Swift 5 is ABI stable, which gives direct compatibility with OS without additional layers and backward compatibility between different versions of Swift, beginning from 5.0 version.

Also the new version has a lot of improvements:

1. Flattened try? with Optional type
   No more double guard.
   

2. Raw strings
   Gives one of the possibility to write a string without escaping characters inside. It's very feature for writing raw RegEx expressions in a clean way.

   

   If you need to embed a variable inside a raw string, you must add hash symbol after backslash: #"(\\|\#(variable))"#

3. String interpolation
   Declaration can be flexible and behave on your own.
   For this, extend the protocol StringInterpolation with own function:
   

4. No recursion inside computed properties
   Avoid additional layer for computations, and have it inside a closure.

It was a part of described features, more is there: https://developer.apple.com/documentation/xcode_release_notes/xcode_10_2_beta_release_notes/swift_5_release_notes_for_xcode_10_2_beta

Article on Medium: https://medium.com/@dimpiax/swift-5-what-is-new-examples-fbb64d525486