GN: Change gen command syntax, support relative dirs.

tools/gn/command_gen.cc

 // Copyright (c) 2013 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 "base/atomicops.h"
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/timer/elapsed_timer.h"
 #include "tools/gn/build_settings.h"
@@ skipping 32 matching lines @@
         base::Bind(&BackgroundDoWrite, target, toolchain));
   }
 }
 
 }  // namespace
 
 const char kGen[] = "gen";
 const char kGen_HelpShort[] =
     "gen: Generate ninja files.";
 const char kGen_Help[] =
-    "gn gen\n"
-    "  Generates ninja files from the current tree.\n"
+    "gn gen: Generate ninja files.\n"
+    "\n"
+    "  gn gen <output_directory>\n"
+    "\n"
+    "  Generates ninja files from the current tree and puts them in the given\n"
+    "  output directory.\n"
+    "\n"
+    "  The output directory can be a source-repo-absolute path name such as:\n"
+    "      //out/foo\n"
+    "  Or it can be a directory relative to the current directory such as:\n"
+    "      out/foo\n"
     "\n"
     "  See \"gn help\" for the common command-line switches.\n";
 
 // Note: partially duplicated in command_gyp.cc.
 int RunGen(const std::vector<std::string>& args) {
   base::ElapsedTimer timer;
 
+  if (args.size() != 1) {
+    Err(Location(), "Need exactly one build directory to generate.",
+        "I expected something more like \"gn gen out/foo\"\n"
+        "You can also see \"gn help gen\".").PrintToStdout();
+    return 1;
+  }
+
   // Deliberately leaked to avoid expensive process teardown.
-  Setup* setup = new Setup;
-  if (!setup->DoSetup())
+  Setup* setup = new Setup();
+  if (!setup->DoSetup(args[0]))
     return 1;
 
   // Cause the load to also generate the ninja files for each target. We wrap
   // the writing to maintain a counter.
   base::subtle::Atomic32 write_counter = 0;
   setup->builder()->set_resolved_callback(
       base::Bind(&ItemResolvedCallback, &write_counter,
                  scoped_refptr<Builder>(setup->builder())));
 
   // Do the actual load. This will also write out the target ninja files.
@@ skipping 17 matching lines @@
             setup->scheduler().input_file_manager()->GetInputFileCount()) +
         " files in " +
         base::IntToString(elapsed_time.InMilliseconds()) + "ms\n";
     OutputString(stats);
   }
 
   return 0;
 }
 
 }  // namespace commands