Normalize Clang forwarded system header paths

cargo/private/cargo_build_script.bzl

@@ -139,11 +139,11 @@ def _prefix_pwd_to_flag(args, flag_variations):
     res = []
     prefix_next_arg = False
     for arg in args:
-        # When expecting a path argument and we see -Xclang, skip over it and
+        # When expecting a path argument, skip Clang forwarding tokens and
         # map the next argument instead.
         # ex: -Xclang -internal-isystem -Xclang path
         # to: -Xclang -internal-isystem -Xclang ${pwd}/path
-        if prefix_next_arg and arg == "-Xclang":
+        if prefix_next_arg and arg in ("-Xclang", "-Xpreprocessor"):
             res.append(arg)
             continue
 
@@ -217,7 +217,7 @@ def _pwd_flags_fsanitize_ignorelist(args):
 
 def _pwd_flags_isystem(args):
     """Prefix execroot-relative paths in -isystem-like arguments with ${pwd}."""
-    return _prefix_pwd_to_flag(args, ["-isystem", "-isystem-after", "-internal-isystem", "-cxx-isystem", "-stdlib++-isystem", "/imsvc"])
+    return _prefix_pwd_to_flag(args, ["-isystem", "-isystem-after", "-internal-isystem", "-internal-externc-isystem", "-cxx-isystem", "-stdlib++-isystem", "/imsvc"])
 
 def _pwd_flags_L(args):
     """Prefix execroot-relative paths in -L arguments with ${pwd}."""

cargo/tests/cargo_build_script/cc_args_and_env/BUILD.bazel

@@ -8,6 +8,7 @@ load(
     "direct_libs_absolute_test",
     "direct_libs_as_flag_operand_test",
     "direct_libs_relative_test",
+    "forwarded_header_paths_test",
     "fsanitize_ignorelist_absolute_test",
     "fsanitize_ignorelist_relative_test",
     "imacros_absolute_test",
@@ -91,3 +92,5 @@ direct_libs_relative_test(name = "direct_libs_relative_test")
 direct_libs_absolute_test(name = "direct_libs_absolute_test")
 
 direct_libs_as_flag_operand_test(name = "direct_libs_as_flag_operand_test")
+
+forwarded_header_paths_test(name = "forwarded_header_paths_test")

cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl

@@ -771,3 +771,26 @@ def direct_libs_as_flag_operand_test(name):
         target_under_test = "%s/cargo_build_script" % name,
         expected_cflags = ["-imacros", "${pwd}/test/relative/libfoo.a", "-B", "${pwd}/test/relative/obj.o"],
     )
+
+def forwarded_header_paths_test(name):
+    """Tests path rewriting through Clang forwarding options.
+
+    Args:
+        name: Prefix for the generated analysis tests.
+    """
+    for forwarding in ["-Xclang", "-Xpreprocessor"]:
+        for category in ["-internal-isystem", "-internal-externc-isystem"]:
+            for kind, path, expected in [
+                ("relative", "test/relative/include", "${pwd}/test/relative/include"),
+                ("absolute", "/test/absolute/include", "/test/absolute/include"),
+            ]:
+                case = name + forwarding + category + "_" + kind
+                cargo_build_script_with_extra_cc_compile_flags(
+                    name = case + "/cargo_build_script",
+                    extra_cc_compile_flags = [forwarding, category, forwarding, path, "-DKEEP=1"],
+                )
+                cc_args_and_env_analysis_test(
+                    name = case,
+                    target_under_test = case + "/cargo_build_script",
+                    expected_cflags = [forwarding, category, forwarding, expected, "-DKEEP=1"],
+                )