How to disable SWIFT_ENABLE_OPAQUE_TYPE_ERASURE explicitly for Debug build

[Kyle-Ye]

The SWIFT_ENABLE_OPAQUE_TYPE_ERASURE is causing some issue on our debug build using Xcode. More specificity, it seems be to associated with -Onone optimization level(which means Debug configuration + a non -Onone will also not add -enable-experimental-feature OpaqueTypeErasure).

The default value in swift-build repo shows it is false. And when invoking xcode by swift package --disable-sandbox launch-xcode to build the project, it is indeed not passing -enable-experimental-feature OpaqueTypeErasure.

However for normal Xcode build and we use -Onone as optimization level, the -enable-experimental-feature OpaqueTypeErasure is always passed.

My question is "Is there any way to disable SWIFT_ENABLE_OPAQUE_TYPE_ERASURE explicitly for a xcodeproj?"

Add --disable-experimental-feature xx does not work since swift compile does not recognize it.

@_typeEraser(AnyAnimal)
protocol Animal {
    var name: String { get }
}

struct AnyAnimal: Animal {
    let name: String

    init(name: String) {
        self.name = name
    }

    public init<V>(erasing v: V) where V: Animal {
        self.name = v.name
    }
}

func makeAnimal() -> some Animal {
    Cat()
}

struct Cat: Animal {
    var name: String { "cat" }
}

let animal = makeAnimal()
print(type(of: animal))
// "Default": Cat
// -enable-experimental-feature OpaqueTypeErasure: AnyAnimal

Reproducible repo

https://github.com/Kyle-Ye/SwiftOpaqueTypeErasureDemo

Other info

swift-build/Sources/SWBUniversalPlatform/Specs/Swift.xcspec

Lines 1229 to 1241 in ff0b14a

	{
	Name = "SWIFT_ENABLE_OPAQUE_TYPE_ERASURE";
	Type = Boolean;
	// rdar://143344956 (Disable opaque type erasure for debug builds)
	DefaultValue = NO;
	CommandLineArgs = {
	YES = (
	"-enable-experimental-feature",
	"OpaqueTypeErasure",
	);
	NO = ();
	};
	},

[Kyle-Ye]

Find the solution: Xcodeproj -> Choose your target -> Add User Defined Setting "SWIFT_ENABLE_OPAQUE_TYPE_ERASURE" -> Set the value to false.

But I still wish:

1. Xcode can add it into the Swift compiler section to make it more clear
2. Align the default value in the repo which is false by now.

[Kyle-Ye]

Set "SWIFT_ENABLE_OPAQUE_TYPE_ERASURE" to NO in User Defined Settings only works for Xcodeproj target.

For swift package executables or Xcodeproj's Swift dependency, it does not work.

How can we disable SWIFT_ENABLE_OPAQUE_TYPE_ERASURE feature (avoid passing -enable-experimental-feature OpaqueTypeErasure) when building a package via Xcode 16?

[jakepetroules]

The reason you're seeing this behavior where the SWIFT_ENABLE_OPAQUE_TYPE_ERASURE default in the source code is different from what you observe is because the default in the current shipping Xcode 16.2 release is still YES.

Please try the Xcode 16.3 beta 1 release (released yesterday), which contains a more up-to-date version of Swift Build where the default value of SWIFT_ENABLE_OPAQUE_TYPE_ERASURE of NO is reflected.

CC @jonathanpenn

[Kyle-Ye]

Fixed: SWIFT_ENABLE_OPAQUE_TYPE_ERASURE is now off by default for all debug builds. If you were previously turning it off in your project, you can remove that override. Previews now enables this selectively and recompiles the edited file with this setting only when running a preview. (140442875)

Ah, I see. Thanks for the update! Did not noticed the new beta release yet.

Set "SWIFT_ENABLE_OPAQUE_TYPE_ERASURE" to NO in User Defined Settings only works for Xcodeproj target.

For swift package executables or Xcodeproj's Swift dependency, it does not work.

How can we disable SWIFT_ENABLE_OPAQUE_TYPE_ERASURE feature (avoid passing -enable-experimental-feature OpaqueTypeErasure) when building a package via Xcode 16?

By making it default to "NO", we can freely enable it in Xcodeproj by setting it to YES or manually add "-enable-experimental-feature OpaqueTypeErasure" in swiftSetting for SPM build.

The originally issue is fixed for Xcode 16.3 Beta 1.

But my second question still apply: How to explicitly set variable like "SWIFT_ENABLE_OPAQUE_TYPE_ERASURE" which is consumed by XB/SB for a Swift package? (There may have other variable we want to explicitly set for a package)

Note

1. We can't bump into Xcode 16.3 Beta right now
2. We may encounter similar issue for a package later.

Is there any existing methods or can we expect a proposal later?

@jakepetroules

[jakepetroules]

But my second question still apply: How to explicitly set variable like "SWIFT_ENABLE_OPAQUE_TYPE_ERASURE" which is consumed by XB/SB for a Swift package? (There may have other variable we want to explicitly set for a package)

You can't set arbitrary build settings for packages at all. There would have to be specific API in the package manifest, and for this particular setting, there isn't.

[Kyle-Ye]

Got it. I know such setting is only existing on xcodebuild/swift-build. Hope one day SwiftPM can allow use to specify them in package manifest.