Add more documentation for GN header checking

tools/gn/command_check.cc

Index: tools/gn/command_check.cc
diff --git a/tools/gn/command_check.cc b/tools/gn/command_check.cc
index 9f89dc060a1fe861b214204bd8338d53d2ac5a12..40931a68709347d82d907c7fd161e29ea40ef57a 100644
--- a/tools/gn/command_check.cc
+++ b/tools/gn/command_check.cc
@@ -16,12 +16,51 @@
 
 namespace commands {
 
+const char kNoGnCheck_Help[] =
+    "nogncheck: Skip an include line from checking.\n"
+    "\n"
+    "  GN's header checker helps validate that the includes match the build\n"
+    "  dependency graph. Sometimes an include might be conditional or\n"
+    "  otherwise problematic, but you want to specifically allow it. In this\n"
+    "  case, it can be whitelisted.\n"
+    "\n"
+    "  Include lines containing the substring \"nogncheck\" will be excluded\n"
+    "  from header checking. The most common case is a conditional include:\n"
+    "\n"
+    "    #if defined(ENABLE_DOOM_MELON)\n"
+    "    #include \"tools/doom_melon/doom_melon.h\"  // nogncheck\n"
+    "    #endif\n"
+    "\n"
+    "  If the build file has a conditional dependency on the corresponding\n"
+    "  target that matches the conditional include, everything will always\n"
+    "  link correctly:\n"
+    "\n"
+    "    source_set(\"mytarget\") {\n"
+    "      ...\n"
+    "      if (enable_doom_melon) {\n"
+    "        defines = [ \"ENABLE_DOOM_MELON\" ]\n"
+    "        deps += [ \"//tools/doom_melon\" ]\n"
+    "      }\n"
+    "\n"
+    "  But GN's header checker does not understand preprocessor directives,\n"
+    "  won't know it matches the build dependencies, and will flag this\n"
+    "  include as incorrect when the condition is false.\n"
+    "\n"
+    "More information\n"
+    "\n"
+    "  The topic \"gn help check\" has general information on how checking\n"
+    "  works and advice on fixing problems. Targets can also opt-out of\n"
+    "  checking, see \"gn help check_includes\".\n";
+
 const char kCheck[] = "check";
 const char kCheck_HelpShort[] =
     "check: Check header dependencies.";
 const char kCheck_Help[] =
     "gn check <out_dir> [<label_pattern>] [--force]\n"
     "\n"
+    "  GN's include header checker validates that the includes for C-like\n"
+    "  source files match the build dependency graph.\n"
+    "\n"
     "  \"gn check\" is the same thing as \"gn gen\" with the \"--check\" flag\n"
     "  except that this command does not write out any build files. It's\n"
     "  intended to be an easy way to manually trigger include file checking.\n"
@@ -31,16 +70,93 @@ const char kCheck_Help[] =
     "  only those matching targets will be checked. See\n"
     "  \"gn help label_pattern\" for details.\n"
     "\n"
+    "Command-specific switches\n"
+    "\n"
+    "  --force\n"
+    "      Ignores specifications of \"check_includes = false\" and checks\n"
+    "      all target's files that match the target label.\n"
+    "\n"
+    "What gets checked\n"
+    "\n"
     "  The .gn file may specify a list of targets to be checked. Only these\n"
     "  targets will be checked if no label_pattern is specified on the\n"
     "  command line. Otherwise, the command-line list is used instead. See\n"
     "  \"gn help dotfile\".\n"
     "\n"
-    "Command-specific switches\n"
+    "  Targets can opt-out from checking with \"check_includes = false\"\n"
+    "  (see \"gn help check_includes\").\n"
     "\n"
-    "  --force\n"
-    "      Ignores specifications of \"check_includes = false\" and checks\n"
-    "      all target's files that match the target label.\n"
+    "  For targets being checked:\n"
+    "\n"
+    "    - GN opens all C-like source files in the targets to be checked and\n"
+    "      scans the top for includes.\n"
+    "\n"
+    "    - Includes with a \"nogncheck\" annotation are skipped (see\n"
+    "      \"gn help nogncheck\").\n"
+    "\n"
+    "    - Only includes using \"quotes\" are checked. <brackets> are assumed\n"
+    "      to be system includes.\n"
+    "\n"
+    "    - Include paths are assumed to be relative to either the source root\n"
+    "      or the \"root_gen_dir\" and must include all the path components.\n"
+    "      (It might be nice in the future to incorporate GN's knowledge of\n"
+    "      the include path to handle other include styles.)\n"
+    "\n"
+    "    - GN does not run the preprocessor so will not understand\n"
+    "      conditional includes.\n"
+    "\n"
+    "    - Only includes matching known files in the build are checked:\n"
+    "      includes matching unknown paths are ignored.\n"
+    "\n"
+    "  For an include to be valid:\n"
+    "\n"
+    "    - The included file must be in the current target, or there must\n"
+    "      be a path following only public dependencies to a target with the\n"
+    "      file in it (\"gn path\" is a good way to diagnose problems).\n"
+    "\n"
+    "    - There can be multiple targets with an included file: only one\n"
+    "      needs to be valid for the include to be allowed.\n"
+    "\n"
+    "    - If there are only \"sources\" in a target, all are considered to\n"
+    "      be public and can be included by other targets with a valid public\n"
+    "      dependency path.\n"
+    "\n"
+    "    - If a target lists files as \"public\", only those files are\n"
+    "      able to be included by other targets. Anything in the sources\n"
+    "      will be considered private and will not be includable regardless\n"
+    "      of dependency paths.\n"
+    "\n"
+    "    - Ouptuts from actions are treated like public sources on that\n"
+    "      target.\n"
+    "\n"
+    "    - A target can include headers from a target that depends on it\n"
+    "      if the other target is annotated accordingly. See\n"
+    "      \"gn help allow_circular_includes_from\".\n"
+    "\n"
+    "Advice on fixing problems\n"
+    "\n"
+    "  If you have a third party project that uses relative includes,\n"
+    "  it's generally best to exclude that target from checking altogether\n"
+    "  via \"check_includes = false\".\n"
+    "\n"
+    "  If you have conditional includes, make sure the build conditions\n"
+    "  and the preprocessor conditions match, and annotate the line with\n"
+    "  \"nogncheck\" (see \"gn help nogncheck\" for an example).\n"
+    "\n"
+    "  If two targets are hopelessly intertwined, use the\n"
+    "  \"allow_circular_includes_from\" annotation. Ideally each should have\n"
+    "  identical dependencies so configs inherited from those dependencies\n"
+    "  are consistent (see \"gn help allow_circular_includes_from\").\n"
+    "\n"
+    "  If you have a standalone header file or files that need to be shared\n"
+    "  between a few targets, you can consider making a source_set listing\n"
+    "  only those headers as public sources. With only header files, the\n"
+    "  source set will be a no-op from a build perspective, but will give a\n"
+    "  central place to refer to those headers. That source set's files\n"
+    "  will still need to pass \"gn check\" in isolation.\n"
+    "\n"
+    "  In rare cases it makes sense to list a header in more than one\n"
+    "  target if it could be considered conceptually a member of both.\n"
     "\n"
     "Examples\n"
     "\n"