[swift-evolution] SE proposal: Add @objc to enum strings

[Derrick Ho]

[TL;DR]
SE-0033 ports objective-c global strings to swift enums, but the reverse
seems to be missing. There should be a swift construct that can be
accessible by objective-c.

[Proposal]
In a previously implemented swift proposal, the concept of NS_STRING_ENUM
and NS_EXTENSIBLE_STRING_ENUM was born.  (see:
https://github.com/apple/swift-evolution/blob/master/proposals/0033-import-objc-constants.md
)

This would enable objective-c global strings to be ported over to swift as
an enum and struct respectively.

I always found it odd that this interoperability only worked in one
direction.  I think that enum strings should be accessible to Objective-c
when it is marked with  the @objc attribute.  When an enum string has @objc
it should be ported over to objective-c as global strings.

// Swift example
@objc
public enum Food: String {
   case Calamari
   case Fish
}

// Objective-c port
typedef NSString *_Nonnull Food;
static Food const Food_Calimari = @"Calimari";
static Food const Food_Fish = @"Fish";

The Objective-c port code could be added as part of the generated header
file or a framework's umbrella header.

When a structs is given the attribute @objcextstring it should become
available to objective-c as a class.  This attribute will put restrictions
on the struct such that it only has static constants and a mandatory
property called rawValue.

// Swift example
@objcextstring
public struct Planets {
public let rawValue: String
public static let Earth = Planets(rawValue: "Earth")
public static let Venus = Planets(rawValue: "Venus")
}

// Objective-c port
@interface Planets: NSObject
+ (instanceType)Earth;
+ (instanceType)Venus;
@end

@implementation Planets
+ (instanceType)Earth {
return [[Planets alloc] initWithRawValue: @"Earth"];
}
+ (instanceType)Venus {
return [[Planets alloc] initWithRawValue:@"Venus"];
}
@end

What do you guys thinks of this proposal?  Do you think it will enhance the
objective-c to swift interoperability?
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