Swift.org – Swift 5.1 Release Process

February 18, 2019

Ted Kremenek is a member of the Swift Core Team and manages the Languages and Runtimes group at Apple.

This post describes the goals, release process, and estimated schedule for Swift 5.1.

Motivation and Goals

The primary goal of Swift 5.1 is for the language to achieve
module stability.

Binary Compatibility

On Apple platforms, since the ABI is now stabilized, Swift 5.1 is binary
compatible with Swift 5.0, and is binary compatible with future releases
of Swift.

On non-Apple platforms (such as Linux) the ABI is not yet fully stable
in order to allow for more vetting. Such vetting will be particularly
needed for the new platforms based on Linux.

Source Compatibility

As with Swift 5.0, we expect that majority of sources that built with
the Swift 5.0 compiler will compile with the Swift 5.1 compiler.
However, it is possible that a bug fix in Swift 5.1 may cause it to
detect errors in code that were not detected before.

Snapshots of Swift 5.1

Downloadable snapshots of the Swift 5.1 release branch will be posted
regularly as part of continuous integration testing.

Once Swift 5.1 is released, the official final builds will also be posted in
addition to the snapshots.

Getting Changes into Swift 5.1

The development of Swift 5.0 required an unusual amount of focus and
attention throughout the time it converged because every issue had to be
evaluated for its permanent ABI impact. Consequently, Swift 5.1 has a
significantly shorter development window than previous releases. This
tighter time constraint is needed to ensure delivering a mature and
stable 5.1 release, with stricter cutoff dates for disruptive changes.

The swift-5.1-branch contains the changes that will be released in Swift
5.1. The branch will be managed as follows:

• The swift-5.1-branch has already been initially cut from master.

• Periodically, the master development branch will be merged into
  swift-5.1-branch until the final branch date.

• March 18, 2019 (final branching): The swift-5.1-branch will have
  changes merged from master one last time. After the final branch date
  there will be a “bake” period in which only select, critical fixes will go
  into the release (via pull requests).

Some notable exceptions to this plan are indicated in the table below.
Each will merge from master into swift-5.1-branch daily. The final
cutoff date for changes to each exception will extend beyond March 18
and will be announced later.

Philosophy on Taking Changes into Swift 5.1

• All language and API changes for Swift 5.1 will go through the Swift
  Evolution process. Evolution
  proposals should aim to be completed by the branch date in order to
  increase their chances of impacting the Swift 5.1 release. Exceptions
  will be considered on a case-by-case basis, particularly if they tie
  in with the core goal of the release.

• Other changes (e.g., bug fixes, diagnostic improvements, SourceKit interface
  improvements) will be accepted based on their risk and impact.

• Low-risk test tweaks will also be accepted late into the release branch if
  it aids in the qualification of the release.

• As the release converges, the criteria for accepted changes will become
  increasingly restrictive.

Impacted Repositories

The following repositories will have a swift-5.1-branch branch to track
sources as part of Swift 5.1 release:

Release Managers

The overall management of the release will be overseen by the following
individuals, who will announce when stricter control of change goes into
effect for the Swift 5.1 release as the release converges:

Please feel free to post on the development forum
or contact Ted Kremenek directly concerning any questions about the release management
process.

Pull Requests for Release Branch

In order for a pull request to be considered for inclusion in the release
branch after the final re-branch from master it must include the following
information:

• Explanation: A description of the issue being fixed or enhancement being
  made. This can be brief, but it should be clear.

• Scope: An assessment of the impact/importance of the change. For
  example, is the change a source-breaking language change, etc.

• SR Issue: The SR if the change fixes/implements an issue/enhancement on
  bugs.swift.org.

• Risk: What is the (specific) risk to the release for taking this change?

• Testing: What specific testing has been done or needs to be done to
  further validate any impact of this change?

• Reviewer: One or more code owners
  for the impacted components should review the change. Technical review can
  be delegated by a code owner or otherwise requested as deemed appropriate or
  useful.

All change going into the swift-5.1-branch (outside changes being merged
in automatically from master) must go through pull requests that are
accepted by the corresponding release manager.