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Build configuration import testing

Available from Swift 4.1

Paul Hudson      @twostraws

Swift 4.1 implemented SE-0075, which introduced a new canImport condition that lets us check whether a specific module can be imported when our code is compiled.

This is particularly important for cross-platform code: if you had a Swift file that implemented one behavior on macOS and another on iOS, or if you needed specific functionality for Linux. For example:

#if canImport(SpriteKit)
   // this will be true for iOS, macOS, tvOS, and watchOS
#else
   // this will be true for other platforms, such as Linux
#endif

Previously you would have had to use inclusive or exclusive tests by operating system, like this:

#if !os(Linux)
   // Matches macOS, iOS, watchOS, tvOS, and any other future platforms
#endif

#if os(macOS) || os(iOS) || os(tvOS) || os(watchOS)
   // Matches only Apple platforms, but needs to be kept up to date as new platforms are added
#endif

The new canImport condition lets us focus on the functionality we care about rather than what platform we're compiling for, thus avoiding a variety of problems.

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Other changes in Swift 4.1…

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