Swift is licensed under the Apache license, which includes a clause stating that all contributors, implicitly in the act of contributing, grant a license to all users to use any component of any patent they own that is necessary in order to use their contributions as part of Swift. That grant is perpetual and irrevocable with regard to both all past contributions and all current contributions. Apple could, in theory, publicly declare at any point that its future contributions will no longer be licensed under the Apache license, and then it could pursue patent infringement claims against any users of versions of the Swift code that incorporate those future contributions. However, (1) that would obviously kill the project, so (2) we have no intention of doing that, and (3) you would know about it in advance if we did because we'd have to be very explicit about it, and finally (4) you'd still be able to freely use all versions of Swift up to that point.
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