[llvm-rc] Remove transitional preprocessing fallback logic

[mstorsjo]

Summary

When preprocessing was integrated to llvm-rc in 2021, this was a
new requirement (previously one could execute llvm-rc without a
suitable preprocessing tool to be available).

As a transitional helper, llvm-rc fell back on skipping preprocessing
if no suitable tool was found (with a warning printed), but users
could pass an llvm-rc specific option to silence the warning, if they
explicitly want to run the tool without preprocessing.

Now 2 years later, remove the transitional helper - error out if
preprocessing failed. The option for disabling preprocessing remains.

[aganea]

LG.

Just a thought: as future work, we should probably have an integration with LLVM_TOOL_LLVM_DRIVER_BUILD?

[mstorsjo]

In D146797#4219456, @aganea wrote:

LG.

Just a thought: as future work, we should probably have an integration with LLVM_TOOL_LLVM_DRIVER_BUILD?

Yes, that would probably be good. I haven't really paid much attention to that build mode myself yet though.

llvm/tools/llvm-rc/llvm-rc.cpp

struct RcOptions {
   bool BeVerbose = false;
   WriterParams Params;
   bool AppendNull = false;
   bool IsDryRun = false;
   // Set the default language; choose en-US arbitrarily.
   unsigned LangId = (/*PrimaryLangId*/ 0x09) | (/*SubLangId*/ 0x01 << 10);
 };
 
-bool preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
+void preprocess(StringRef Src, StringRef Dst, const RcOptions &Opts,
                 const char *Argv0) {
   std::string Clang;
   if (Opts.PrintCmdAndExit || !Opts.PreprocessCmd.empty()) {
     Clang = "clang";
   } else {
     ErrorOr<std::string> ClangOrErr = findClang(Argv0, Opts.Triple);
     if (ClangOrErr) {
       Clang = *ClangOrErr;
     } else {
-      errs() << "llvm-rc: Unable to find clang, skipping preprocessing."
+      errs() << "llvm-rc: Unable to find clang for preprocessing."
              << "\n";
       StringRef OptionName =
           Opts.IsWindres ? "--no-preprocess" : "-no-preprocess";
-      errs()
-          << "Pass " << OptionName
-          << " to disable preprocessing. This will be an error in the future."
-          << "\n";
-      return false;
+      errs() << "Pass " << OptionName << " to disable preprocessing.\n";
+      fatalError("llvm-rc: Unable to preprocess.");
     }
   }
 
   SmallVector<StringRef, 8> Args = {
       Clang, "--driver-mode=gcc", "-target", Opts.Triple, "-E",
       "-xc", "-DRC_INVOKED"};
   if (!Opts.PreprocessCmd.empty()) {
     Args.clear();
if (Opts.PrintCmdAndExit)
       exit(0);
   }
   // The llvm Support classes don't handle reading from stdout of a child
   // process; otherwise we could avoid using a temp file.
   int Res = sys::ExecuteAndWait(Args[0], Args);
   if (Res) {
     fatalError("llvm-rc: Preprocessing failed.");
   }
-  return true;
 }
 
 static std::pair<bool, std::string> isWindres(llvm::StringRef Argv0) {
   StringRef ProgName = llvm::sys::path::stem(Argv0);
   // x86_64-w64-mingw32-windres -> x86_64-w64-mingw32, windres
   // llvm-rc -> "", llvm-rc
   // aarch64-w64-mingw32-llvm-windres-10.exe -> aarch64-w64-mingw32, llvm-windres
   ProgName = ProgName.rtrim("0123456789.-");

 }
 
 void doRc(std::string Src, std::string Dest, RcOptions &Opts,
           const char *Argv0) {
   std::string PreprocessedFile = Src;
   if (Opts.Preprocess) {
     std::string OutFile = createTempFile("preproc", "rc");
     TempPreprocFile.setFile(OutFile);
-    if (preprocess(Src, OutFile, Opts, Argv0))
-      PreprocessedFile = OutFile;
+    preprocess(Src, OutFile, Opts, Argv0);
   }
 
   // Read and tokenize the input file.
   ErrorOr<std::unique_ptr<MemoryBuffer>> File =
       MemoryBuffer::getFile(PreprocessedFile);
   if (!File) {
     fatalError("Error opening file '" + Twine(PreprocessedFile) +
                "': " + File.getError().message());