Give llvm-lib rudimentary help output.

[thakis]

lib/ToolDrivers/llvm-lib/LibDriver.cpp

126

If this is invoked via bin/lld-link /lib /? then the output is

OVERVIEW: LLVM Lib

USAGE: /lib [options] <inputs>

which is imho ok.

[pcc]

LGTM

Is it worth adding a test?

[thakis]

Thanks! Landed with a test in r337084.

lib/ToolDrivers/llvm-lib/LibDriver.cpp

llvm::errs() << "missing arg value for \""
                  << Args.getArgString(MissingIndex) << "\", expected "
                  << MissingCount
                  << (MissingCount == 1 ? " argument.\n" : " arguments.\n");
     return 1;
   }
   for (auto *Arg : Args.filtered(OPT_UNKNOWN))
     llvm::errs() << "ignoring unknown argument: " << Arg->getSpelling() << "\n";
 
+  // Handle /help
+  if (Args.hasArg(OPT_help)) {
+    Table.PrintHelp(outs(), ArgsArr[0], "LLVM Lib");
+    return 0;
+  }
+
   // If no input files, silently do nothing to match lib.exe.
   if (!Args.hasArgNoClaim(OPT_INPUT))
     return 0;
 
   std::vector<StringRef> SearchPaths = getSearchPaths(&Args, Saver);
 
   // Create a NewArchiveMember for each input file.
   std::vector<NewArchiveMember> Members;

lib/ToolDrivers/llvm-lib/Options.td

 include "llvm/Option/OptParser.td"
 
 // lib.exe accepts options starting with either a dash or a slash.
 
 // Flag that takes no arguments.
 class F<string name> : Flag<["/", "-", "-?"], name>;
 
 // Flag that takes one argument after ":".
 class P<string name, string help> :
       Joined<["/", "-", "-?"], name#":">, HelpText<help>;
 
 def libpath: P<"libpath", "Object file search path">;
 def out    : P<"out", "Path to file to write output">;
 
-def llvmlibthin : F<"llvmlibthin">;
+def llvmlibthin : F<"llvmlibthin">,
+    HelpText<"Make .lib point to .obj files instead of copying their contents">;
+
+def help : F<"help">;
+def help_q : Flag<["/?", "-?"], "">, Alias<help>;
 
 //==============================================================================
 // The flags below do nothing. They are defined only for lib.exe compatibility.
 //==============================================================================
 
 class QF<string name> : Joined<["/", "-", "-?"], name#":">;
 
 def ignore : QF<"ignore">;
 def machine: QF<"machine">;
 def nologo : F<"nologo">;