wasmkit in 6.2 toolchain is not universal on macOS

[wti]

Description

.../bin/wasmkit is built only for x86 in release/6.2 toolchain for macOS.

All other tools (except docc) are built as universal binaries.

This causes swift run ... to fail with a bad CPU on Apple silicon for wasm examples (recently described on the swift.org blog).

(fix proposed below)

Reproduction

• Download and install macOS toolchains from https://www.swift.org/install
  • Pick any version starting with swift-6.2-DEVELOPMENT-SNAPSHOT-2025-05-15-a.xctoolchain
• Check binary type: file usr/bin/wasmkit
• RESULT: Mach-O 64-bit executable x86_64
• Should be: universal binary with 2 architectures...

Expected behavior

• Check binary type: file usr/bin/wasmkit
• Expected result: universal binary with 2 architectures: [x86_64: .. ] [arm64]

Environment

Impacts macOS arm64, not x86.

Additional information

Workaround

• directly build and run wasmkit-cli from https://github.com/swiftwasm/WasmKit

Rough fix: build with --arch x86_64 --arch arm64 in wasmkit.py

The Swiftpm build command for wasmkit-cli should include --arch x86_64 --arch arm64 when the target is macOS.

• In file: /swift/utils/swift_build_support/swift_build_support/products/wasmkit.py
• See function: def run_swift_build
• Currently constructing rpath_args per target, so inject arch at the same time.

The following injects the parameters at about the right place (but it should be corrected, see below):

    build_os = host_target.split('-')[0]
    if set_installation_rpath and not host_target.startswith('macos'):
        # Library rpath for swift, dispatch, Foundation, etc. when installing
        rpath_args = [
            '--disable-local-rpath', '-Xswiftc', '-no-toolchain-stdlib-rpath',
            '-Xlinker', '-rpath', '-Xlinker', '$ORIGIN/../lib/swift/' + build_os
        ]
    else:
-        rpath_args = [] // <-------------------------------------- fix here
+       rpath_args = ['--arch', 'x86_64', '--arch', 'arm64']

    build_args = [
        swift_build,
        '--product', swiftpm_package_product_name,
        '--package-path', os.path.join(product.source_dir),
        '--build-path', product.build_dir,
        '--configuration', 'release',
    ] + rpath_args

Issues with the proposed fix

• Untested
• This does not deal with other platforms and assumes macOS is the only multi-arch platform.
• The if logic should add flags for macOS even if set_installation_rpath is true
• Probably should use another variable (arches_args) or rename rpath_args for clarity.

Related: docc binary

• docc similarly is not a universal binary
• If/since all toolchain targets should be universal, consider a configuration-driven unified fix?

[wti]

Context FYI: #81504

cc @MaxDesiatov

(If cc is not appropriate, sorry, I won't do it again.)

[MaxDesiatov]

@wti great catch, thanks for filing this bug report!