[1] 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.
- Extend the legacy compile features (compiler_input_flags,
  compiler_output_flags, dependency_file, user_compile_flags) to the C++20
  module actions, so the generic -c / -o / -MD -MF / user-copt flags are
  emitted for module interface compile, codegen and dependency scanning
  the same way they are for ordinary compiles. Toolchains that use these
  features then need no module-specific source/output/dependency/copt args
  of their own.
- 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.

The C++20 modules pipeline lives only in rules_cc's compile.bzl (Bazel's
native rules do not implement it), and was previously untested here:
Bazel's CppModulesConfiguredTargetTest only checks the
--experimental_cpp_modules / cpp_modules gating, and the shell
integration test self-skips unless a host clang >= 17 is present. Add an
analysis test that drives the module interface compile + dependency
scanning actions with a mock toolchain (no real compiler needed) and
asserts the scanning action emits both the P1689 .ddi file and the
Make-style .ddi.d dependency file. This test fails without the fixes in
the preceding commit.

Author: fmeum

cc/toolchains/actions/BUILD

@@ -272,6 +272,15 @@ cc_action_type_set(
     ],
 )
 
+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/args/compile_flags/BUILD

@@ -37,7 +37,10 @@ cc_feature(
 
 cc_args(
     name = "user_compile_flags",
-    actions = ["//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     args = ["{user_compile_flags}"],
     format = {"user_compile_flags": "//cc/toolchains/variables:user_compile_flags"},
     iterate_over = "//cc/toolchains/variables:user_compile_flags",

cc/toolchains/args/compiler_input_flags/BUILD

@@ -13,7 +13,10 @@ cc_feature(
 
 cc_args(
     name = "flags",
-    actions = ["//cc/toolchains/actions:compile_actions_without_header_parsing"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions_without_header_parsing",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     args = [
         "-c",
         "{source_file}",

cc/toolchains/args/compiler_output_flags/BUILD

@@ -34,7 +34,10 @@ cc_args(
 
 cc_args(
     name = "output_flags",
-    actions = ["//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     args = [
         "-o",
         "{output_file}",

cc/toolchains/args/dependency_file/BUILD

@@ -10,7 +10,10 @@ cc_feature(
 
 cc_args(
     name = "flags",
-    actions = ["//cc/toolchains/actions:compile_actions"],
+    actions = [
+        "//cc/toolchains/actions:compile_actions",
+        "//cc/toolchains/actions:cpp20_module_actions",
+    ],
     args = [
         "-MD",
         "-MF",

cc/toolchains/legacy_file_group.bzl

@@ -28,6 +28,9 @@ LEGACY_FILE_GROUPS = {
         Label("//cc/toolchains/actions:c_compile"),
         Label("//cc/toolchains/actions:cpp_compile"),
         Label("//cc/toolchains/actions:cpp_header_parsing"),
+        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),
 )
 
@@ -424,6 +443,7 @@ 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),
@@ -485,7 +505,10 @@ 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),
 )
 
@@ -566,7 +589,10 @@ 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)),
 )
 
@@ -581,6 +607,8 @@ 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",