Add --help flag to rungenmain, fixing #5323 (#6354)

tools/RunGenMain.cpp

@@ -108,6 +108,9 @@ Usage: $NAME$ argument=value [argument=value... ] [flags]
 
 Flags:
 
+ --help:
+ print this message and exit.
+
  --describe:
  print names and types of all arguments to stdout and exit.
 
@@ -325,6 +328,13 @@ int main(int argc, char **argv) {
  std::vector<std::string> v = split_string(p, "=");
  std::string flag_name = v[0];
  std::string flag_value = v.size() > 1 ? v[1] : "";
+ // Check for the help flag early so that it takes
+ // precedence over other errors that occur before full
+ // argument parsing.
+ if (flag_name == "help") {
+ usage(argv[0]);
+ return 0;
+ }
  if (v.size() > 2) {
  fail() << "Invalid argument: " << argv[i];
  }