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diff --git a/gn/tools/gn/command_check.cc b/gn/tools/gn/command_check.cc
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+++ b/gn/tools/gn/command_check.cc
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <stddef.h>
+
+#include "base/command_line.h"
+#include "base/strings/stringprintf.h"
+#include "tools/gn/commands.h"
+#include "tools/gn/header_checker.h"
+#include "tools/gn/setup.h"
+#include "tools/gn/standard_out.h"
+#include "tools/gn/switches.h"
+#include "tools/gn/target.h"
+#include "tools/gn/trace.h"
+
+namespace commands {
+
+const char kNoGnCheck_Help[] =
+    R"(nogncheck: Skip an include line from checking.
+
+  GN's header checker helps validate that the includes match the build
+  dependency graph. Sometimes an include might be conditional or otherwise
+  problematic, but you want to specifically allow it. In this case, it can be
+  whitelisted.
+
+  Include lines containing the substring "nogncheck" will be excluded from
+  header checking. The most common case is a conditional include:
+
+    #if defined(ENABLE_DOOM_MELON)
+    #include "tools/doom_melon/doom_melon.h"  // nogncheck
+    #endif
+
+  If the build file has a conditional dependency on the corresponding target
+  that matches the conditional include, everything will always link correctly:
+
+    source_set("mytarget") {
+      ...
+      if (enable_doom_melon) {
+        defines = [ "ENABLE_DOOM_MELON" ]
+        deps += [ "//tools/doom_melon" ]
+      }
+
+  But GN's header checker does not understand preprocessor directives, won't
+  know it matches the build dependencies, and will flag this include as
+  incorrect when the condition is false.
+
+More information
+
+  The topic "gn help check" has general information on how checking works and
+  advice on fixing problems. Targets can also opt-out of checking, see
+  "gn help check_includes".
+)";
+
+const char kCheck[] = "check";
+const char kCheck_HelpShort[] = "check: Check header dependencies.";
+const char kCheck_Help[] =
+    R"(gn check <out_dir> [<label_pattern>] [--force]
+
+  GN's include header checker validates that the includes for C-like source
+  files match the build dependency graph.
+
+  "gn check" is the same thing as "gn gen" with the "--check" flag except that
+  this command does not write out any build files. It's intended to be an easy
+  way to manually trigger include file checking.
+
+  The <label_pattern> can take exact labels or patterns that match more than
+  one (although not general regular expressions). If specified, only those
+  matching targets will be checked. See "gn help label_pattern" for details.
+
+Command-specific switches
+
+  --force
+      Ignores specifications of "check_includes = false" and checks all
+      target's files that match the target label.
+
+What gets checked
+
+  The .gn file may specify a list of targets to be checked. Only these targets
+  will be checked if no label_pattern is specified on the command line.
+  Otherwise, the command-line list is used instead. See "gn help dotfile".
+
+  Targets can opt-out from checking with "check_includes = false" (see
+  "gn help check_includes").
+
+  For targets being checked:
+
+    - GN opens all C-like source files in the targets to be checked and scans
+      the top for includes.
+
+    - Includes with a "nogncheck" annotation are skipped (see
+      "gn help nogncheck").
+
+    - Only includes using "quotes" are checked. <brackets> are assumed to be
+      system includes.
+
+    - Include paths are assumed to be relative to any of the "include_dirs" for
+      the target (including the implicit current dir).
+
+    - GN does not run the preprocessor so will not understand conditional
+      includes.
+
+    - Only includes matching known files in the build are checked: includes
+      matching unknown paths are ignored.
+
+  For an include to be valid:
+
+    - The included file must be in the current target, or there must be a path
+      following only public dependencies to a target with the file in it
+      ("gn path" is a good way to diagnose problems).
+
+    - There can be multiple targets with an included file: only one needs to be
+      valid for the include to be allowed.
+
+    - If there are only "sources" in a target, all are considered to be public
+      and can be included by other targets with a valid public dependency path.
+
+    - If a target lists files as "public", only those files are able to be
+      included by other targets. Anything in the sources will be considered
+      private and will not be includable regardless of dependency paths.
+
+    - Outputs from actions are treated like public sources on that target.
+
+    - A target can include headers from a target that depends on it if the
+      other target is annotated accordingly. See "gn help
+      allow_circular_includes_from".
+
+Advice on fixing problems
+
+  If you have a third party project that is difficult to fix or doesn't care
+  about include checks it's generally best to exclude that target from checking
+  altogether via "check_includes = false".
+
+  If you have conditional includes, make sure the build conditions and the
+  preprocessor conditions match, and annotate the line with "nogncheck" (see
+  "gn help nogncheck" for an example).
