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Re:Swift 5.0 release process by coder543 on Tuesday September 25, 2018 @05:30PM (#18070656)
The Swift 5 release process document specifically implies that some Swift 4 code will not compile on Swift 5. This discussion is about source compatibility, not binary compatibility, which neither Rust nor Go offer AFAIK.

The number 5 _also_ implies significant breaking changes. Otherwise, why wouldn't it be Swift 4.3?

Removing Swift 3 compatibility mode (which was part of Swift 4) is itself a major source-compatibility breaking change, even if nothing else.

Re:Swift 5.0 release process by slavapestov on Tuesday September 25, 2018 @05:33PM (#18070682)
So far there are only a handful of small breaking changes between -swift-version 4 and -swift-version 5 on the master branch. The only ones I can think of are a type safety hole getting plugged and some warnings for corner cases around access control that got promoted to errors.
Re:Swift 5.0 release process by coder543 on Tuesday September 25, 2018 @05:37PM (#18070710)
If that's the case, that's encouraging to hear.
Re:Swift 5.0 release process by saagarjha on Tuesday September 25, 2018 @05:34PM (#18070687)
"Source compatibility" for Swift means that the compiler provides a flag with that will allow it to compiler earlier versions of Swift.