Add warning suppression mapping file to the build

This adds an (empty) warning suppression mapping file, per the linked bug. It controls its use by adding a new gn arg pointing to the file, and passing the WSM to each clang compilation if the arg is set. Since the arg is set in our `.gn` file, consumers of our `build/` directory will see no change, but can choose to use their own WSM file if they wish.

It also contains some hacks/configs to make sure the file gets uploaded by reclient/siso.

So far, the file is unused, but we'll start filling it out as we make progress enabling warnings.

Change-Id: I0493109c1c365643b97de937286ca7d0a4f89729
Bug: 404297941
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6344734
Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Devon Loehr <dloehr@google.com>
Cr-Commit-Position: refs/heads/main@{#1438081}

.gn

@@ -74,6 +74,7 @@ default_args = {
   devtools_visibility = [ "*" ]
 
   clang_unsafe_buffers_paths = "//build/config/unsafe_buffers_paths.txt"
+  clang_warning_suppression_file = "//build/config/warning_suppression.txt"
 }
 
 # These are the targets to skip header checking by default. The files in targets

build/config/BUILDCONFIG.gn

@@ -184,6 +184,7 @@ declare_args() {
   # Unsafe buffers. Location of file used by plugins to track portions of
   # the codebase which have been made manifestly safe.
   clang_unsafe_buffers_paths = ""
+  clang_warning_suppression_file = ""
 }
 
 # ==============================================================================

build/config/OWNERS

@@ -5,3 +5,8 @@ per-file cast.gni=file://build/config/chromecast/OWNERS
 
 per-file unsafe_buffers_paths.txt=arthursonzogni@chromium.org
 per-file unsafe_buffers_paths.txt=tsepez@chromium.org
+
+per-file warning_suppression.txt=dloehr@google.com
+per-file warning_suppression.txt=thakis@chromium.org
+per-file warning_suppression.txt=hans@chromium.org
+per-file warning_suppression.txt=tsepez@chromium.org

build/config/clang/clang.gni

@@ -51,18 +51,26 @@ lld_emit_indexes_and_imports =
     lld_emit_indexes_and_imports && is_a_target_toolchain
 
 # TODO(crbug.com/326584510): Reclient does not upload `inputs` from C/C++
-# targets. This file is added to `inputs` for all C targets in
-# //build/config/BUILDCONFIG.gn via //build/config/clang:unsafe_buffers.
-# We work around the bug in Reclient by specifying the file here.
-#
-# This is a comma-delimited list of paths relative to the source tree root. The
-# leading space is important, if the string is non-empty. :)
-rbe_bug_326584510_missing_inputs = ""
+# targets. We work around the bug in Reclient by
+# specifying the files here.
+rbe_bug_326584510_missing_input_list = []
 if (clang_use_chrome_plugins && defined(clang_unsafe_buffers_paths) &&
     "$clang_unsafe_buffers_paths" != "") {
   if (rbe_exec_root != rebase_path("//")) {
     assert(!use_siso, "Can't use non-default rbe_exec_root with siso.")
   }
-  from_exec_root = rebase_path(clang_unsafe_buffers_paths, rbe_exec_root)
-  rbe_bug_326584510_missing_inputs = " -inputs=$from_exec_root"
+  rbe_bug_326584510_missing_input_list +=
+      [ rebase_path(clang_unsafe_buffers_paths, rbe_exec_root) ]
 }
+if (defined(clang_warning_suppression_file) &&
+    "$clang_warning_suppression_file" != "") {
+  if (rbe_exec_root != rebase_path("//")) {
+    assert(!use_siso, "Can't use non-default rbe_exec_root with siso.")
+  }
+  rbe_bug_326584510_missing_input_list +=
+      [ rebase_path(clang_warning_suppression_file, rbe_exec_root) ]
+}
+
+# The leading space is important, if the string is non-empty.
+rbe_bug_326584510_missing_inputs =
+    " -inputs=" + string_join(",", rbe_bug_326584510_missing_input_list)

build/config/compiler/BUILD.gn

@@ -312,6 +312,7 @@ config("compiler") {
     ":compiler_cpu_abi",
     ":compiler_codegen",
     ":compiler_deterministic",
+    ":clang_warning_suppression",
   ]
 
   # Here we enable -fno-delete-null-pointer-checks, which makes various nullptr
@@ -1650,6 +1651,31 @@ config("clang_revision") {
   }
 }
 
+# Controls the usage of a warning suppression mapping (WSM) file to suppress
+# warnings based on the path of the file they come from. It's controlled by the
+# `clang_warning_suppression_file` gn argument , which points to a text file
+# defining which warnings should be suppressed where.
+# See //build/config/warning_suppression.txt for an example file; this is the
+# file used by Chromium.
+#
+# To use a different file, or to use this functionality outside of chromium,
+# set the `clang_warning_suppression_file` argument to point to the new file,
+# e.g. by setting in the the project's .gn file.
+config("clang_warning_suppression") {
+  # Some build configs use older versions of clang that don't support WSMs
+  if (!is_nacl && default_toolchain != "//build/toolchain/cros:target" &&
+      !llvm_android_mainline && is_clang &&
+      clang_warning_suppression_file != "") {
+    from_build_root =
+        rebase_path(clang_warning_suppression_file, root_build_dir)
+    inputs = [ clang_warning_suppression_file ]
+    cflags = [
+      "-Xclang",
+      "--warning-suppression-mappings=" + from_build_root,
+    ]
+  }
+}
+
 config("rustc_revision") {
   if (rustc_revision != "") {
     # Similar to the above config, this is here so that all files get recompiled

build/config/siso/clang_all.star

@@ -87,15 +87,19 @@ __input_deps = {
     ],
     "third_party/llvm-build/Release+Asserts/bin/clang": [
         "build/config/unsafe_buffers_paths.txt",
+        "build/config/warning_suppression.txt",
     ],
     "third_party/llvm-build/Release+Asserts/bin/clang++": [
         "build/config/unsafe_buffers_paths.txt",
+        "build/config/warning_suppression.txt",
     ],
     "third_party/llvm-build/Release+Asserts/bin/clang-cl": [
         "build/config/unsafe_buffers_paths.txt",
+        "build/config/warning_suppression.txt",
     ],
     "third_party/llvm-build/Release+Asserts/bin/clang-cl.exe": [
         "build/config/unsafe_buffers_paths.txt",
+        "build/config/warning_suppression.txt",
     ],
     "third_party/llvm-build/Release+Asserts/bin/lld-link": [
         "build/config/c++/libc++.natvis",

build/config/warning_suppression.txt

@@ -0,0 +1,9 @@
+# Copyright 2025 The Chromium Project. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Warning suppression mappings are listed here on a per-warning basis.
+# Upstream docs: https://clang.llvm.org/docs/WarningSuppressionMappings.html
+# We'll put the policy here too when it's written.
+# Don't use this file until the policy is written.
+# See crbug.com/404297941 for more information.