[swift-evolution] Package manager support for versioning (both language and tools)

[Rick Ballard]

As part of Swift source compatibility, the Swift compiler added a new flag for controlling what Swift language version should be used to compile with. Currently there is no good way to specify this for Swift packages. We've got a proposal ready that provides a mechanism for controlling this. I'd appreciate any feedback the community would like addressed before this goes through evolution review.

The Swift language version support proposal can be viewed and commented on at https://github.com/rballard/swift-evolution/pull/2/files

Additionally, we are proposing to introduce a "Swift minimum tools version" mechanism for Swift packages. This mechanism will allow us to evolve the package manager – adding new API, and doing a one-time revision of our existing API for Swift 4 – without breaking the package ecosystem. It will also manage the Swift 3–>4 language transition for Package.swift files themselves. This is a pretty lengthy proposal, but I'd appreciate any important feedback before this goes to evolution review. There is also one decision left to make before this proposal is final; I lay out several possibilities for how we could store the "Swift tools version". I'd appreciate feedback on which of the possibilities we should pick before I bring this to review.

The Swift tools version support proposal can be viewed and commented on at https://github.com/rballard/swift-evolution/pull/1/files

Absent any serious feedback which requires us to make major revisions, I'm expecting to bring both of these to Swift evolution review early next week.

Thank you,

	- Rick