[Bug]: Incompatible with warnings as errors when built through Xcode

[Obbut]

I have done the following

• I have searched the existing issues
• If possible, I've reproduced the issue using the 'main' branch of this project

Steps to reproduce

1. Create an Xcode project with a target that depends on Containerization
2. Enable SWIFT_TREAT_WARNINGS_AS_ERRORS = YES on the target
3. Build

The issue does not occur when building via SPM with .treatAllWarnings(as: .error).

To quickly reproduce this using Xcodegen:

mkdir -p /tmp/containerization-zstd-repro/Sources/MyLib

cat > /tmp/containerization-zstd-repro/Sources/MyLib/MyLib.swift << 'EOF'
import Containerization

public struct MyLib {
    public init() {}
}
EOF

cat > /tmp/containerization-zstd-repro/project.yml << 'EOF'
name: ZstdRepro
options:
  bundleIdPrefix: com.test
packages:
  containerization:
    url: https://github.com/apple/containerization
    exactVersion: "0.25.1"
targets:
  ZstdRepro:
    type: framework
    platform: macOS
    deploymentTarget: "26.0"
    sources:
      - Sources/MyLib
    dependencies:
      - package: containerization
        product: Containerization
    settings:
      SWIFT_TREAT_WARNINGS_AS_ERRORS: YES
EOF

cd /tmp/containerization-zstd-repro
xcodegen generate
xcodebuild -project ZstdRepro.xcodeproj -scheme ZstdRepro -destination 'platform=macOS' build

Current behavior

The build fails with errors originating from zstd's module.modulemap:

<unknown>:0: error: definition of configuration macro 'ZSTDLIB_VISIBLE' has no effect on the import of 'libzstd'; pass '-DZSTDLIB_VISIBLE=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zstd.h:33:13: note: macro was defined here
#    define ZSTDLIB_VISIBLE __attribute__ ((visibility ("default")))
            ^
<unknown>:0: error: definition of configuration macro 'ZSTDLIB_HIDDEN' has no effect on the import of 'libzstd'; pass '-DZSTDLIB_HIDDEN=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zstd.h:41:13: note: macro was defined here
#    define ZSTDLIB_HIDDEN __attribute__ ((visibility ("hidden")))
            ^
<unknown>:0: error: definition of configuration macro 'ZSTD_CLEVEL_DEFAULT' has no effect on the import of 'libzstd'; pass '-DZSTD_CLEVEL_DEFAULT=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zstd.h:134:11: note: macro was defined here
#  define ZSTD_CLEVEL_DEFAULT 3
          ^
<unknown>:0: error: definition of configuration macro 'ZDICTLIB_VISIBLE' has no effect on the import of 'libzstd'; pass '-DZDICTLIB_VISIBLE=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zdict.h:28:13: note: macro was defined here
#    define ZDICTLIB_VISIBLE __attribute__ ((visibility ("default")))
            ^
<unknown>:0: error: definition of configuration macro 'ZDICTLIB_HIDDEN' has no effect on the import of 'libzstd'; pass '-DZDICTLIB_HIDDEN=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zdict.h:36:13: note: macro was defined here
#    define ZDICTLIB_HIDDEN __attribute__ ((visibility ("hidden")))
            ^
<unknown>:0: error: definition of configuration macro 'ZSTDERRORLIB_VISIBLE' has no effect on the import of 'libzstd'; pass '-DZSTDERRORLIB_VISIBLE=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zstd_errors.h:24:13: note: macro was defined here
#    define ZSTDERRORLIB_VISIBLE __attribute__ ((visibility ("default")))
            ^
<unknown>:0: error: definition of configuration macro 'ZSTDERRORLIB_HIDDEN' has no effect on the import of 'libzstd'; pass '-DZSTDERRORLIB_HIDDEN=...' on the command line to configure the module
/Users/robbert/Library/Developer/Xcode/DerivedData/ZstdRepro-cmbtortrvhygntcubcdbmijtgeye/SourcePackages/checkouts/zstd/lib/zstd_errors.h:32:13: note: macro was defined here
#    define ZSTDERRORLIB_HIDDEN __attribute__ ((visibility ("hidden")))

The root cause is fixed in facebook/zstd#4531, but Containerization pins zstd at exact: "1.5.7" which does not include the fix.

Expected behavior

Importing Containerization should not produce warnings, so that Xcode targets can use warnings-as-errors without issue.

Environment

- OS: macOS 26.3
- Xcode: 26.3
- Swift: 6.2

Relevant log output

N/A

Code of Conduct

• I agree to follow this project's Code of Conduct

[jglogan]

@Obbut could you test out fix #554 by trying to build you Xcode project against a local path dependency to a clone of https://github.com/jglogan/containerization/tree/encapsulate-libzstd?

I threw together a quick Xcode project and was able to see the warnings and build failure (with treat-swift-warnings-as-errors) without this fix, and a totally clean build with no warnings afterwards.

[Obbut]

@jglogan I can confirm that #554 resolved the issue. Thanks!