Support building with Clang-CL on Windows.

See https://bazel.build/configure/windows#clang.

.bazelrc

@@ -46,4 +46,8 @@ build --//private:treat_warnings_as_errors
 
 build --compilation_mode=dbg --host_compilation_mode=dbg
 
+build:clang-cl --compiler=clang-cl --host_compiler=clang-cl
+build:clang-cl --extra_toolchains='@local_config_cc//:cc-toolchain-x64_windows-clang-cl'
+build:clang-cl --extra_execution_platforms='//elisp/private:windows_clang_cl'
+
 import %workspace%/c-std.bazelrc

.github/actions/set-up/action.yaml

@@ -58,6 +58,8 @@ runs:
         # --incompatible_enforce_starlark_utf8 was added in Bazel 8.1.
         # FIXME: Add it to the project’s .bazelrc once we drop support for
         # Bazel 8.0 and below.
+        # See https://bazel.build/configure/windows#clang for how to configure
+        # Bazel to use Clang on Windows.
         bazelrc: |
           common --announce_rc
           common --show_progress_rate_limit=10
@@ -73,6 +75,10 @@ runs:
             inputs.bazel-version == 'latest' &&
             'common --incompatible_enforce_starlark_utf8=error'
           }}
+          ${{
+            runner.os == 'Windows' && inputs.cc == 'clang' &&
+            'build --config=clang-cl'
+          }}
         # Use disk cache to speed up runs.
         disk-cache: ${{inputs.bazel-version}}-${{inputs.cc}}
         repository-cache: true

.github/workflows/bazel-test.yaml

@@ -41,8 +41,6 @@ jobs:
             cc: msvc
           - os: windows-latest
             cc: gcc
-          - os: windows-latest
-            cc: clang
           # Exclude non-default compilers for now.  We should add them later.
           - os: macos-latest
             cc: gcc

elisp/private/BUILD

@@ -189,6 +189,15 @@ selects.config_setting_group(
     visibility = ["//visibility:private"],
 )
 
+selects.config_setting_group(
+    name = "msvc_or_clang_cl",
+    match_any = [
+        "@rules_cc//cc/compiler:msvc-cl",
+        "@rules_cc//cc/compiler:clang-cl",
+    ],
+    visibility = ["//visibility:private"],
+)
+
 # keep
 bzl_library(
     name = "module_config_info",
@@ -242,6 +251,17 @@ bzl_library(
     visibility = ["//elisp:__pkg__"],
 )
 
+# https://bazel.build/configure/windows#clang
+platform(
+    name = "windows_clang_cl",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:windows",
+        "@rules_cc//cc/private/toolchain:clang-cl",
+    ],
+    visibility = ["//visibility:private"],
+)
+
 bzl_library(
     name = "cc_config",
     srcs = ["cc_config.bzl"],

elisp/private/cc_config.bzl

@@ -23,7 +23,7 @@ FEATURES = select({
 
 # Shared C++ compilation options.
 COPTS = select({
-    Label("@rules_cc//cc/compiler:msvc-cl"): [
+    Label(":msvc_or_clang_cl"): [
         "/W4",
         "/utf-8",
         "/permissive-",
@@ -48,10 +48,20 @@ COPTS = select({
         "--system-header-prefix=upb/",
     ],
     Label("//conditions:default"): [],
+}) + select({
+    Label("@rules_cc//cc/compiler:clang-cl"): [
+        # Allow Clang-CL to include windows.h without error.
+        "-Wno-error=ignored-attributes",
+        "-Wno-error=ignored-pragma-intrinsic",
+        "-Wno-error=macro-redefined",
+        "-Wno-error=pragma-pack",
+        "-Wno-error=unknown-pragmas",
+    ],
+    Label("//conditions:default"): [],
 })
 
 CXXOPTS = select({
-    Label("@rules_cc//cc/compiler:msvc-cl"): [],
+    Label(":msvc_or_clang_cl"): [],
     Label("@rules_cc//cc/compiler:gcc"): [
         # GCC appears to treat some moves as redundant that are in fact
         # necessary.
@@ -61,7 +71,7 @@ CXXOPTS = select({
 })
 
 CONLYOPTS = select({
-    Label("@rules_cc//cc/compiler:msvc-cl"): [],
+    Label(":msvc_or_clang_cl"): [],
     Label(":gcc_or_clang"): ["-Wvla"],
 })
 

elisp/private/cc_launcher_config.bzl

@@ -27,7 +27,7 @@ LAUNCHER_FEATURES = FEATURES
 LAUNCHER_COPTS = COPTS + CXXOPTS
 LAUNCHER_DEFINES = DEFINES
 LAUNCHER_LINKOPTS = LINKOPTS + select({
-    Label("@rules_cc//cc/compiler:msvc-cl"): ["/SUBSYSTEM:CONSOLE"],
+    Label(":msvc_or_clang_cl"): ["/SUBSYSTEM:CONSOLE"],
     Label(":gcc_or_clang"): [],
 })
 

elisp/proto/BUILD

@@ -140,7 +140,7 @@ elisp_cc_module(
     name = "module",
     srcs = ["module.c"],
     copts = COPTS + CONLYOPTS + select({
-        "@rules_cc//cc/compiler:msvc-cl": ["/wd4100"],
+        "//elisp/private:msvc_or_clang_cl": ["/wd4100"],
         "//elisp/private:gcc_or_clang": ["-fvisibility=hidden"],
     }),
     features = FEATURES + ["-default_link_libs"],

examples/ext/.bazelrc

@@ -12,4 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+build:clang-cl --compiler=clang-cl --host_compiler=clang-cl
+build:clang-cl --extra_toolchains='@local_config_cc//:cc-toolchain-x64_windows-clang-cl'
+build:clang-cl --extra_execution_platforms='@phst_rules_elisp//elisp/private:windows_clang_cl'
+
 import %workspace%/../../c-std.bazelrc

examples/ext/MODULE.bazel

@@ -34,3 +34,9 @@ elisp.http_archive(
     ],
 )
 use_repo(elisp, "dash")
+
+# Needed for building with Clang-CL on Windows.
+bazel_dep(name = "rules_cc", version = "0.1.1", dev_dependency = True)
+
+cc_configure = use_extension("@rules_cc//cc:extensions.bzl", "cc_configure_extension", dev_dependency = True)
+use_repo(cc_configure, "local_config_cc")