Enable Swift 6 Language Mode

[dfed]

Apollo V2 is currently enableExperimentalFeature swift settings that are meant for Package files using the Swift 5.9 and 5.10 toolchains. The packages in this repo are all using swift-tools-version:6.1.

Per Enabling Complete Concurrency Checking:

We could change enableExperimentalFeature to enableUpcomingFeature, as suggested by the above. But based on local testing, the targets in the Package.swift files in this repo already build in Swift 6 Language Mode, so I enabled it.

Feel free to close this out if it's not what you want just yet – I know I'm jumping the gun going straight to a PR here. Note that this PR is against the v2-alpha branch, which may not be the right place. It seemed like the most up-to-date branch, but I may have missed something.

[AnthonyMDev]

I actually had done this the other day locally, and then reverted it because I couldn't find any clear documentation of how this affected compatibility with Swift 5 code. Maybe you are more familiar with this than I am.

If we merge this PR and enable Swift 6 language mode, will the library still be able to be linked to user's code if they are using Swift 5? My understanding is that the answer is yes, but I haven't been able to find a definitive answer to that question, so I was hesitant to make the switch.

[AnthonyMDev]

Also, changing this to be based on the v2-staging branch. :)

[dfed]

If we merge this PR and enable Swift 6 language mode, will the library still be able to be linked to user's code if they are using Swift 5? My understanding is that the answer is yes, but I haven't been able to find a definitive answer to that question, so I was hesitant to make the switch.

Yes! Swift 5 can depend on Swift 6 code, and visa versa. The bigger constraint is your swift-tools-version: by setting swift-tools-version:6.1, you are dropping support for Xcode versions that do not have the Swift 6.1 compiler available (Xcode 16.2 and earlier). My guess is that's intentional though.