[lld] Fixes wrong Twine uses in FileNode::errStr() and in LayoutPass.cpp

[rafaelauler]

Summary

This patch fixes forbidden uses of Twine. It should only be used as an intermediary value, but never stored.

[ruiu]

LGTM. Is there any place we need to fix for this incorrect use of Twine?

[rafaelauler]

Sorry for sending this in another patch. I double checked the lld tree and now I think I covered all bad cases of Twine. Could you please commit it?

[kledzik]

Is there some C++ language feature we can use to mark the Twine class in a way that causes a warning/error on these incorrect usages?

[ruiu]

Committed as r218088.

[rafaelauler]

Thanks, Ruiu

Kledzik, I can't think of anything...

include/lld/Core/InputGraph.h

public:
   /// \brief Casting support
   static inline bool classof(const InputElement *a) {
     return a->kind() == InputElement::Kind::File;
   }
 
   /// \brief create an error string for printing purposes
   virtual std::string errStr(std::error_code errc) {
     std::string msg = errc.message();
-    Twine twine = Twine("Cannot open ") + _path + ": " + msg;
-    return twine.str();
+    return (Twine("Cannot open ") + _path + ": " + msg).str();
   }
 
   /// \brief Get the list of files
   range<InputGraph::FileIterT> files() {
     return make_range(_files.begin(), _files.end());
   }
 
   /// \brief add a file to the list of files

lib/Passes/LayoutPass.cpp

 #define DEBUG_TYPE "LayoutPass"
 
 static bool compareAtoms(const LayoutPass::SortKey &,
                          const LayoutPass::SortKey &);
 
 #ifndef NDEBUG
 // Return "reason (leftval, rightval)"
 static std::string formatReason(StringRef reason, int leftVal, int rightVal) {
-  Twine msg =
-      Twine(reason) + " (" + Twine(leftVal) + ", " + Twine(rightVal) + ")";
-  return msg.str();
+  return (Twine(reason) + " (" + Twine(leftVal) + ", " + Twine(rightVal) + ")")
+      .str();
 }
 
 // Less-than relationship of two atoms must be transitive, which is, if a < b
 // and b < c, a < c must be true. This function checks the transitivity by
 // checking the sort results.
 static void checkTransitivity(std::vector<LayoutPass::SortKey> &vec) {
   for (auto i = vec.begin(), e = vec.end(); (i + 1) != e; ++i) {
     for (auto j = i + 1; j != e; ++j) {
static void checkNoCycleInFollowonChain(const Registry &registry,
   }
 }
 
 static void checkReachabilityFromRoot(AtomToAtomT &followOnRoots,
                                       const DefinedAtom *atom) {
   if (!atom) return;
   auto i = followOnRoots.find(atom);
   if (i == followOnRoots.end()) {
-    Twine msg(Twine("Atom <") + atomToDebugString(atom)
-              + "> has no follow-on root!");
-    llvm_unreachable(msg.str().c_str());
+    llvm_unreachable(((Twine("Atom <") + atomToDebugString(atom) +
+                       "> has no follow-on root!"))
+                         .str()
+                         .c_str());
   }
   const DefinedAtom *ap = i->second;
   while (true) {
     const DefinedAtom *next = followOnRoots[ap];
     if (!next) {
-      Twine msg(Twine("Atom <" + atomToDebugString(atom)
-                      + "> is not reachable from its root!"));
-      llvm_unreachable(msg.str().c_str());
+      llvm_unreachable((Twine("Atom <" + atomToDebugString(atom) +
+                              "> is not reachable from its root!"))
+                           .str()
+                           .c_str());
     }
     if (next == ap)
       return;
     ap = next;
   }
 }
 
 static void printDefinedAtoms(const MutableFile::DefinedAtomRange &atomRange) {