[swift-evolution] Proposal: Allow @objc(name) on enum declarations

[Kevin Ballard]

Swift allows for placing @objc on an enum that has an Int raw type in order to expose it to Obj-C. But it doesn't currently let you rename the enum when exposing it to Obj-C. This is particularly problematic when exposing a Swift enum that's nested in a struct/class, as the nesting resolves ambiguity in Swift but is not present in Obj-C.

Example:

import Foundation

class Foo: NSObject {
    @objc enum Bar: Int {
        case One, Two
    }
}

This generates the following:

SWIFT_CLASS("_TtC7unnamed3Foo")
@interface Foo : NSObject
- (nonnull instancetype)init OBJC_DESIGNATED_INITIALIZER;
@end

typedef SWIFT_ENUM(NSInteger, Bar) {
  BarOne = 0,
  BarTwo = 1,
};

I'd like to resolve this by saying @objc(FooBar) but that emits an error.

I'm also going to submit a separate proposal saying we should change the default naming here so the enum is named FooBar, but these two proposals go hand-in-hand (there are cases where you might want to rename a root-level enum, to add a prefix for disambiguation in Obj-C, or you may want to selectively opt out of the proposed renaming rules by forcing your nested enum to use just its name in Obj-C).

-Kevin Ballard