TableGen: Add some more helpful error messages

[nhaehnle]

Summary

Some fairly simple changes to start with.

[craig.topper]

LGTM

llvm/trunk/lib/TableGen/TGParser.cpp

if (!RV)
     return Error(Loc, "Value '" + ValName->getAsUnquotedString() +
                  "' unknown!");
 
   // Do not allow assignments like 'X = X'.  This will just cause infinite loops
   // in the resolution machinery.
   if (BitList.empty())
     if (VarInit *VI = dyn_cast<VarInit>(V))
       if (VI->getNameInit() == ValName && !AllowSelfAssignment)
-        return true;
+        return Error(Loc, "Recursion / self-assignment forbidden");
 
   // If we are assigning to a subset of the bits in the value... then we must be
   // assigning to a field of BitsRecTy, which must have a BitsInit
   // initializer.
   //
   if (!BitList.empty()) {
     BitsInit *CurVal = dyn_cast<BitsInit>(RV->getValue());
     if (!CurVal)
if (!BitList.empty()) {
     V = BitsInit::get(NewBits);
   }
 
   if (RV->setValue(V)) {
     std::string InitType;
     if (BitsInit *BI = dyn_cast<BitsInit>(V))
       InitType = (Twine("' of type bit initializer with length ") +
                   Twine(BI->getNumBits())).str();
+    else if (TypedInit *TI = dyn_cast<TypedInit>(V))
+      InitType = (Twine("' of type '") + TI->getType()->getAsString()).str();
     return Error(Loc, "Value '" + ValName->getAsUnquotedString() +
                           "' of type '" + RV->getType()->getAsString() +
-                 "' is incompatible with initializer '" + V->getAsString() +
-                 InitType + "'");
+                          "' is incompatible with initializer '" +
+                          V->getAsString() + InitType + "'");
   }
   return false;
 }
 
 /// AddSubClass - Add SubClass as a subclass to CurRec, resolving its template
 /// args as SubClass's template arguments.
 bool TGParser::AddSubClass(Record *CurRec, SubClassReference &SubClass) {
   Record *SC = SubClass.Rec;
if (!EltTy) {
         TokError("No type for list");
         return nullptr;
       }
       DeducedEltTy = GivenListTy->getElementType();
     } else {
       // Make sure the deduced type is compatible with the given type
       if (GivenListTy) {
         if (!EltTy->typeIsConvertibleTo(GivenListTy->getElementType())) {
-          TokError("Element type mismatch for list");
+          TokError(Twine("Element type mismatch for list: element type '") +
+                   EltTy->getAsString() + "' not convertible to '" +
+                   GivenListTy->getElementType()->getAsString());
           return nullptr;
         }
       }
       DeducedEltTy = EltTy;
     }
 
     return ListInit::get(Vals, DeducedEltTy);
   }