Let the rules-based cc_toolchain be driven by a non-Xcode clang

test/layering_check/BUILD

@@ -1,5 +1,6 @@
 load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test", "objc_library")
+load("//test/rules:action_command_line_test.bzl", "action_command_line_test")
 
 package(features = ["layering_check"])
 
@@ -37,6 +38,74 @@ cc_test(
     ],
 )
 
+# From Clang 14 on, -std=c++20 switches the driver to C++20 modules
+# semantics: including a header a module map covers *non-textually* becomes
+# "module 'cxx20_modular' is needed but has not been provided" -- unless the
+# toolchain passes -fno-cxx-modules (//toolchain:no_cxx_modules_flags, gated
+# on use_module_maps). wrapped_clang's generated map and the per-target maps
+# Bazel writes are all-textual and immune, but the system module map of a
+# hermetic toolchain driving this configuration is not.
+cc_library(
+    name = "cxx20_non_textual_module_map",
+    srcs = ["modular.cpp"],
+    hdrs = ["modular.h"],
+    additional_compiler_inputs = ["cxx20.modulemap"],
+    copts = [
+        "-std=c++20",
+        "-fmodule-map-file=$(execpath cxx20.modulemap)",
+    ],
+    # use_module_maps alone, without layering_check: strict-decluse would
+    # (correctly) reject the include, since nothing declares a dependency on
+    # the map's module. The fix is gated on use_module_maps, not on
+    # layering_check itself.
+    features = [
+        "-layering_check",
+        "use_module_maps",
+    ],
+)
+
+build_test(
+    name = "cxx20_non_textual_module_map_test",
+    targets = [":cxx20_non_textual_module_map"],
+)
+
+# The flag is emitted exactly when use_module_maps is enabled: by itself, by
+# way of layering_check, and never without. Note the build test above cannot
+# fail under Xcode's clang, which does not switch to C++20 modules semantics
+# the way upstream Clang 14+ does -- these assert the flag so the fix is
+# visible under the tools this repo tests with.
+action_command_line_test(
+    name = "no_cxx_modules_flags_from_module_maps_test",
+    expected_argv = [
+        "-Xclang",
+        "-fno-cxx-modules",
+        "-Wno-module-import-in-extern-c",
+    ],
+    mnemonic = "CppCompile",
+    tags = ["requires_rules_based_toolchain"],
+    target_under_test = ":cxx20_non_textual_module_map",
+)
+
+action_command_line_test(
+    name = "no_cxx_modules_flags_from_layering_check_test",
+    expected_argv = [
+        "-Xclang",
+        "-fno-cxx-modules",
+        "-Wno-module-import-in-extern-c",
+    ],
+    mnemonic = "CppCompile",
+    tags = ["requires_rules_based_toolchain"],
+    target_under_test = ":good_layering_check",
+)
+
+action_command_line_test(
+    name = "no_cxx_modules_flags_off_without_module_maps_test",
+    mnemonic = "CppCompile",
+    not_expected_argv = ["-fno-cxx-modules"],
+    tags = ["requires_rules_based_toolchain"],
+    target_under_test = ":disabled_bad_layering_check",
+)
+
 objc_library(
     name = "bad_layering_check_objc",
     srcs = ["c.m"],

test/layering_check/cxx20.modulemap

@@ -0,0 +1,7 @@
+// A module map whose header is *not* textual, like the umbrella-directory
+// system module maps a hermetic toolchain generates for its own headers --
+// unlike the all-textual maps wrapped_clang injects and Bazel writes per
+// target, which C++20 modules semantics leave alone.
+module "cxx20_modular" [system] {
+  header "modular.h"
+}

test/layering_check/modular.cpp

@@ -0,0 +1,3 @@
+#include "modular.h"
+
+int uses_modular() { return from_modular_header(); }

test/layering_check/modular.h

@@ -0,0 +1,6 @@
+#ifndef TEST_LAYERING_CHECK_MODULAR_H_
+#define TEST_LAYERING_CHECK_MODULAR_H_
+
+inline int from_modular_header() { return 20; }
+
+#endif  // TEST_LAYERING_CHECK_MODULAR_H_

test/test_data/toolchain_configs/darwin_arm64.json

@@ -1006,6 +1006,45 @@
           "type_name": "flag_set",
           "with_features": []
         },
+        {
+          "actions": [
+            "c++-compile",
+            "c++-header-parsing",
+            "c++-module-codegen",
+            "c++-module-compile",
+            "clif-match",
+            "linkstamp-compile",
+            "lto-backend",
+            "objc++-compile"
+          ],
+          "flag_groups": [
+            {
+              "expand_if_available": null,
+              "expand_if_equal": null,
+              "expand_if_false": null,
+              "expand_if_not_available": null,
+              "expand_if_true": null,
+              "flag_groups": [],
+              "flags": [
+                "-Xclang",
+                "-fno-cxx-modules",
+                "-Wno-module-import-in-extern-c"
+              ],
+              "iterate_over": null,
+              "type_name": "flag_group"
+            }
+          ],
+          "type_name": "flag_set",
+          "with_features": [
+            {
+              "features": [
+                "use_module_maps"
+              ],
+              "not_features": [],
+              "type_name": "with_feature_set"
+            }
+          ]
+        },
         {
           "actions": [
             "assemble",

