Swift 3.1 is intended to be source compatible with Swift 3.0. It will contain a few additive enhancements to the core language as well as improvements to the Swift Package Manager, Swift on Linux, and general quality improvements to the compiler and Standard Library.
A strong goal of Swift 3.1 is that it be backwards compatible with Swift 3.0 code; however, the exception will be bug fixes to the compiler that reject code that shouldn't have been accepted.
Snapshots of Swift 3.1
Previous releases of Swift have had “Developer Previews”, e.g. “Preview 1”, “Preview 2”, etc., that represent stabilized snapshots of a Swift release as it converges. Developer previews have often been irregularly spaced apart, and have sometimes not provided enough granularity for the Swift community to try out new features or verify bug fixes in a release as it converges.
For Swift 3.1 there will instead be daily downloadable snapshots of the release branch. Snapshots will be produced as part of continuous integration testing. The cadence of downloadable snapshots will thus be more frequent and granular. Snapshots will be posted daily, assuming tests are passing.
Once Swift 3.1 is released, official final builds will also be posted in addition to the snapshots.
Getting Changes into Swift 3.1
Swift 3.1 is intended to be limited in scope, with the desire to move focus early in 2017 to the development of Swift 4. To meet this goal, Swift 3.1 will include changes in mainline development (i.e. the master branch) only until January 16. After that date there will be a “bake” period in which only select, critical fixes will go into the swift-3.1-branch and move master on to Swift 4 development.
More details can be found in the post linked below...