Require build directories exist for some GN commands.

tools/gn/setup.cc

Index: tools/gn/setup.cc
diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index 02ab3ca912de485883510b413ca202a974c88e59..7ffe2f89d1d8447e02e9b9822841bc05f10ef47c 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -225,7 +225,7 @@ Setup::Setup()
 Setup::~Setup() {
 }
 
-bool Setup::DoSetup(const std::string& build_dir) {
+bool Setup::DoSetup(const std::string& build_dir, bool force_create) {
   CommandLine* cmdline = CommandLine::ForCurrentProcess();
 
   scheduler_.set_verbose_logging(cmdline->HasSwitch(kSwitchVerbose));
@@ -241,8 +241,11 @@ bool Setup::DoSetup(const std::string& build_dir) {
     return false;
   if (!FillOtherConfig(*cmdline))
     return false;
-  if (!FillBuildDir(build_dir))  // Must be after FillSourceDir to resolve.
+
+  // Must be after FillSourceDir to resolve.
+  if (!FillBuildDir(build_dir, !force_create))
     return false;
+
   if (fill_arguments_) {
     if (!FillArguments(*cmdline))
       return false;
@@ -438,7 +441,7 @@ bool Setup::FillSourceDir(const CommandLine& cmdline) {
   return true;
 }
 
-bool Setup::FillBuildDir(const std::string& build_dir) {
+bool Setup::FillBuildDir(const std::string& build_dir, bool require_exists) {
   SourceDir resolved =
       SourceDirForCurrentDirectory(build_settings_.root_path()).
           ResolveRelativeDir(build_dir);
@@ -451,6 +454,21 @@ bool Setup::FillBuildDir(const std::string& build_dir) {
 
   if (scheduler_.verbose_logging())
     scheduler_.Log("Using build dir", resolved.value());
+
+  if (require_exists) {
+    base::FilePath build_dir_path = build_settings_.GetFullPath(resolved);
+    if (!base::PathExists(build_dir_path.Append(
+            FILE_PATH_LITERAL("build.ninja")))) {
+      Err(Location(), "Not a build directory.",
+          "This command requires an existing build directory. I interpreted "
+          "your input\n\"" + build_dir + "\" as:\n  " +
+          FilePathToUTF8(build_dir_path) +
+          "\nwhich doesn't seem to contain a previously-generated build.")
+          .PrintToStdout();
+      return false;
+    }
+  }
+
   build_settings_.SetBuildDir(resolved);
   return true;
 }