macOS 10.14.4 also includes Swift 5. The main new feature of Swift 5 is that Swift is ABI stable.
Simply said, ABI stability allows swift binaries to use a Swift library on the system instead of having to bundle the libraries with them. This will, of course reduce the size of the binaries.
For example, my desktoppr tool compiles to 6.5MB with Swift 4.2 and 56KB with Swift 5.
However, command line tools built with Xcode 10.2 now rely on the Swift library to be available on the system. macOS 10.14.4 and future versions will include the libraries, but older macOS versions did not. There is no option in Xcode 10.2 to keep the old behavior of bundling the libraries in the tool.
This means that when you re-build a tool in Xcode 10.2 with Swift it will not run on older macOS version:
$ sw_vers -productVersion
10.13.6
$ ./desktoppr
dyld: Library not loaded: @rpath/libswiftAppKit.dylib
Referenced from: /Users/armin/Desktop/desktoppr
Reason: image not found
Abort trap: 6
Apple provides an installer for the Swift libraries for “earlier versions of macOS.” (The package installer declares a minimum OS version of 10.9.)
The libraries are installed in /usr/lib/swift, where your binaries can find them:
$ ./desktoppr
/Library/Desktop Pictures/BoringBlueDesktop.png
Note: these libraries are used by compiled binaries. Installing the runtime libriaries will not allow you to run swift script files, i.e. text files starting with the #!/usr/bin/swift shebang. You still need to install Xcode or the Developer Command Line tools for that.
So far, only command line tools written and compiled in Xcode 10.2 will require the libraries. Application bundles will continue to include their own libraries.
As Swift gets updated, you will need to update the installed libraries as well. You can get the installed version of the Runtime with pkgutil:
$ pkgutil --info com.apple.pkg.SwiftRuntimeForCommandLineTools
package-id: com.apple.pkg.SwiftRuntimeForCommandLineTools
version: 10.2.0.0.1.1552586384
volume: /
location: /
install-time: 1553789052
groups: com.apple.FindSystemFiles.pkg-group
However, on a Mac with 10.14.4 the swift libraries are present but not installed by the same installer package, they are part of the entire system:
$ pkgutil --file-info /usr/lib/swift/libswiftFoundation.dylib
volume: /
path: /usr/lib/swift/libswiftFoundation.dylib
pkgid: com.apple.pkg.update.os.10.14.4.18E226
pkg-version: 1.0.0.0.1.1553145155
install-time: 1553765876
uid: 0
gid: 0
mode: 755
If/When Apple updates the runtime libraries, this might be a challenge to track and update properly.
Note: While I used my tool desktoppr as an example, I have not updated the version available for download to Swift 5 yet. So, that is still built with the “old” Swift and Xcode and should work everywhere without the runtime libraries.
Nevertheless, it should be prudent for MacAdmins to install the Swift 5 runtime libraries on their fleet. At the very least be aware that these errors can occur and how to fix them.
