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Fix macOS 27 SDK linking with Apple ld #388

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tameware merged 1 commit into from
Sep 19, 2026
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Fix macOS 27 SDK linking with Apple ld #388

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6 changes: 6 additions & 0 deletions .bazelrc
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Expand Up @@ -37,6 +37,12 @@ build --tool_java_runtime_version=remotejdk_25
# Platform-specific C++ standard flags
build:macos --cxxopt=-std=c++20
build:macos --host_cxxopt=-std=c++20
# Apple's ld (MODULE.bazel extra_link_flags --ld-path=/usr/bin/ld) does not
# accept lld's --start-lib/--end-lib. Disabling this feature is a no-op for
# toolchains that never advertised it (wasm/emscripten) and only skips an
# lld grouping optimization elsewhere. Do not put --ld-path here: it would
# leak into wasm transitions via enable_platform_specific_config.
build:macos --features=-supports_start_end_lib
build:linux --cxxopt=-std=c++20
build:linux --host_cxxopt=-std=c++20
# Windows MSVC C++20 comes from the patched rules_cc default_cpp_std feature
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10 changes: 10 additions & 0 deletions MODULE.bazel
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Expand Up @@ -76,6 +76,16 @@ llvm.toolchain(
# When bumping these versions, also update .bazelrc build:tsan_macos rpath
# (clang/N) and confirm Xcode ships the same major; see docs/BUILD_SYSTEM.md.
# Keep MSAN_LLVM_VERSION / MSAN_DISTRIBUTION below on the same LLVM release.
# LLVM 21 ld64.lld cannot parse MacOSX 27 SDK TBD files that list
# arm64e.x1. Apple's ld can; last --ld-path wins over the toolchain default
# --ld-path=ld64.lld. Pair with .bazelrc --features=-supports_start_end_lib
# because Apple ld rejects lld's --start-lib/--end-lib. Scoped to this
# toolchain so wasm/emscripten keeps wasm-ld (do not put --ld-path in
# build:macos --linkopt). See docs/BUILD_SYSTEM.md (macOS SDK section).
extra_link_flags = {
"darwin-aarch64": ["--ld-path=/usr/bin/ld"],
"darwin-x86_64": ["--ld-path=/usr/bin/ld"],
},
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llvm_versions = {
"darwin-aarch64": "21.1.8",
"linux-x86_64": "21.1.8",
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11 changes: 11 additions & 0 deletions docs/BUILD_SYSTEM.md
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Expand Up @@ -64,6 +64,17 @@ If you see runtime loader failures after a toolchain or OS change:
2. Re-resolve Bazel toolchains (`bazelisk shutdown`, then `bazelisk clean --expunge`).
3. Re-run `bazelisk test //...` to confirm runtime compatibility.

**Xcode 27 / MacOSX 27 SDK TBD files.** The macOS 27 SDK lists `arm64e.x1` in
system `.tbd` stubs. LLVM 21's `ld64.lld` rejects those files (`unknown
architecture arm64e.x1`) and then reports a cascade of missing libc++ /
libSystem symbols. Native Darwin links therefore use Apple's `/usr/bin/ld`
via `llvm.toolchain.extra_link_flags` in `MODULE.bazel`, and `.bazelrc`
disables Bazel's `supports_start_end_lib` feature (Apple ld does not accept
`--start-lib` / `--end-lib`). That `--ld-path` override is toolchain-scoped
on purpose: putting it in `build:macos --linkopt` would leak into wasm
transitions and replace `wasm-ld`. Drop both workarounds when LLVM's Mach-O
linker can parse `arm64e.x1` TBD targets.

### AddressSanitizer, ThreadSanitizer, UndefinedBehaviorSanitizer, and MemorySanitizer

Sanitizer builds use `--config=asan`, `--config=tsan`, `--config=ubsan`, or
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