+
+  If two targets are hopelessly intertwined, use the
+  "allow_circular_includes_from" annotation. Ideally each should have identical
+  dependencies so configs inherited from those dependencies are consistent (see
+  "gn help allow_circular_includes_from").
+
+  If you have a standalone header file or files that need to be shared between
+  a few targets, you can consider making a source_set listing only those
+  headers as public sources. With only header files, the source set will be a
+  no-op from a build perspective, but will give a central place to refer to
+  those headers. That source set's files will still need to pass "gn check" in
+  isolation.
+
+  In rare cases it makes sense to list a header in more than one target if it
+  could be considered conceptually a member of both.
+
+Examples
+
+  gn check out/Debug
+      Check everything.
+
+  gn check out/Default //foo:bar
+      Check only the files in the //foo:bar target.
+
+  gn check out/Default "//foo/*
+      Check only the files in targets in the //foo directory tree.
+)";
+
+int RunCheck(const std::vector<std::string>& args) {
+  if (args.size() != 1 && args.size() != 2) {
+    Err(Location(), "You're holding it wrong.",
+        "Usage: \"gn check <out_dir> [<target_label>]\"")
+        .PrintToStdout();
+    return 1;
+  }
+
+  // Deliberately leaked to avoid expensive process teardown.
+  Setup* setup = new Setup();
+  if (!setup->DoSetup(args[0], false))
+    return 1;
+  if (!setup->Run())
+    return 1;
+
+  std::vector<const Target*> all_targets =
+      setup->builder().GetAllResolvedTargets();
+
+  bool filtered_by_build_config = false;
+  std::vector<const Target*> targets_to_check;
+  if (args.size() > 1) {
+    // Compute the targets to check.
+    std::vector<std::string> inputs(args.begin() + 1, args.end());
+    UniqueVector<const Target*> target_matches;
+    UniqueVector<const Config*> config_matches;
+    UniqueVector<const Toolchain*> toolchain_matches;
+    UniqueVector<SourceFile> file_matches;
+    if (!ResolveFromCommandLineInput(setup, inputs, false, &target_matches,
+                                     &config_matches, &toolchain_matches,
+                                     &file_matches))
+      return 1;
+
+    if (target_matches.size() == 0) {
+      OutputString("No matching targets.\n");
+      return 1;
+    }
+    targets_to_check.insert(targets_to_check.begin(), target_matches.begin(),
+                            target_matches.end());
+  } else {
+    // No argument means to check everything allowed by the filter in
+    // the build config file.
+    if (setup->check_patterns()) {
+      FilterTargetsByPatterns(all_targets, *setup->check_patterns(),
+                              &targets_to_check);
+      filtered_by_build_config = targets_to_check.size() != all_targets.size();
+    } else {
+      // No global filter, check everything.
+      targets_to_check = all_targets;
+    }
+  }
+
+  const base::CommandLine* cmdline = base::CommandLine::ForCurrentProcess();
+  bool force = cmdline->HasSwitch("force");
+
+  if (!CheckPublicHeaders(&setup->build_settings(), all_targets,
+                          targets_to_check, force))
+    return 1;
+
+  if (!base::CommandLine::ForCurrentProcess()->HasSwitch(switches::kQuiet)) {
+    if (filtered_by_build_config) {
+      // Tell the user about the implicit filtering since this is obscure.
+      OutputString(base::StringPrintf(
+          "%d targets out of %d checked based on the check_targets defined in"
+          " \".gn\".\n",
+          static_cast<int>(targets_to_check.size()),
+          static_cast<int>(all_targets.size())));
+    }
+    OutputString("Header dependency check OK\n", DECORATION_GREEN);
+  }
+  return 0;
+}
+
+bool CheckPublicHeaders(const BuildSettings* build_settings,
+                        const std::vector<const Target*>& all_targets,
+                        const std::vector<const Target*>& to_check,
+                        bool force_check) {
+  ScopedTrace trace(TraceItem::TRACE_CHECK_HEADERS, "Check headers");
+
+  scoped_refptr<HeaderChecker> header_checker(
+      new HeaderChecker(build_settings, all_targets));
+
+  std::vector<Err> header_errors;
+  header_checker->Run(to_check, force_check, &header_errors);
+  for (size_t i = 0; i < header_errors.size(); i++) {
+    if (i > 0)
+      OutputString("___________________\n", DECORATION_YELLOW);
+    header_errors[i].PrintToStdout();
+  }
+  return header_errors.empty();
+}
+
+}  // namespace commands