I'm wondering if this means that the major work on Swift the language is complete? I've avoided it since it seems every year the language or the API changes... has it now settled? I really didn't want to go back rewriting code every year.
UPDATE: Another comment here says that code compatibility is already fixed with Swift 3+.
The changes are mostly syntactic and like API changes. And Xcode has an integrated migration tool that's not too bad. I ported approx. 23KLOC of Swift (http://audiokit.io) from 2 to 3 in like 3 hours. And the changes are generally for the better.
> The Swift 4 compiler will include a language compatibility flag -swift-version that accepts 3 or 4 as arguments and controls the compatibility mode (SR- 2582 <https://bugs.swift.org/browse/SR-2582>).
> The -swift-version 3 compatibility mode has the goal of source compatibility as much as practically possible with Swift 3.x (including 3, 3.0.x, 3.x).
> The -swift-version 4 compatibility allows approved source code breakages from Swift 3.x (including 3, 3.0.x, 3.x).
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mo...
TL;DR: there may be breaking source changes between Swift 3 and Swift 4, but the Swift 4 compiler will still be capable of compiling Swift 3 code. There's no indication whether the Swift 5 compiler will still be able to compile Swift 3 code, so you'll likely still have to migrate your codebase within a year or two.