$ nix build -f . swiftPackages.cxx-interop-test -L
swift-cxx-interop-test> Running phase: unpackPhase
swift-cxx-interop-test> unpacking source archive /nix/store/m30j9w6nrkaddslrfwyhbkq28jcs2bqw-src
swift-cxx-interop-test> source root is src
swift-cxx-interop-test> Unpacking SwiftPM dependencies
swift-cxx-interop-test> Running phase: patchPhase
swift-cxx-interop-test> Running phase: updateAutotoolsGnuConfigScriptsPhase
swift-cxx-interop-test> Running phase: configurePhase
swift-cxx-interop-test> no configure script, doing nothing
swift-cxx-interop-test> Running phase: buildPhase
swift-cxx-interop-test> SwiftPM flags: -j 16 -c release -Xswiftc -module-cache-path -Xswiftc /nix/var/nix/builds/nix-build-swift-cxx-interop-test.drv-0/b/module-cache -Xswiftc -I -Xswiftc /nix/store/9gz91ivdymjlk4dhnfs2wri3h3z7y3vv-swift-testing-6.2.3-include/lib/swift/macosx/testing -Xswiftc -I -Xswiftc /nix/store/y3w9x0ywz7d6ya9d95i9i5apbwq6dwmf-swift-stdlib-6.2.3-dev/lib/swift/macosx -Xswiftc -I -Xswiftc /nix/store/9gz91ivdymjlk4dhnfs2wri3h3z7y3vv-swift-testing-6.2.3-include/lib/swift/macosx/testing -Xswiftc -I -Xswiftc /nix/store/y3w9x0ywz7d6ya9d95i9i5apbwq6dwmf-swift-stdlib-6.2.3-dev/lib/swift/macosx
swift-cxx-interop-test> warning: /var/empty/Library/org.swift.swiftpm/configuration is not accessible or not writable, disabling user-level cache features.
swift-cxx-interop-test> warning: /var/empty/Library/org.swift.swiftpm/security is not accessible or not writable, disabling user-level cache features.
swift-cxx-interop-test> warning: 'src': Warning: supplying the --target arm64-apple-macosx14.0 != arm64-apple-darwin argument to a nix-wrapped compiler may not work correctly - cc-wrapper is currently not designed with multi-target compilers in mind. You may want to use an un-wrapped compiler instead.
swift-cxx-interop-test> Building for production...
swift-cxx-interop-test> [0/4] Write sources
swift-cxx-interop-test> [1/4] Write swift-version-2FDF012AFD0A72F3.txt
swift-cxx-interop-test> [3/5] Compiling CxxInteropTest main.swift
swift-cxx-interop-test> [3/5] Write Objects.LinkFileList
swift-cxx-interop-test> Warning: supplying the --target arm64-apple-macosx10.13 != arm64-apple-darwin argument to a nix-wrapped compiler may not work correctly - cc-wrapper is currently not designed with multi-target compilers in mind. You may want to use an un-wrapped compiler instead.
swift-cxx-interop-test> clang: warning: overriding '-mmacos-version-min=14.0' option with '--target=arm64-apple-macosx10.13' [-Woverriding-option]
swift-cxx-interop-test> [4/5] Linking CxxInteropTest
swift-cxx-interop-test> Build complete! (2.32s)
swift-cxx-interop-test> Running phase: installPhase
swift-cxx-interop-test> warning: /var/empty/Library/org.swift.swiftpm/configuration is not accessible or not writable, disabling user-level cache features.
swift-cxx-interop-test> warning: /var/empty/Library/org.swift.swiftpm/security is not accessible or not writable, disabling user-level cache features.
swift-cxx-interop-test> warning: 'src': Warning: supplying the --target arm64-apple-macosx14.0 != arm64-apple-darwin argument to a nix-wrapped compiler may not work correctly - cc-wrapper is currently not designed with multi-target compilers in mind. You may want to use an un-wrapped compiler instead.
swift-cxx-interop-test> warning: /var/empty/Library/org.swift.swiftpm/configuration is not accessible or not writable, disabling user-level cache features.
swift-cxx-interop-test> warning: /var/empty/Library/org.swift.swiftpm/security is not accessible or not writable, disabling user-level cache features.
swift-cxx-interop-test> Running phase: fixupPhase
swift-cxx-interop-test> checking for references to /nix/var/nix/builds/nix-build-swift-cxx-interop-test.drv-0/b/ in /nix/store/jrw7fkjigg64ika9w5dxv2w1m3b4gms9-swift-cxx-interop-test...
swift-cxx-interop-test> patching script interpreter paths in /nix/store/jrw7fkjigg64ika9w5dxv2w1m3b4gms9-swift-cxx-interop-test
swift-cxx-interop-test> stripping (with command strip and flags -S) in  /nix/store/jrw7fkjigg64ika9w5dxv2w1m3b4gms9-swift-cxx-interop-test/bin
swift-cxx-interop-test> Running phase: installCheckPhase
swift-cxx-interop-test> Hello, world!

$ nix develop -f . swiftPackages.cxx-interop-test -L
$ command -v clang
/nix/store/3fbjimjwwxk682a9gkbr2nss0yf232mf-clang-wrapper-21.1.8/bin/clang