completed. The client machine is angry.
 localizedFailureReason: Optional("The client machine is angry.")
 localizedRecoverySuggestion: Optional("Apologize to the machine")
 */