Swift 4.2 Release Process

Swift 4.2 is meant to be a waypoint towards achieving ABI stability in Swift 5.

Swift 4.2 will include numerous under-the-hood ABI changes as part of the effort to stabilize the Swift ABI. It is valuable to incrementally roll out ABI changes — many of which are performance related — to provide ample time for user feedback in assessing these changes before they are locked into the final ABI.

Swift 4.2 will also include numerous bug fixes, as well as have a goal of some focused improvements on compile-time performance.

→ swift.org/blog/4.2-release-process/

Swift 4.2 serves as a step towards ABI stability in Swift 5, incorporating ABI changes for performance, bug fixes, and targeted improvements to compile-time speed.

It maintains source compatibility with Swift 4.0 and 4.1 for most code, though not binary compatibility, with the swift-4.2-branch cut from master soon and regular merges every two weeks until the final cutoff on April 20, 2018, after which only critical fixes are allowed via pull requests. Some repositories like swift-package-manager have extended deadlines up to July 2018.

Language and API changes require Swift Evolution approval, with increasingly strict acceptance criteria as the release nears, overseen by designated release managers.