SwiftPM package always produces warnings when `import libzstd`

[yyaroshevich]

Describe the bug Import libzstd into Swift file causes compilation warnings.

To Reproduce Steps to reproduce the behavior:

1. Clone repro repository git clone https://github.com/yyaroshevich/zstd-warning-swift.git
2. Compile project (cd zstd-warning-swift; xcodebuild -project ./ZStdWarning.xcodeproj)
3. Observe compilation is failed due to warnings (warnings as errors are enabled)

Expected behavior No warnings produced when import libzstd.

Screenshots and charts

Desktop (please complete the following information):

• OS: Mac OS X 13.0.1
• Version libzstd: dev branch & 1.5.2
• Compiler: Swift 5.7, XCode 14.1

[yyaroshevich]

I've found out that the warnings are gone if 4 of that macroses are removed from https://github.com/facebook/zstd/blob/dev/lib/module.modulemap This is just an observation I wanted to share.

[yyaroshevich]

Hello @felixhandte! Could you please take a look at this issue (maybe giving hint where to proceed with investigation). It seems like the warnings appeared since changes introduced by https://github.com/facebook/zstd/pull/2953. If I locally revert modulemap to:

module libzstd [extern_c] {
    header "zstd.h"
    export *
}

the warning disappears, but it's likely incorrect per se. So it seems that either problem in modulemap file itself or in way Package.swift is defined. So, could you please help with this issue?

[terrelln]

These are all macros that if they aren't defined, zstd.h and friends define them. I guess that Swift then sees those macros as defined, and errors.

Looking into this.

[yyaroshevich]

Hello there! Is there any updates on this issue (maybe something not reflected here)?

[terrelln]

@yyaroshevich we don't really know how to fix it, and don't have the setup to reproduce it easily. If you find a fix we'd happily accept a PR!

[gmilos]

@terrelln the cause are the macros defined as config_macros in module.modulemap here: https://github.com/facebook/zstd/blob/97291fc5020a8994019ab76cf0cda83a9824374c/lib/module.modulemap#L4-L24. They aren't really config macros, as they are defined here: https://clang.llvm.org/docs/Modules.html#configuration-macros-declaration

(config macros need to be "binary" and cannot be #define-d the way zstd macros are).

AFICT there is no harm in just removing the config_macros block from the module.modulemap. It's broken currently anyway. Would you be able to do it for us?