Add macOS (xcode) support to swift-syntax-rs build

MODULE.bazel

@@ -227,12 +227,17 @@ use_repo(
 # patched prebuilt toolchain wired up via `swift_deps` above.
 #
 # Bazel cannot auto-select between Linux distributions, so the toolchain is
-# registered explicitly for the distribution our CI runs on (ubuntu24.04,
-# x86_64). Add further platforms here if CI grows to cover them.
+# registered explicitly for the distributions/platforms our CI runs on:
+#   - ubuntu24.04 / x86_64  (Linux CI)
+#   - xcode                  (macOS CI — covers both Apple Silicon and Intel)
 #
-# We register the `exec` toolchain (normal host compilation, targeting
-# linux/x86_64) rather than the `embedded` one, which targets `os:none`
-# (Embedded Swift) and would not match a normal host build.
+# NOTE: The macOS toolchain is distributed as a `.pkg` archive that can only
+# be extracted on a macOS host. Do not build the `xcode` toolchain on Linux.
+#
+# We register the `exec` toolchains (normal host compilation) rather than the
+# `embedded` ones, which target `os:none` (Embedded Swift) and would not
+# match a normal host build. Bazel selects the toolchain matching the host
+# platform automatically.
 #
 # The Swift version is read from `unified/swift-syntax-rs/.swift-version`, which
 # is the single source of truth shared with the local `cargo` build (via
@@ -246,10 +251,12 @@ use_repo(
     swift,
     "swift_toolchain",
     "swift_toolchain_ubuntu24.04",
+    "swift_toolchain_xcode",
 )
 
 register_toolchains(
     "@swift_toolchain//:swift_toolchain_exec_ubuntu24.04",
+    "@swift_toolchain//:swift_toolchain_exec_xcode",
 )
 
 node = use_extension("@rules_nodejs//nodejs:extensions.bzl", "node")

unified/swift-syntax-rs/BUILD.bazel

@@ -42,11 +42,16 @@ rust_binary(
     name = "swift-syntax-parse",
     srcs = ["src/main.rs"],
     # The Swift toolchain propagates a runfiles-relative RPATH to its runtime
-    # `.so`s via `swift_syntax_ffi`'s `CcInfo`, but (unlike `swift_binary`)
-    # `rust_binary` does not copy those libraries into runfiles. Add the
-    # toolchain files as `data` so the runtime is found at execution time.
-    # (rules_swift does not yet support statically linking the runtime.)
-    data = ["@swift_toolchain_ubuntu24.04//:files"],
+    # `.so`s/`.dylib`s via `swift_syntax_ffi`'s `CcInfo`, but (unlike
+    # `swift_binary`) `rust_binary` does not copy those libraries into runfiles.
+    # Add the toolchain files as `data` so the runtime is found at execution
+    # time. (rules_swift does not yet support statically linking the runtime.)
+    # Select the correct per-platform toolchain repo: `xcode` on macOS,
+    # `ubuntu24.04` on Linux.
+    data = select({
+        "@platforms//os:macos": ["@swift_toolchain_xcode//:files"],
+        "//conditions:default": ["@swift_toolchain_ubuntu24.04//:files"],
+    }),
     edition = "2024",
     deps = [":swift_syntax_rs"],
 )
@@ -55,6 +60,9 @@ rust_test(
     name = "swift_syntax_rs_test",
     size = "small",
     crate = ":swift_syntax_rs",
-    data = ["@swift_toolchain_ubuntu24.04//:files"],
+    data = select({
+        "@platforms//os:macos": ["@swift_toolchain_xcode//:files"],
+        "//conditions:default": ["@swift_toolchain_ubuntu24.04//:files"],
+    }),
     edition = "2024",
 )

unified/swift-syntax-rs/README.md

@@ -144,10 +144,12 @@ Requirements:
 - **`clang`** must be installed on the runner. `rules_swift` requires the Bazel
   CC toolchain to use clang; the repo's `.bazelrc` already sets
   `--repo_env=CC=clang`, so no extra flags are needed.
-- The registered Swift toolchain currently targets **ubuntu24.04 / x86_64**
-  only (Bazel cannot auto-select between Linux distributions). Add more
-  platforms in `MODULE.bazel` (`swift.toolchain` + `register_toolchains`) if CI
-  grows to cover them.
+- The registered Swift toolchains cover **ubuntu24.04 / x86_64** and
+  **macOS / `xcode`** (both Apple Silicon and Intel). Bazel automatically
+  selects the toolchain matching the host platform.
+- **macOS only:** the macOS toolchain is distributed as a `.pkg` archive that
+  can only be fetched and extracted on a macOS host. Building the `xcode`
+  toolchain on Linux is not supported.
 
 The Swift compiler version is read from [`.swift-version`](.swift-version) by
 both the Bazel toolchain (`swift.toolchain(swift_version_file = …)`) and the

unified/swift-syntax-rs/build.rs

@@ -73,7 +73,7 @@ fn swift_runtime_dir() -> Option<PathBuf> {
     let close = value_start.find('"')?;
     let resource_path = &value_start[..close];
 
-    Some(PathBuf::from(resource_path).join("linux"))
+    Some(PathBuf::from(resource_path).join(if cfg!(target_os = "macos") { "macosx" } else { "linux" }))
 }
 
 /// The `swift` driver to invoke: `$SWIFT` if set, otherwise `swift` from `PATH`.