[pseudo-gen] Add -o flag, make --grammar required

[thakis]

Summary

Virtually all LLVM tools accept a -o flag, so add one. This will make it
possible to possibly add a --write-if-changed flag later. It also makes it
so that the file isn't partially written if the tool oesn't run successfully.

Marking --grammar as Required allows removing some manual
verification code for it.

[hokein]

Thanks!

clang-tools-extra/pseudo/gen/Main.cpp

 // generate a necessary data bits to statically construct core pieces (Grammar,
 // LRTable etc) of the LR parser.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang-pseudo/Grammar.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/FileSystem.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/ToolOutputFile.h"
 #include <algorithm>
 
 using llvm::cl::desc;
 using llvm::cl::init;
 using llvm::cl::opt;
+using llvm::cl::Required;
+using llvm::cl::value_desc;
 using llvm::cl::values;
 
 namespace {
 enum EmitType {
   EmitSymbolList,
   EmitGrammarContent,
 };
 
 opt<std::string> Grammar("grammar", desc("Parse a BNF grammar file."),
-                         init(""));
+                         Required);
 opt<EmitType>
     Emit(desc("which information to emit:"),
          values(clEnumValN(EmitSymbolList, "emit-symbol-list",
                            "Print nonterminal symbols (default)"),
                 clEnumValN(EmitGrammarContent, "emit-grammar-content",
                            "Print the BNF grammar content as a string")));
+
+opt<std::string> OutputFilename("o", init("-"), desc("Output"),
+                                value_desc("file"));
+
 std::string readOrDie(llvm::StringRef Path) {
   llvm::ErrorOr<std::unique_ptr<llvm::MemoryBuffer>> Text =
       llvm::MemoryBuffer::getFile(Path);
   if (std::error_code EC = Text.getError()) {
     llvm::errs() << "Error: can't read grammar file '" << Path
                  << "': " << EC.message() << "\n";
     ::exit(1);
   }
   return Text.get()->getBuffer().str();
 }
 } // namespace
 
 int main(int argc, char *argv[]) {
   llvm::cl::ParseCommandLineOptions(argc, argv, "");
-  if (!Grammar.getNumOccurrences()) {
-    llvm::errs() << "Grammar file must be provided!\n";
-    return 1;
-  }
 
   std::string GrammarText = readOrDie(Grammar);
   std::vector<std::string> Diags;
   auto G = clang::pseudo::Grammar::parseBNF(GrammarText, Diags);
 
   if (!Diags.empty()) {
     llvm::errs() << llvm::join(Diags, "\n");
     return 1;
   }
-  switch (Emit) {
 
+  std::error_code EC;
+  llvm::ToolOutputFile Out{OutputFilename, EC, llvm::sys::fs::OF_None};
+  if (EC) {
+    llvm::errs() << EC.message() << '\n';
+    return 1;
+  }
+
+  switch (Emit) {
   case EmitSymbolList:
     for (clang::pseudo::SymbolID ID = 0; ID < G->table().Nonterminals.size();
          ++ID) {
       std::string Name = G->symbolName(ID).str();
       // translation-unit -> translation_unit
       std::replace(Name.begin(), Name.end(), '-', '_');
-      llvm::outs() << (llvm::formatv("NONTERMINAL({0}, {1})\n", Name, ID));
+      Out.os() << llvm::formatv("NONTERMINAL({0}, {1})\n", Name, ID);
     }
     break;
   case EmitGrammarContent:
     for (llvm::StringRef Line : llvm::split(GrammarText, '\n')) {
-      llvm::outs() << '"';
-      llvm::outs().write_escaped((Line + "\n").str());
-      llvm::outs() << "\"\n";
+      Out.os() << '"';
+      Out.os().write_escaped((Line + "\n").str());
+      Out.os() << "\"\n";
     }
     break;
   }
 
+  Out.keep();
+
   return 0;
 }

clang-tools-extra/pseudo/include/CMakeLists.txt

 # The cxx.bnf grammar file
 set(cxx_bnf ${CMAKE_CURRENT_SOURCE_DIR}/../lib/cxx.bnf)
 
 # Generate inc files.
 set(cxx_symbols_inc ${CMAKE_CURRENT_BINARY_DIR}/CXXSymbols.inc)
 add_custom_command(OUTPUT ${cxx_symbols_inc}
    COMMAND "${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/pseudo-gen"
      --grammar ${cxx_bnf}
      --emit-symbol-list
-     > ${cxx_symbols_inc}
+     -o ${cxx_symbols_inc}
    COMMENT "Generating nonterminal symbol file for cxx grammar..."
    DEPENDS pseudo-gen
    VERBATIM)
 
 set(cxx_bnf_inc ${CMAKE_CURRENT_BINARY_DIR}/CXXBNF.inc)
 add_custom_command(OUTPUT ${cxx_bnf_inc}
    COMMAND "${CMAKE_RUNTIME_OUTPUT_DIRECTORY}/pseudo-gen"
      --grammar ${cxx_bnf}
      --emit-grammar-content
-     > ${cxx_bnf_inc}
+     -o ${cxx_bnf_inc}
    COMMENT "Generating bnf string file for cxx grammar..."
    DEPENDS pseudo-gen
    VERBATIM)
 
 # add_custom_command does not create a new target, we need to deine a target
 # explicitly, so that other targets can depend on it.
 add_custom_target(cxx_gen
     DEPENDS ${cxx_symbols_inc} ${cxx_bnf_inc}
     VERBATIM)