From 0e26117868b7dcb926974d4613dca781678792cf Mon Sep 17 00:00:00 2001
From: Corentin Kerisit <corentin.kerisit@gmail.com>
Date: Thu, 8 Oct 2026 13:22:45 +0200
Subject: [PATCH 1/2] Normalize Clang forwarded system header paths

---
 cargo/private/cargo_build_script.bzl           |  6 +++---
 .../cc_args_and_env/BUILD.bazel                |  3 +++
 .../cc_args_and_env/cc_args_and_env_test.bzl   | 18 ++++++++++++++++++
 3 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/cargo/private/cargo_build_script.bzl b/cargo/private/cargo_build_script.bzl
index 730ec65f43..cb9f0c518c 100644
--- a/cargo/private/cargo_build_script.bzl
+++ b/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}."""
diff --git a/cargo/tests/cargo_build_script/cc_args_and_env/BUILD.bazel b/cargo/tests/cargo_build_script/cc_args_and_env/BUILD.bazel
index a2e39c9855..8966f22194 100644
--- a/cargo/tests/cargo_build_script/cc_args_and_env/BUILD.bazel
+++ b/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")
diff --git a/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl b/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl
index 2f492ea49d..18366f5978 100644
--- a/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl
+++ b/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl
@@ -771,3 +771,21 @@ 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):
+    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"],
+                )

From b192590e28027285b78e7a4f559cc773cd453835 Mon Sep 17 00:00:00 2001
From: Corentin Kerisit <corentin.kerisit@gmail.com>
Date: Thu, 8 Oct 2026 13:25:52 +0200
Subject: [PATCH 2/2] Document forwarded header test macro

---
 .../cc_args_and_env/cc_args_and_env_test.bzl                 | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl b/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl
index 18366f5978..b80ccaf37a 100644
--- a/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl
+++ b/cargo/tests/cargo_build_script/cc_args_and_env/cc_args_and_env_test.bzl
@@ -773,6 +773,11 @@ def direct_libs_as_flag_operand_test(name):
     )
 
 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 [