Add support for visionOS

[noahpistilli]

Adds compilation support for visionOS. All tests pass. Closes #150.

I was unsure if I had to edit Utilities/expand-availability.py, so I didn't touch it.

[glessard]

I would prefer to do this by setting a compilation variable in the package manifest. This os||os||os||os||os list was already unwieldy, and it's only getting worse.

[noahpistilli]

Perhaps something like

swiftSettings: [
      .define("APPLE", .when(platforms: [Platform.macOS, Platform.iOS, Platform.watchOS, Platform.tvOS, Platform.visionOS])),

in the package manifest?

[rauhul]

Perhaps something like

swiftSettings: [

      .define("APPLE", .when(platforms: [Platform.macOS, Platform.iOS, Platform.watchOS, Platform.tvOS, Platform.visionOS])),

in the package manifest?

If you do go this route, note that Apple has non Darwin uses of Swift such as the triple: armv7em-apple-none-macho.

You could name this define DARWIN instead?

[glessard]

This is great, thanks for helping with this!

[glessard]

@swift-ci please test

[glessard]

The testing failure involves a change in Swift 6.0. Will need to fix this separately.

[glessard]

@noahpistilli I think the approach is good, but the symbol "DARWIN" may be a bit too prone to collisions, given that it could possibly be passed from the command-line by any package that uses this. May I suggest something less likely have collisions, such as "SYSTEM_PACKAGE_DARWIN"?

[glessard]

@swift-ci please test

[karwa]

Wouldn't it be better to use canImport(Darwin)?

Especially since a lot of these are literally guarding imports of the Darwin module and uses of symbols exported by it...

[rauhul]

Wouldn't it be better to use canImport(Darwin)?

Especially since a lot of these are literally guarding imports of the Darwin module and uses of symbols exported by it...

This seems abundantly obvious now that you say it...

[glessard]

@swift-ci please test

[glessard]

#if canImport() has a namespacing issue; nothing particular prevents a Linux distribution from having a module named Darwin. We've avoided using that approach for that reason. It would be nicer to be able to check specifically for the thing we need, and to be honest in this package Darwin isn't it.

[karwa]

#if canImport() has a namespacing issue; nothing particular prevents a Linux distribution from having a module named Darwin. We've avoided using that approach for that reason.

It’s only an issue if this package were to introduce a dependency on another package named “Darwin”, or if a third-party toolchain were to include a Swift module named “Darwin” in its standard installation.

That’s sufficiently unlikely that I don’t think it needs to be a concern. Such a toolchain would quickly encounter widespread problems across the Swift ecosystem and would likely choose a different name.

Importantly, the official toolchain would not encounter issues when building for Apple platforms. That’s the only thing I think we need to be concerned about right now.