[clang-tblgen] Automatically document options values

This is a port of f5c6667 to clang's tablegen,
with a better wording.

Differential Revision: https://reviews.llvm.org/D123682

Author: serge-sans-paille

clang/utils/TableGen/ClangOptionDocEmitter.cpp

@@ -238,6 +238,8 @@ void emitOptionWithArgs(StringRef Prefix, const Record *Option,
   }
 }
 
+constexpr StringLiteral DefaultMetaVarName = "<arg>";
+
 void emitOptionName(StringRef Prefix, const Record *Option, raw_ostream &OS) {
   // Find the arguments to list after the option.
   unsigned NumArgs = getNumArgsForKind(Option->getValueAsDef("Kind"), Option);
@@ -247,7 +249,7 @@ void emitOptionName(StringRef Prefix, const Record *Option, raw_ostream &OS) {
   if (HasMetaVarName)
     Args.push_back(std::string(Option->getValueAsString("MetaVarName")));
   else if (NumArgs == 1)
-    Args.push_back("<arg>");
+    Args.push_back(DefaultMetaVarName.str());
 
   // Fill up arguments if this option didn't provide a meta var name or it
   // supports an unlimited number of arguments. We can't see how many arguments
@@ -341,8 +343,30 @@ void emitOption(const DocumentedOption &Option, const Record *DocInfo,
   OS << "\n\n";
 
   // Emit the description, if we have one.
+  const Record *R = Option.Option;
   std::string Description =
-      getRSTStringWithTextFallback(Option.Option, "DocBrief", "HelpText");
+      getRSTStringWithTextFallback(R, "DocBrief", "HelpText");
+
+  if (!isa<UnsetInit>(R->getValueInit("Values"))) {
+    if (!Description.empty() && Description.back() != '.')
+      Description.push_back('.');
+
+    StringRef MetaVarName;
+    if (!isa<UnsetInit>(R->getValueInit("MetaVarName")))
+      MetaVarName = R->getValueAsString("MetaVarName");
+    else
+      MetaVarName = DefaultMetaVarName;
+
+    SmallVector<StringRef> Values;
+    SplitString(R->getValueAsString("Values"), Values, ",");
+    Description += (" " + MetaVarName + " must be '").str();
+    if (Values.size() > 1) {
+      Description += join(Values.begin(), Values.end() - 1, "', '");
+      Description += "' or '";
+    }
+    Description += (Values.back() + "'.").str();
+  }
+
   if (!Description.empty())
     OS << Description << "\n\n";
 }

llvm/utils/TableGen/OptRSTEmitter.cpp

@@ -85,15 +85,13 @@ void EmitOptRST(RecordKeeper &Records, raw_ostream &OS) {
       if (!isa<UnsetInit>(R->getValueInit("Values"))) {
         SmallVector<StringRef> Values;
         SplitString(R->getValueAsString("Values"), Values, ",");
-        HelpText += (" " + MetaVarName + " can be ").str();
+        HelpText += (" " + MetaVarName + " must be '").str();
 
-        if (Values.size() == 1) {
-          HelpText += ("'" + Values.front() + "'.").str();
-        } else {
-          HelpText += "one of '";
+        if (Values.size() > 1) {
           HelpText += join(Values.begin(), Values.end() - 1, "', '");
-          HelpText += ("' or '" + Values.back() + "'.").str();
+          HelpText += "' or '";
         }
+        HelpText += (Values.front() + "'.").str();
       }
 
       if (!HelpText.empty()) {