String Enums.m

//Obj-C
typedef NSString * ABCMessageField NS_STRING_ENUM;
extern ABCMessageField const ABCMessageFieldSubject;
extern ABCMessageField const ABCMessageFieldFrom;

//Expected Swift (according to https://developer.apple.com/library/content/documentation/Swift/Conceptual/BuildingCocoaApps/InteractingWithCAPIs.html#//apple_ref/doc/uid/TP40014216-CH8-ID17)
struct ABCMessageField: RawRepresentable {
    typealias RawValue = String
    
    init(rawValue: RawValue)
    var rawValue: RawValue { get }
    
    static var subject: ABCMessageField { get }
    static var from: ABCMessageField { get }
}

//Actual Swift
public struct ABCMessageField : RawRepresentable, Equatable, Hashable, Comparable {
    public init(rawValue: String)
}
public static let subject: ABCMessageField
public static let from: ABCMessageField