Add GN variables for controlling header checking.

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index c6cc06895518d054c54c6417af8d2c391126cd15..1bf301af1c90f6dada7e2e9f3bb440f974ddc1a8 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -262,6 +262,41 @@ const char kAllDependentConfigs_Help[] =
     "  See also \"direct_dependent_configs\".\n"
     COMMON_ORDERING_HELP;
 
+const char kAllowCircularIncludesFrom[] = "allow_circular_includes_from";
+const char kAllowCircularIncludesFrom_HelpShort[] =
+    "allow_circular_includes_from: [label list] Permit includes from deps.";
+const char kAllowCircularIncludesFrom_Help[] =
+    "allow_circular_includes_from: Permit includes from deps.\n"
+    "\n"
+    "  A list of target labels. Must be a subset of the target's \"deps\".\n"
+    "  These targets will be permitted to include headers from the current\n"
+    "  target despite the dependency going in the opposite direction.\n"
+    "\n"
+    "Tedious exposition\n"
+    "\n"
+    "  Normally, for a file in target A to include a file from target B,\n"
+    "  A must list B as a dependency. This invariant is enforced by the\n"
+    "  \"gn check\" command (and the --check flag to \"gn gen\").\n"
+    "\n"
+    "  Sometimes, two targets might be the same unit for linking purposes\n"
+    "  (two source sets or static libraries that would always be linked\n"
+    "  together in a final executable or shared library). In this case,\n"
+    "  you want A to be able to include B's headers, and B to include A's\n"
+    "  headers.\n"
+    "\n"
+    "  This list, if specified, lists which of the dependencies of the\n"
+    "  current target can include header files from the current target.\n"
+    "  That is, if A depends on B, B can only include headers from A if it is\n"
+    "  in A's allow_circular_includes_from list.\n"
+    "\n"
+    "Example\n"
+    "\n"
+    "  source_set(\"a\") {\n"
+    "    deps = [ \":b\", \":c\" ]\n"
+    "    allow_circular_includes_from = [ \":b\" ]\n"
+    "    ...\n"
+    "  }\n";
+
 const char kArgs[] = "args";
 const char kArgs_HelpShort[] =
     "args: [string list] Arguments passed to an action.";
@@ -311,6 +346,28 @@ const char kCflagsObjCC_HelpShort[] =
     "cflags_objcc: [string list] Flags passed to the Objective C++ compiler.";
 const char* kCflagsObjCC_Help = kCommonCflagsHelp;
 
+const char kCheckIncludes[] = "check_includes";
+const char kCheckIncludes_HelpShort[] =
+    "check_includes: [boolean] Controls whether a target's files are checked.";
+const char kCheckIncludes_Help[] =
+    "check_includes: [boolean] Controls whether a target's files are checked.\n"
+    "\n"
+    "  When true (the default), the \"gn check\" command (as well as\n"
+    "  \"gn gen\" with the --check flag) will check this target's sources\n"
+    "  and headers for proper dependencies.\n"
+    "\n"
+    "  When false, the files in this target will be skipped by default.\n"
+    "  This does not affect other targets that depend on the current target,\n"
+    "  it just skips checking the includes of the current target's files.\n"
+    "\n"
+    "Example\n"
+    "\n"
+    "  source_set(\"busted_includes\") {\n"
+    "    # This target's includes are messed up, exclude it from checking.\n"
+    "    check_includes = false\n"
+    "    ...\n"
+    "  }\n";
+
 const char kConfigs[] = "configs";
 const char kConfigs_HelpShort[] =
     "configs: [label list] Configs applying to this target.";
@@ -831,12 +888,14 @@ const VariableInfoMap& GetTargetVariables() {
   static VariableInfoMap info_map;
   if (info_map.empty()) {
     INSERT_VARIABLE(AllDependentConfigs)
+    INSERT_VARIABLE(AllowCircularIncludesFrom)
     INSERT_VARIABLE(Args)
     INSERT_VARIABLE(Cflags)
     INSERT_VARIABLE(CflagsC)
     INSERT_VARIABLE(CflagsCC)
     INSERT_VARIABLE(CflagsObjC)
     INSERT_VARIABLE(CflagsObjCC)
+    INSERT_VARIABLE(CheckIncludes)
     INSERT_VARIABLE(Configs)
     INSERT_VARIABLE(Data)
     INSERT_VARIABLE(Datadeps)