test/test_data/toolchain_configs/darwin_arm64e.json

@@ -1006,6 +1006,45 @@
           "type_name": "flag_set",
           "with_features": []
         },
+        {
+          "actions": [
+            "c++-compile",
+            "c++-header-parsing",
+            "c++-module-codegen",
+            "c++-module-compile",
+            "clif-match",
+            "linkstamp-compile",
+            "lto-backend",
+            "objc++-compile"
+          ],
+          "flag_groups": [
+            {
+              "expand_if_available": null,
+              "expand_if_equal": null,
+              "expand_if_false": null,
+              "expand_if_not_available": null,
+              "expand_if_true": null,
+              "flag_groups": [],
+              "flags": [
+                "-Xclang",
+                "-fno-cxx-modules",
+                "-Wno-module-import-in-extern-c"
+              ],
+              "iterate_over": null,
+              "type_name": "flag_group"
+            }
+          ],
+          "type_name": "flag_set",
+          "with_features": [
+            {
+              "features": [
+                "use_module_maps"
+              ],
+              "not_features": [],
+              "type_name": "with_feature_set"
+            }
+          ]
+        },
         {
           "actions": [
             "assemble",

test/test_data/toolchain_configs/darwin_x86_64.json

@@ -1006,6 +1006,45 @@
           "type_name": "flag_set",
           "with_features": []
         },
+        {
+          "actions": [
+            "c++-compile",
+            "c++-header-parsing",
+            "c++-module-codegen",
+            "c++-module-compile",
+            "clif-match",
+            "linkstamp-compile",
+            "lto-backend",
+            "objc++-compile"
+          ],
+          "flag_groups": [
+            {
+              "expand_if_available": null,
+              "expand_if_equal": null,
+              "expand_if_false": null,
+              "expand_if_not_available": null,
+              "expand_if_true": null,
+              "flag_groups": [],
+              "flags": [
+                "-Xclang",
+                "-fno-cxx-modules",
+                "-Wno-module-import-in-extern-c"
+              ],
+              "iterate_over": null,
+              "type_name": "flag_group"
+            }
+          ],
+          "type_name": "flag_set",
+          "with_features": [
+            {
+              "features": [
+                "use_module_maps"
+              ],
+              "not_features": [],
+              "type_name": "with_feature_set"
+            }
+          ]
+        },
         {
           "actions": [
             "assemble",

test/test_data/toolchain_configs/ios_arm64.json

@@ -1006,6 +1006,45 @@
           "type_name": "flag_set",
           "with_features": []
         },
+        {
+          "actions": [
+            "c++-compile",
+            "c++-header-parsing",
+            "c++-module-codegen",
+            "c++-module-compile",
+            "clif-match",
+            "linkstamp-compile",
+            "lto-backend",
+            "objc++-compile"
+          ],
+          "flag_groups": [
+            {
+              "expand_if_available": null,
+              "expand_if_equal": null,
+              "expand_if_false": null,
+              "expand_if_not_available": null,
+              "expand_if_true": null,
+              "flag_groups": [],
+              "flags": [
+                "-Xclang",
+                "-fno-cxx-modules",
+                "-Wno-module-import-in-extern-c"
+              ],
+              "iterate_over": null,
+              "type_name": "flag_group"
+            }
+          ],
+          "type_name": "flag_set",
+          "with_features": [
+            {
+              "features": [
+                "use_module_maps"
+              ],
+              "not_features": [],
+              "type_name": "with_feature_set"
+            }
+          ]
+        },
         {
           "actions": [
             "assemble",

test/test_data/toolchain_configs/ios_arm64e.json

@@ -1006,6 +1006,45 @@
           "type_name": "flag_set",
           "with_features": []
         },
+        {
+          "actions": [
+            "c++-compile",
+            "c++-header-parsing",
+            "c++-module-codegen",
+            "c++-module-compile",
+            "clif-match",
+            "linkstamp-compile",
+            "lto-backend",
+            "objc++-compile"
+          ],
+          "flag_groups": [
+            {
+              "expand_if_available": null,
+              "expand_if_equal": null,
+              "expand_if_false": null,
+              "expand_if_not_available": null,
+              "expand_if_true": null,
+              "flag_groups": [],
+              "flags": [
+                "-Xclang",
+                "-fno-cxx-modules",
+                "-Wno-module-import-in-extern-c"
+              ],
+              "iterate_over": null,
+              "type_name": "flag_group"
+            }
+          ],
+          "type_name": "flag_set",
+          "with_features": [
+            {
+              "features": [
+                "use_module_maps"
+              ],
+              "not_features": [],
+              "type_name": "with_feature_set"
+            }
+          ]
+        },
         {
           "actions": [
             "assemble",

test/test_data/toolchain_configs/ios_sim_arm64.json

@@ -1006,6 +1006,45 @@
           "type_name": "flag_set",
           "with_features": []
         },
+        {
+          "actions": [
+            "c++-compile",
+            "c++-header-parsing",
+            "c++-module-codegen",
+            "c++-module-compile",
+            "clif-match",
+            "linkstamp-compile",
+            "lto-backend",
+            "objc++-compile"
+          ],
+          "flag_groups": [
+            {
+              "expand_if_available": null,
+              "expand_if_equal": null,
+              "expand_if_false": null,
+              "expand_if_not_available": null,
+              "expand_if_true": null,
+              "flag_groups": [],
+              "flags": [
+                "-Xclang",
+                "-fno-cxx-modules",
+                "-Wno-module-import-in-extern-c"
+              ],
+              "iterate_over": null,
+              "type_name": "flag_group"
+            }
+          ],
+          "type_name": "flag_set",
+          "with_features": [
+            {
+              "features": [
+                "use_module_maps"
+              ],
+              "not_features": [],
+              "type_name": "with_feature_set"
+            }
+          ]
+        },
         {
           "actions": [
             "assemble",