Support C++20 modules in the rules-based toolchain API

The rules-based cc/toolchains toolchain API was missing several pieces
needed to drive Bazel's C++20 modules pipeline (clang-scan-deps -> .ddi
-> aggregate-ddi -> generate-modmap -> module compile):

- Add a cpp20_module_actions action type set grouping the module
  interface compile, codegen and dependency scanning actions. These are
  deliberately kept out of compile_actions/source_compile_actions so that
  flags incompatible with C++20 modules can keep targeting only the
  non-module compiles.
- Register the cpp_module_output_file and cpp_module_modmap_file build
  variables and make dependency_file, output_file, source_file and
  user_compile_flags available to the C++20 module actions.
- Include the C++20 module actions in the compiler_files legacy file
  group so their tools (e.g. the dependency scanner) are available to the
  compile actions.
- Fix _create_scan_deps_action to pass needs_include_validation and to
  not request a .d dotd file (the scanner emits a P1689 .ddi file), and
  let _create_compile_action forward the C++20 module output/modmap
  arguments to the underlying action.

Author: fmeum

cc/private/compile/compile.bzl

@@ -527,21 +527,9 @@ def _create_scan_deps_action(
         ddi_file,
         ddi_output_name,
         progress_message_prefix):
+    # The dependency scanner (e.g. clang-scan-deps) emits the P1689 .ddi file,
+    # not a Make-style .d dependency file, so no dotd file is requested here.
     dotd_file = None
-    if (
-        dotd_files_enabled(language, cpp_configuration, feature_configuration) and
-        _use_dotd_file(feature_configuration, source_artifact)
-    ):
-        dotd_file = _get_compile_output_file(
-            action_construction_context,
-            label,
-            configuration = configuration,
-            output_name = _cc_internal.get_artifact_name_for_category(
-                cc_toolchain = cc_toolchain,
-                category = artifact_category.INCLUDED_FILE_LIST,
-                output_name = ddi_output_name,
-            ),
-        )
     specific_compile_build_variables = get_specific_compile_build_variables(
         feature_configuration,
         use_pic = use_pic,
@@ -579,6 +567,7 @@ def _create_scan_deps_action(
         compile_build_variables = compile_variables,
         action_name = ACTION_NAMES.cpp_module_deps_scanning,
         toolchain_type = _starlark_cc_semantics.toolchain,
+        needs_include_validation = _starlark_cc_semantics.needs_include_validation(language),
         progress_message_prefix = progress_message_prefix,
     )
 
@@ -2319,11 +2308,24 @@ def _create_compile_action(
         progress_message_prefix = None,
         source = None,
         toolchain_type = None,
-        use_pic = False):
+        use_pic = False,
+        additional_outputs = [],
+        module_files = None,
+        modmap_file = None,
+        modmap_input_file = None):
     # TODO(bgorshenev): use bazel_features checks
     version_dependent_kwargs = {}
     if progress_message_prefix:
         version_dependent_kwargs["progress_message_prefix"] = progress_message_prefix
+
+    # Provide the C++20 module args conditionally as they require a recent
+    # version of Bazel.
+    if modmap_file:
+        version_dependent_kwargs["additional_outputs"] = additional_outputs
+        version_dependent_kwargs["module_files"] = module_files
+        version_dependent_kwargs["modmap_file"] = modmap_file
+        version_dependent_kwargs["modmap_input_file"] = modmap_input_file
+
     return _cc_internal.create_cc_compile_action(
         action_construction_context = action_construction_context,
         action_name = action_name,

cc/toolchains/actions/BUILD

@@ -272,6 +272,31 @@ cc_action_type_set(
     ],
 )
 
+# The C++20 module actions: module interface compilation, module codegen and
+# module dependency scanning. These are deliberately kept out of
+# `compile_actions`/`source_compile_actions` so that flags incompatible with
+# C++20 modules can keep targeting only the non-module compiles. Toolchains that
+# support C++20 modules typically combine this set with `compile_actions` or
+# `source_compile_actions` on the relevant `cc_args`, e.g.
+#
+#     actions = [
+#         "//cc/toolchains/actions:compile_actions",
+#         "//cc/toolchains/actions:cpp20_module_actions",
+#     ]
+#
+# The module interface compilation and codegen actions are ordinary C++ compiles
+# and the dependency scanning action reuses the regular compiler command line,
+# so toolchain flags that apply to compiles (target, sysroot, include search
+# paths, ...) generally need to apply to these actions as well.
+cc_action_type_set(
+    name = "cpp20_module_actions",
+    actions = [
+        ":cpp20_module_compile",
+        ":cpp20_module_codegen",
+        ":cpp_module_deps_scanning",
+    ],
+)
+
 cc_action_type_set(
     name = "compile_actions_without_header_parsing",
     actions = [

cc/toolchains/legacy_file_group.bzl

@@ -28,6 +28,11 @@ LEGACY_FILE_GROUPS = {
         Label("//cc/toolchains/actions:c_compile"),
         Label("//cc/toolchains/actions:cpp_compile"),
         Label("//cc/toolchains/actions:cpp_header_parsing"),
+        # C++20 module actions: their tools (e.g. the dependency scanner) must
+        # be part of compiler_files so they are available to compile actions.
+        Label("//cc/toolchains/actions:cpp20_module_compile"),
+        Label("//cc/toolchains/actions:cpp20_module_codegen"),
+        Label("//cc/toolchains/actions:cpp_module_deps_scanning"),
     ],
     # There are no actions listed for coverage and objcopy in action_names.bzl.
     "coverage_files": [],

cc/toolchains/variables/BUILD

@@ -2,6 +2,22 @@ load("//cc/toolchains/impl:variables.bzl", "cc_builtin_variables", "cc_variable"
 
 package(default_visibility = ["//visibility:public"])
 
+cc_variable(
+    name = "cpp_module_modmap_file",
+    actions = [
+        "//cc/toolchains/actions:cpp_compile",
+        "//cc/toolchains/actions:cpp20_module_compile",
+        "//cc/toolchains/actions:cpp20_module_codegen",
+    ],
+    type = types.option(types.file),
+)
+
+cc_variable(
+    name = "cpp_module_output_file",
+    actions = ["//cc/toolchains/actions:cpp20_module_compile"],
+    type = types.option(types.file),
+)
+
 cc_variable(
     name = "cs_fdo_instrument_path",
     actions = [
@@ -19,7 +35,10 @@ cc_variable(
 
 cc_variable(
     name = "dependency_file",
-    actions = ["//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     type = types.option(types.file),
 )
 
@@ -412,6 +431,7 @@ cc_variable(
     name = "output_file",
     actions = [
         "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
         "//cc/toolchains/actions:strip",
     ],
     type = types.option(types.file),
@@ -473,7 +493,10 @@ cc_variable(
 
 cc_variable(
     name = "source_file",
-    actions = ["//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     type = types.option(types.file),
 )
 
@@ -554,7 +577,10 @@ cc_variable(
 
 cc_variable(
     name = "user_compile_flags",
-    actions = ["//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     type = types.option(types.list(types.string)),
 )
 
@@ -569,6 +595,8 @@ cc_builtin_variables(
     srcs = [
         ":attr_linkopts",
         ":cc_library_exec_paths",
+        ":cpp_module_modmap_file",
+        ":cpp_module_output_file",
         ":cs_fdo_instrument_path",
         ":def_file_path",
         ":dep_linkopts",