Breaking change: prohibit using Bazel+MSVC to build protobuf

An opt-out flag `--define=protobuf_allow_msvc=true` will be available until our 2026 breaking release 34.0. See #20085 for more details.

#test-continuous

PiperOrigin-RevId: 718525948

Author: mkruskal-google

.bazelrc

@@ -40,3 +40,8 @@ common --noenable_bzlmod
 build --features=layering_check
 
 common --repo_env=BAZEL_NO_APPLE_CPP_TOOLCHAIN=1
+
+common --enable_platform_specific_config
+
+# Use clang-cl by default on Windows (see https://github.com/protocolbuffers/protobuf/issues/20085).
+build:windows --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl --extra_execution_platforms=//build_defs:x64_windows-clang-cl --host_platform=//build_defs:x64_windows-clang-cl

.github/workflows/test_cpp.yml

@@ -412,7 +412,11 @@ jobs:
             bazel: build --cpu=darwin_arm64 //src/... //third_party/utf8_range/... //conformance:conformance_framework_tests
           - name: Windows Bazel
             os: windows-2022
-            cache_key: windows-2022-bazel7
+            cache_key: windows-2022-msvc-cl
+            bazel: test //src/...  @com_google_protobuf_examples//... --config=msvc-cl --test_tag_filters=-conformance --build_tag_filters=-conformance --define=protobuf_allow_msvc=true
+          - name: Windows Bazel clang-cl
+            os: windows-2022
+            cache_key: windows-2022-clang-cl
             bazel: test //src/...  @com_google_protobuf_examples//... --test_tag_filters=-conformance --build_tag_filters=-conformance
     name: ${{ matrix.continuous-only && inputs.continuous-prefix || '' }} ${{ matrix.name }}
     runs-on: ${{ matrix.os }}
@@ -524,6 +528,7 @@ jobs:
         if: ${{ matrix.install-flags && (!matrix.continuous-only || inputs.continuous-run) }}
         uses: protocolbuffers/protobuf-ci/bash@v4
         with:
+          bazel-version: 7.1.2
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
           command: >-
             cmake . -DCMAKE_CXX_STANDARD=17 ${{ matrix.install-flags }}
@@ -550,6 +555,7 @@ jobs:
         uses: protocolbuffers/protobuf-ci/bash@v4
         with:
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
+          bazel-version: 7.1.2
           command: >-
             cmake . -DCMAKE_CXX_STANDARD=17 ${{ matrix.flags }}
             ${{ env.SCCACHE_CMAKE_FLAGS }} -Dprotobuf_ALLOW_CCACHE=ON

.github/workflows/test_csharp.yml

@@ -74,6 +74,7 @@ jobs:
       - name: Run tests
         uses: protocolbuffers/protobuf-ci/bash@v4
         with:
+          bazel-version: 7.1.2
           credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
           command: |
             dotnet build csharp/src/Google.Protobuf.sln

build_defs/BUILD.bazel

@@ -18,12 +18,36 @@ create_compiler_config_setting(
     value = "msvc-cl",
 )
 
-# Caveat: clang-cl support in protobuf is only best-effort / untested for now.
 create_compiler_config_setting(
     name = "config_clang_cl",
     value = "clang-cl",
 )
 
+platform(
+    name = "x64_windows-clang-cl",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:windows",
+        "@bazel_tools//tools/cpp:clang-cl",
+    ],
+)
+
+platform(
+    name = "x64_windows-msvc-cl",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:windows",
+        "@bazel_tools//tools/cpp:msvc",
+    ],
+)
+
+config_setting(
+    name = "protobuf_allow_msvc",
+    values = {
+        "define": "protobuf_allow_msvc=true",
+    },
+)
+
 selects.config_setting_group(
     name = "config_msvc",
     match_any = [

ci/Windows.bazelrc

@@ -5,3 +5,7 @@ build --cxxopt=/std:c++17 --host_cxxopt=/std:c++17
 startup --output_user_root=C:/tmp --windows_enable_symlinks
 common --enable_runfiles
 
