[llvm-rc] Default to writing the output next to the input, if no output is specified

[mstorsjo]

Summary

This matches what rc.exe does if no output is specified.

[amccarth]

LGTM

test/tools/llvm-rc/tag-menu.test

 ; RUN: llvm-rc /FO %t %p/Inputs/tag-menu.rc
 ; RUN: llvm-readobj %t | FileCheck %s --check-prefix=MENU
 
+; Test running llvm-rc without an explicit output file.
+; RUN: cp %p/Inputs/tag-menu.rc %t.implicit.rc
+; RUN: llvm-rc %t.implicit.rc
+; RUN: llvm-readobj %t.implicit.res | FileCheck --check-prefix=MENU %s
+
 ; MENU: Resource type (int): 4
 ; MENU-NEXT: Resource name (string): CHECKRECURSION
 ; MENU-NEXT: Data version: 0
 ; MENU-NEXT: Memory flags: 0x1030
 ; MENU-NEXT: Language ID: 1033
 ; MENU-NEXT: Version (major): 0
 ; MENU-NEXT: Version (minor): 0
 ; MENU-NEXT: Characteristics: 0

tools/llvm-rc/llvm-rc.cpp

 
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/InitLLVM.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MemoryBuffer.h"
+#include "llvm/Support/Path.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include <system_error>
 
 using namespace llvm;
#include "ResourceScriptTokenList.def"
   Params.Include = InputArgs.getAllArgValues(OPT_INCLUDE);
   Params.NoInclude = InputArgs.getAllArgValues(OPT_NOINCLUDE);
 
   std::unique_ptr<ResourceFileWriter> Visitor;
   bool IsDryRun = InputArgs.hasArg(OPT_DRY_RUN);
 
   if (!IsDryRun) {
     auto OutArgsInfo = InputArgs.getAllArgValues(OPT_FILEOUT);
+    if (OutArgsInfo.empty()) {
+      SmallString<128> OutputFile = InputFile;
+      llvm::sys::path::replace_extension(OutputFile, "res");
+      OutArgsInfo.push_back(OutputFile.str());
+    }
+
     if (OutArgsInfo.size() != 1)
       fatalError(
-          "Exactly one output file should be provided (using /FO flag).");
+          "No more than one output file should be provided (using /FO flag).");
 
     std::error_code EC;
     auto FOut =
         llvm::make_unique<raw_fd_ostream>(OutArgsInfo[0], EC, sys::fs::F_RW);
     if (EC)
       fatalError("Error opening output file '" + OutArgsInfo[0] +
                  "': " + EC.message());
     Visitor = llvm::make_unique<ResourceFileWriter>(Params, std::move(FOut));