Swift 3.1 fixes a bug in the Swift Package Manager that prevented overriding the macOS deployment target.

When you run swift build on macOS, the package manager currently (as of Swift 3.0 and 3.1) hardcodes the deployment target to macOS 10.10.¹ In Swift 3.0 it was impossible to override this due to a bug in the order arguments were evaluated.

Because of this, you could not easily² compile code that required newer APIs. As an example, consider a very simple package with just a few lines of code in a single source file. The program uses the new units and measurements types introduced in macOS 10.12 to convert a value from km/h to m/s:

// main.swift
import Foundation

let kph = Measurement(value: 100,
    unit: UnitSpeed.kilometersPerHour)
let mps = kph.converted(to: .metersPerSecond)
print("\(kph) is \(mps)")

Building this with swift build on macOS (with Swift 3.0 or 3.1) produces an error because the API is not available on macOS 10.10:

$ swift build
Compile Swift Module 'Units' (1 sources)
main.swift:3:11: error: 'Measurement' is only available on OS X 10.12 or newer
let kph = Measurement(value: 100,
          ^
main.swift:3:11: note: add 'if #available' version check
let kph = Measurement(value: 100,
          ^
...
<unknown>:0: error: build had 1 command failures
error: exit(1): /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift-build-tool -f .build/debug.yaml

In Swift 3.1 you can override the deployment target on the command line like this:

$ swift build -Xswiftc "-target" \
    -Xswiftc "x86_64-apple-macosx10.12"
Compile Swift Module 'Units' (1 sources)
Linking ./.build/debug/Units

And now you can run the executable:

$ .build/debug/Units
100.0 km/h is 27.7778 m/s

Conclusion

Aside from the deployment target, another common use case for custom build settings is to pass a DEBUG flag to the compiler so that you can use #if DEBUG/#endif blocks in your code — the package manager doesn’t do this automatically in debug builds. You can do it with swift build -Xswiftc "-D" -Xswiftc "DEBUG".

This is still far from ideal — you have to pass the command line arguments manually every time you invoke swift build or swift test — but at least it’s possible.

The ability to specify custom build settings in the package manifest is part of the Swift 4 roadmap for the package manager. My guess is we’ll see a Swift Evolution proposal for this soon.

You an verify this by adding this code snippet to your main.swift file and then building and running the package:
```
#if os(macOS)
    print("macOS deployment target:", __MAC_OS_X_VERSION_MIN_REQUIRED)
#endif
```
If run on macOS, this will print:
```
macOS deployment target: 101000
```
↩︎
You’d have to wrap all code that depended on the newer APIs in if #available(macOS 10.12, iOS 10.0, tvOS 10.0, watchOS 3.0, *) { ... } (or similar) blocks. ↩︎

Ole Begemann

Swift Package Manager macOS deployment target override

Conclusion