+build --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl
+build:clang-cl --extra_execution_platforms=//build_defs:x64_windows-clang-cl --host_platform=//build_defs:x64_windows-clang-cl
+build:msvc-cl --extra_execution_platforms=//build_defs:x64_windows-msvc-cl --host_platform=//build_defs:x64_windows-msvc-cl
+build --config=clang-cl

examples/.bazelrc

@@ -3,6 +3,7 @@ common --enable_platform_specific_config
 build:linux --cxxopt=-std=c++17 --host_cxxopt=-std=c++17
 build:macos --cxxopt=-std=c++17 --host_cxxopt=-std=c++17
 
+build:windows --extra_toolchains=@local_config_cc//:cc-toolchain-x64_windows-clang-cl --extra_execution_platforms=//:x64_windows-clang-cl
 common:windows --cxxopt=/std:c++17 --host_cxxopt=/std:c++17 --enable_runfiles
 
 build --experimental_remote_cache_eviction_retries=5

examples/BUILD.bazel

@@ -185,3 +185,12 @@ pkg_files(
     strip_prefix = strip_prefix.from_root(""),
     visibility = ["//visibility:public"],
 )
+
+platform(
+    name = "x64_windows-clang-cl",
+    constraint_values = [
+        "@platforms//cpu:x86_64",
+        "@platforms//os:windows",
+        "@bazel_tools//tools/cpp:clang-cl",
+    ],
+)

examples/MODULE.bazel

@@ -7,7 +7,12 @@ local_path_override(
 )
 
 bazel_dep(name = "bazel_skylib", version = "1.7.1")
+bazel_dep(name = "platforms", version = "0.0.8")
 bazel_dep(name = "rules_cc", version = "0.0.17")
 bazel_dep(name = "rules_java", version = "8.6.1")
 bazel_dep(name = "rules_pkg", version = "1.0.1")
 bazel_dep(name = "rules_python", version = "1.0.0")
+
+# For clang-cl configuration
+cc_configure = use_extension("@bazel_tools//tools/cpp:cc_configure.bzl", "cc_configure_extension")
+use_repo(cc_configure, "local_config_cc")

examples/WORKSPACE

@@ -1,5 +1,7 @@
 workspace(name = "com_google_protobuf_examples")
 
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
 # This protobuf repository is required for proto_library rule.
 # It provides the protocol compiler binary (i.e., protoc).
 #
@@ -26,6 +28,16 @@ local_repository(
     path = "..",
 )
 
+# Bazel platform rules, for clang-cl.
+http_archive(
+    name = "platforms",
+    sha256 = "3a561c99e7bdbe9173aa653fd579fe849f1d8d67395780ab4770b1f381431d51",
+    urls = [
+        "https://mirror.bazel.build/github.com/bazelbuild/platforms/releases/download/0.0.7/platforms-0.0.7.tar.gz",
+        "https://github.com/bazelbuild/platforms/releases/download/0.0.7/platforms-0.0.7.tar.gz",
+    ],
+)
+
 load("@com_google_protobuf//:protobuf_deps.bzl", "protobuf_deps")
 
 protobuf_deps()

src/google/protobuf/stubs/BUILD.bazel

@@ -22,6 +22,12 @@ cc_library(
         "status_macros.h",
     ],
     copts = COPTS,
+    defines = ["GOOGLE_PROTOBUF_USING_BAZEL=1"] + select({
+        "//build_defs:protobuf_allow_msvc": [
+            "GOOGLE_PROTOBUF_MSVC_BAZEL_OVERRIDE=1",
+        ],
+        "//conditions:default": [],
+    }),
     linkopts = LINK_OPTS,
     strip_include_prefix = "/src",
     deps = [

src/google/protobuf/stubs/port.h

@@ -31,6 +31,13 @@
 #include <byteswap.h>  // IWYU pragma: export
 #endif
 
+#if defined(_MSC_VER) && !defined(__clang__) && \
+    defined(GOOGLE_PROTOBUF_USING_BAZEL) &&     \
+    !defined(GOOGLE_PROTOBUF_MSVC_BAZEL_OVERRIDE)
+#error \
+    "Protobuf will be dropping support for MSVC + Bazel in 34.0.  To continue using it until then, use the flag --define=protobuf_allow_msvc=true.  For feedback or discussion, see github.com/protocolbuffers/protobuf/issues/20085."
+#endif
+
 // Legacy: some users reference these (internal-only) macros even though we
 // don't need them any more.
 #if defined(_MSC_VER) && defined(PROTOBUF_USE_DLLS)