#error in Swift (how to mark compile-time error)

Question

What is swift replacement of traditional c-style #error keyword?

I need it to raise compile-time error when pre-defines failed:

#if CONFIG1
    ...
#elseif CONFIG2
    ...
#else
    #error "CONFIG not defined"
#endif

Answer 1

According to the documentation, there is no specific #error macro. However it is possible to the program from compiling.

The way to do this is to define the variable you will be using inside the the #if/#endif clause. If no clause matches then the variable will be undefined and the program will not compile.

Raising an error at the failure site is possible with workarounds. Entering a plain string in the #else clause, which will generate a syntax error. Using @available will generate a compiler warning.

#if CONFIG1
    let config = // Create config 1
#elseif CONFIG2
    let config = // Create config 2
#else
    // Compilation fails due to config variable undefined errors elsewhere in the program.

    // Explicit syntax error to describe the scenario.
    Config not specified.

    // This generates a compiler warning.
    @available(iOS, deprecated=1.0, message="Config not defined")
#endif

// Use config here, e.g.
let foo = config["fooSize"]

Answer 2

The main idea of #error is causing a compilation error when something is missing, as swift doesn't have yet a similar preprocessor statement, just force the compilation error, like this code.

#if CONFIG1
...
#elseif CONFIG2
...
#else
    fatalError("CONFIG not defined")
    callingANonExistingFunctionForBreakingTheCompilation()
#endif

Remember that C/C++ won't verify the syntax of non-matching blocks, but Swift will, so this is the reason I'm calling a function rather than just writing a message.

Another option is using your own tag for generating the error and checking it before its compilation, like what this guy did here

Answer 3

Great news - if you're using Swift 4.2 or newer you can now use #error() and #warning()

For example:

let someBoolean = true

#warning("implement real logic in the variable above") // this creates a yellow compiler warning

#error("do not pass go, do not collect $200") // this creates a red compiler error & prevents code from compiling

Check out the implemented proposal here https://github.com/apple/swift-evolution/blob/master/proposals/0196-diagnostic-directives.md