#error

Swift has an #if system for conditional compilation, but there's no #error, so there's no obvious way to halt compilation. (Each branch must be valid syntax, so you can't simply write the word "error".) Easy workaround: import a library that doesn't exist.

#if os(macOS)
    import AppKit
#elseif os(iOS)
    import UIKit
#else
    import CompileTimeErrorPlease
#endif

UPDATE: The Swift team has just accepted SE-0196, which adds #warning and #error, so this is available in Swift 4.2 now!