Add support for --swiftversion latest

[calda]

This PR adds support for specifying --swiftversion latest, to automatically use the most recent language version supported by SwiftFormat. Implements #1226.

[bachand]

@calda thanks for adding this. When we use SwiftFormat in our project I think it's still best to specify the Swift version manually since we may not always be on the latest version of Swift supported by SwiftFormat. What do you think?

[calda]

Either is fine with me

[bachand]

I feel like what would be nice is if we could pass the Swift version to the SwiftFormat tool at runtime (maybe we can?). Then our tooling that wraps SwiftFormat can determine the current version of Xcode we're using and pass it to SwiftFormat.

[nicklockwood]

@calda hmm, I realize the concept of stable vs beta Swift versions complicates this more than I anticipated. I assumed latest would just be the most recent version that SwiftFormat knows about.

I did previously have an idea to try to automatically detect the Swift version, either by inspecting the files for modern features, or looking for a .xcodeproj or some other source of info. Maybe that's a better way to go?

[calda]

If you prefer to just use swiftVersions.last (5.7) instead, that's fine with me.

[calda]

or looking for a .xcodeproj or some other source of info

This is an interesting idea. For packages with a Package.swift I wonder if the swift-tools-version would be a good source for this info. As far as I can tell .xcodeproj config files don't include info about the minimum Swift version.

[bachand]

I feel like what would be nice is if we could pass the Swift version to the SwiftFormat tool at runtime (maybe we can?). Then our tooling that wraps SwiftFormat can determine the current version of Xcode we're using and pass it to SwiftFormat.

It looks like this is already supported 👍

or looking for a .xcodeproj or some other source of info

This is an interesting idea. For packages with a Package.swift I wonder if the swift-tools-version would be a good source for this info. As far as I can tell .xcodeproj config files don't include info about the minimum Swift version.

I also agree that an .xcodeproj may not have enough information for inferring the Swift version since you could be opening the project in different versions of Xcode with different Swift versions. Unless the .xcodeproj file stored the latest Xcode version with which it was saved?

When you point SwiftFormat at a directory corresponding to a Swift package using the swift-tools-version is a cool idea.