bazel: scope Xcode-coupled flags to Swift builds via --config=swift_macos

Author: Copilot

.bazelrc

@@ -12,15 +12,21 @@ common --override_module=semmle_code=%workspace%/misc/bazel/semmle_code_stub
 
 build --repo_env=CC=clang --repo_env=CXX=clang++
 
-# Use apple_support's xcode_config: its version strings match the ones
-# `rules_swift`'s `system_sdk` selects are keyed on.
-build:macos --xcode_version_config=@local_config_xcode//:host_xcodes
-
-# Force apple_support's CC toolchains over Bazel's built-in `local_config_cc`
-# (whose `target_gnu_system_name` is literally "local", which breaks
-# `xcode_swift_toolchain`'s triple parsing). Root-level `--extra_toolchains`
-# beats module-registered toolchains; only matches Apple platforms.
-build:macos --extra_toolchains=@local_config_apple_cc_toolchains//:all
+# macOS Xcode-coupled flags, scoped to Swift builds only so that non-Swift
+# builds (C/C++/Rust/...) work with just the Command Line Tools. Enable
+# with `--config=swift_macos` (see `unified/swift-syntax-rs/README.md`; CI
+# for the `swift/` package passes it in `.github/workflows/swift.yml`).
+#
+# - `--xcode_version_config` selects `apple_support`'s xcode_config, whose
+#   version strings match the ones `rules_swift`'s `system_sdk` selects are
+#   keyed on (Bazel's built-in `host_xcodes` does not match).
+# - `--extra_toolchains` forces `apple_support`'s CC toolchain ahead of
+#   Bazel's built-in `local_config_cc` (whose `target_gnu_system_name` is
+#   literally "local", which breaks `xcode_swift_toolchain`'s triple
+#   parsing). Root-level `--extra_toolchains` beats module-registered
+#   toolchains; only matches Apple platforms.
+build:swift_macos --xcode_version_config=@local_config_xcode//:host_xcodes
+build:swift_macos --extra_toolchains=@local_config_apple_cc_toolchains//:all
 
 # Disable Android SDK auto-detection (we don't use it, and rules_android has Bazel 9 compatibility issues)
 build --repo_env=ANDROID_HOME=

.github/workflows/swift.yml

@@ -44,12 +44,18 @@ jobs:
           sudo apt-get install -y uuid-dev zlib1g-dev
       - name: Build Swift extractor
         shell: bash
+        env:
+          # On macOS, opt into the Xcode-coupled toolchain wiring only for
+          # Swift builds. Empty on Linux (hermetic swift.org toolchain).
+          BAZEL_SWIFT_CONFIG: ${{ runner.os == 'macOS' && '--config=swift_macos' || '' }}
         run: |
-          bazel run :install
+          bazel run $BAZEL_SWIFT_CONFIG :install
       - name: Run Swift tests
         shell: bash
+        env:
+          BAZEL_SWIFT_CONFIG: ${{ runner.os == 'macOS' && '--config=swift_macos' || '' }}
         run: |
-          bazel test ... --test_tag_filters=-override --test_output=errors
+          bazel test $BAZEL_SWIFT_CONFIG ... --test_tag_filters=-override --test_output=errors
   clang-format:
     runs-on: ubuntu-latest
     steps:

MODULE.bazel

@@ -35,7 +35,8 @@ bazel_dep(name = "rules_swift", version = "4.0.0-rc4")
 bazel_dep(name = "swift-syntax", version = "602.0.0.bcr.2")
 # Direct dep on `apple_support` (transitively pulled in by `rules_swift`) so we
 # can `use_repo` `local_config_xcode` and `local_config_apple_cc_toolchains`
-# below, needed to make `xcode_swift_toolchain` work on macOS. See `.bazelrc`.
+# below, needed by the `swift_macos` bazelrc config that makes
+# `xcode_swift_toolchain` work on macOS. See `.bazelrc`.
 bazel_dep(name = "apple_support", version = "2.6.1")
 
 bazel_dep(name = "zstd", version = "1.5.7.bcr.1")
@@ -248,15 +249,17 @@ register_toolchains(
 )
 
 # `apple_support`'s xcode_config, selected via `--xcode_version_config` in
-# `.bazelrc`. Needed because rules_swift's `system_sdk` selects use Xcode
-# version strings from `apple_support`'s `xcode_locator.m`, which Bazel's
-# built-in `host_xcodes` does not match.
+# the `swift_macos` bazelrc config (see `.bazelrc`). Needed because
+# rules_swift's `system_sdk` selects use Xcode version strings from
+# `apple_support`'s `xcode_locator.m`, which Bazel's built-in `host_xcodes`
+# does not match.
 xcode_configure = use_extension("@apple_support//xcode:xcode_configure.bzl", "xcode_configure_extension")
 use_repo(xcode_configure, "local_config_xcode")
 
 # `apple_support`'s CC toolchains, forced ahead of Bazel's built-in
-# `local_config_cc` via `--extra_toolchains` in `.bazelrc`. Must be visible
-# from the root module for the flag to resolve.
+# `local_config_cc` via `--extra_toolchains` in the `swift_macos` bazelrc
+# config (see `.bazelrc`). Must be visible from the root module for the flag
+# to resolve.
 apple_cc_configure = use_extension("@apple_support//crosstool:setup.bzl", "apple_cc_configure_extension")
 use_repo(apple_cc_configure, "local_config_apple_cc_toolchains")
 

unified/swift-syntax-rs/README.md

@@ -139,6 +139,11 @@ bazel test  //unified/swift-syntax-rs:swift_syntax_rs_test
 bazel run   //unified/swift-syntax-rs:swift-syntax-parse < some.swift
 ```
 
+On **macOS**, add `--config=swift_macos` to any of the above so that
+`rules_swift`'s `xcode_swift_toolchain` gets the Xcode-coupled CC toolchain
+and xcode_config it needs. The flag is opt-in so that non-Swift builds on
+macOS don't require a full Xcode.app install.
+
 Requirements:
 
 - **`clang`** must be installed on the runner. `rules_swift` requires the Bazel