Pitch: [SwiftPM] Swift Language Version Per Target

[hborla (Holly Borla)]

I think this is an important pitch, because incremental migration per-target is critical for the Swift 6 migration, including for packages. I have a few minor comments/questions

@xedin

  public static func swiftVersion(

Should we call this swiftLanguageVersion since the term "Swift version" is ambiguous between "language" and "tools"?

@xedin

Since this is a new API, all existing packages will use the default behavior - version specified at the package level when set or determined based on the tools version.

It's worth clarifying what the default language version actually is when using Swift 6.0 tools, because I think it's debatable. Personally, I don't think the default when using Swift 6.0 tools should be Swift 6 mode - that's not the default in the compiler, because otherwise source compatibility won't be maintained for existing code. I can see an argument that maybe the SwiftPM behavior should be different because you need to explicitly change the package manifest to require 6.0 tools, but I also don't think using 6.0 tools is an indication that the package wants to use the Swift 6 language mode by default.

@xedin

Add a new setting for 'known-safe' flags, of which -swift-version could be the first. This seems less user-friendly and error prone than re-using SwiftLanguageVersion, which has known language versions as its cases (with a plaintext escape hatch when necessary).

I agree with the choice to use SwiftLanguageVersion in the API here. Will you also add a v6 static let? e.g.

    /// Swift language version 6.
    public static let v6 = SwiftLanguageVersion(uncheckedString: "6")