introduce a new flag --experimental_cpp20_modules

src/main/java/com/google/devtools/build/lib/rules/cpp/CppConfiguration.java

@@ -880,10 +880,24 @@ public boolean experimentalCcImplementationDepsForStarlark(StarlarkThread thread
     return experimentalCcImplementationDeps();
   }
 
+  @StarlarkMethod(
+      name = "experimental_cpp20_modules",
+      documented = false,
+      useStarlarkThread = true)
+  public boolean experimentalCpp20ModulesForStarlark(StarlarkThread thread)
+      throws EvalException {
+    CcModule.checkPrivateStarlarkificationAllowlist(thread);
+    return experimentalCpp20Modules();
+  }
+
   public boolean experimentalCcImplementationDeps() {
     return cppOptions.experimentalCcImplementationDeps;
   }
 
+  public boolean experimentalCpp20Modules() {
+    return cppOptions.experimentalCpp20Modules;
+  }
+
   public boolean getExperimentalCppCompileResourcesEstimation() {
     return cppOptions.experimentalCppCompileResourcesEstimation;
   }

src/main/java/com/google/devtools/build/lib/rules/cpp/CppOptions.java

@@ -1038,6 +1038,19 @@ public Label getMemProfProfileLabel() {
       help = "If enabled, cc_library targets can use attribute `implementation_deps`.")
   public boolean experimentalCcImplementationDeps;
 
+  @Option(
+      name = "experimental_cpp20_modules",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.UNDOCUMENTED,
+      effectTags = {
+        OptionEffectTag.LOADING_AND_ANALYSIS,
+        OptionEffectTag.EXECUTION,
+        OptionEffectTag.CHANGES_INPUTS
+      },
+      metadataTags = {OptionMetadataTag.EXPERIMENTAL},
+      help = "If enabled, cc_binary and cc_library targets can use attribute `module_interfaces`.")
+  public boolean experimentalCpp20Modules;
+
   @Option(
       name = "experimental_link_static_libraries_once",
       defaultValue = "true",

src/main/starlark/builtins_bzl/common/builtin_exec_platforms.bzl

@@ -278,6 +278,7 @@ bazel_fragments["CppOptions"] = fragment(
         "//command_line_option:target libcTop label",
         "//command_line_option:experimental_link_static_libraries_once",
         "//command_line_option:experimental_cc_implementation_deps",
+        "//command_line_option:experimental_cpp20_modules",
         "//command_line_option:start_end_lib",
         "//command_line_option:experimental_inmemory_dotd_files",
         "//command_line_option:incompatible_disable_legacy_cc_provider",

src/main/starlark/builtins_bzl/common/cc/attrs.bzl

@@ -97,6 +97,8 @@ C++ Standard has no restriction about module interface file extension
 <li>MSVC use ixx </li>
 </ul>
 </p>
+<p>For now usage is guarded by the flag
+<code>--experimental_cpp20_modules</code>.</p>
         """,
     ),
     "data": attr.label_list(

src/main/starlark/builtins_bzl/common/cc/cc_binary.bzl

@@ -487,7 +487,8 @@ def cc_binary_impl(ctx, additional_linkopts, force_linkstatic = False):
         unsupported_features = disabled_features,
     )
 
-    cc_helper.check_cpp20_module(ctx, feature_configuration)
+    semantics.check_can_module_interfaces(ctx)
+    cc_helper.check_cpp20_modules(ctx, feature_configuration)
 
     all_deps = ctx.attr.deps + semantics.get_cc_runtimes(ctx, _is_link_shared(ctx))
     compilation_context_deps = [dep[CcInfo].compilation_context for dep in all_deps if CcInfo in dep]

src/main/starlark/builtins_bzl/common/cc/cc_helper.bzl

@@ -955,7 +955,6 @@ def _map_to_list(m):
         result.append((k, v))
     return result
 
-
 # "srcs" attribute is a LABEL_LIST in cc_rules, which might also contain files.
 def _calculate_artifact_label_map(srcs, attr):
     artifact_label_map = {}
@@ -971,6 +970,7 @@ def _calculate_artifact_label_map(srcs, attr):
                             attr = attr,
                         )
     return artifact_label_map
+
 # Returns a list of (Artifact, Label) tuples. Each tuple represents an input source
 # file and the label of the rule that generates it (or the label of the source file itself if it
 # is an input file).
@@ -1216,7 +1216,8 @@ def _should_use_pic(ctx, cc_toolchain, feature_configuration):
             cc_common.is_enabled(feature_configuration = feature_configuration, feature_name = "prefer_pic_for_opt_binaries")
         )
     )
-def _check_cpp20_module(ctx, feature_configuration):
+
+def _check_cpp20_modules(ctx, feature_configuration):
     if len(ctx.files.module_interfaces) > 0 and not cc_common.is_enabled(
         feature_configuration = feature_configuration,
         feature_name = "cpp20_modules",
@@ -1290,5 +1291,5 @@ cc_helper = struct(
     package_source_root = _package_source_root,
     tokenize = _tokenize,
     should_use_pic = _should_use_pic,
-    check_cpp20_module = _check_cpp20_module,
+    check_cpp20_modules = _check_cpp20_modules,
 )

src/main/starlark/builtins_bzl/common/cc/cc_library.bzl

@@ -37,7 +37,8 @@ def _cc_library_impl(ctx):
         unsupported_features = ctx.disabled_features,
     )
 
-    cc_helper.check_cpp20_module(ctx, feature_configuration)
+    semantics.check_can_module_interfaces(ctx)
+    cc_helper.check_cpp20_modules(ctx, feature_configuration)
 
     precompiled_files = cc_helper.build_precompiled_files(ctx = ctx)
 

src/main/starlark/builtins_bzl/common/cc/semantics.bzl

@@ -117,6 +117,11 @@ def _check_can_use_implementation_deps(ctx):
     if (not experimental_cc_implementation_deps and ctx.attr.implementation_deps):
         fail("requires --experimental_cc_implementation_deps", attr = "implementation_deps")
 
+def _check_can_module_interfaces(ctx):
+    experimental_cpp20_modules = ctx.fragments.cpp.experimental_cpp20_modules()
+    if (not experimental_cpp20_modules and ctx.attr.module_interfaces):
+        fail("requires --experimental_cpp20_modules", attr = "module_interfaces")
+
 _WINDOWS_PLATFORM = Label("@platforms//os:windows")  # Resolve the label within builtins context
 
 def _get_linkstatic_default_for_test():
@@ -161,6 +166,7 @@ semantics = struct(
     get_grep_includes = _get_grep_includes,
     get_implementation_deps_allowed_attr = _get_implementation_deps_allowed_attr,
     check_can_use_implementation_deps = _check_can_use_implementation_deps,
+    check_can_module_interfaces = _check_can_module_interfaces,
     get_linkstatic_default_for_test = _get_linkstatic_default_for_test,
     get_runtimes_toolchain = _get_runtimes_toolchain,
     get_test_malloc_attr = _get_test_malloc_attr,