Implement proper handling for use_libcpp with bzlmod enabled

Author: martin4861

MODULE.bazel.lock

@@ -206,6 +206,15 @@ config_setting(
     visibility = ["//visibility:public"],
 )
 
+config_setting(
+    name = "_enable_bzlmod_not_libcpp",
+    flag_values = {
+        ":enable_bzlmod": "true",
+        ":use_libcpp": "false",
+    },
+    visibility = ["//visibility:public"],
+)
+
 py_binary(
     name = "fix_include_guards",
     srcs = [

cc/BUILD.bazel

@@ -117,8 +117,8 @@ cc_toolchain_config(
     abi_version = "clang",
     builtin_sysroot = select({
         "@rules_swiftnav//cc:_use_libcpp": None,
-        # Remove once bzlmod is enabled by default
-        "@rules_swiftnav//cc:_enable_bzlmod": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
+        # When bzlmod is enabled but libcpp is not, use the bzlmod path
+        "@rules_swiftnav//cc:_enable_bzlmod_not_libcpp": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
         "//conditions:default": "external/aarch64-sysroot",
     }),
     compiler = "clang",

cc/toolchains/llvm/aarch64-linux/BUILD.bazel

@@ -103,8 +103,8 @@ cc_toolchain_config(
     abi_version = "clang",
     builtin_sysroot = select({
         "@rules_swiftnav//cc:_use_libcpp": None,
-        # Remove once bzlmod is enabled by default
-        "@rules_swiftnav//cc:_enable_bzlmod": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
+        # When bzlmod is enabled but libcpp is not, use the bzlmod path
+        "@rules_swiftnav//cc:_enable_bzlmod_not_libcpp": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
         "//conditions:default": "external/aarch64-sysroot",
     }),
     compiler = "clang",

cc/toolchains/llvm/x86_64-aarch64-linux/BUILD.bazel

@@ -117,8 +117,8 @@ cc_toolchain_config(
     abi_version = "clang",
     builtin_sysroot = select({
         "@rules_swiftnav//cc:_use_libcpp": None,
-        # Remove once bzlmod is enabled by default
-        "@rules_swiftnav//cc:_enable_bzlmod": "external/rules_swiftnav++swift_cc_toolchain_extension+x86_64-sysroot",
+        # When bzlmod is enabled but libcpp is not, use the bzlmod path
+        "@rules_swiftnav//cc:_enable_bzlmod_not_libcpp": "external/rules_swiftnav++swift_cc_toolchain_extension+x86_64-sysroot",
         "//conditions:default": "external/x86_64-sysroot",
     }),
     compiler = "clang",

cc/toolchains/llvm/x86_64-linux/BUILD.bazel

@@ -117,8 +117,8 @@ cc_toolchain_config(
     abi_version = "clang",
     builtin_sysroot = select({
         "@rules_swiftnav//cc:_use_libcpp": None,
-        # Remove once bzlmod is enabled by default
-        "@rules_swiftnav//cc:_enable_bzlmod": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
+        # When bzlmod is enabled but libcpp is not, use the bzlmod path
+        "@rules_swiftnav//cc:_enable_bzlmod_not_libcpp": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
         "//conditions:default": "external/aarch64-sysroot",
     }),
     compiler = "clang",

cc/toolchains/llvm20/aarch64-linux/BUILD.bazel

@@ -102,8 +102,9 @@ cc_toolchain_config(
     abi_libc_version = "glibc_unknown",
     abi_version = "clang",
     builtin_sysroot = select({
-        # Remove once bzlmod is enabled by default
-        "@rules_swiftnav//cc:_enable_bzlmod": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
+        # _use_libcpp case needs to be implemented properly here!
+        # When bzlmod is enabled but libcpp is not, use the bzlmod path
+        "@rules_swiftnav//cc:_enable_bzlmod_not_libcpp": "external/rules_swiftnav++swift_cc_toolchain_extension+aarch64-sysroot",
         "//conditions:default": "external/aarch64-sysroot",
     }),
     compiler = "clang",

cc/toolchains/llvm20/x86_64-aarch64-linux/BUILD.bazel

@@ -117,8 +117,8 @@ cc_toolchain_config(
     abi_version = "clang",
     builtin_sysroot = select({
         "@rules_swiftnav//cc:_use_libcpp": None,
-        # Remove once bzlmod is enabled by default
-        "@rules_swiftnav//cc:_enable_bzlmod": "external/rules_swiftnav++swift_cc_toolchain_extension+x86_64-sysroot",
+        # When bzlmod is enabled but libcpp is not, use the bzlmod path
+        "@rules_swiftnav//cc:_enable_bzlmod_not_libcpp": "external/rules_swiftnav++swift_cc_toolchain_extension+x86_64-sysroot",
         "//conditions:default": "external/x86_64-sysroot",
     }),
     compiler = "clang",