libexpr: quote reserved keys when printing

This fixes a bug in commands like `nix eval' which would emit invalid attribute
sets if they contained reserved keywords such as "assert", "let", etc.

These keywords will not be quoted when printed, making them valid expressions.
All keywords recognized by the lexer are quoted except "or", which does not
require quotation.

src/libexpr/eval.cc

@@ -94,7 +94,6 @@ RootValue allocRootValue(Value * v)
 #endif
 }
 
-
 void Value::print(const SymbolTable & symbols, std::ostream & str,
     std::set<const void *> * seen) const
 {
@@ -122,7 +121,13 @@ void Value::print(const SymbolTable & symbols, std::ostream & str,
         else {
             str << "{ ";
             for (auto & i : attrs->lexicographicOrder(symbols)) {
-                str << symbols[i->name] << " = ";
+                // Quote reserved keywords so that the output can be
+                // re-evaluated later without upsetting the lexer.
+                if (isReservedKeyword(symbols[i->name])) {
+                    str << "\"" << symbols[i->name] << "\" = ";
+                } else {
+                    str << symbols[i->name] << " = ";
+                }
                 i->value->print(symbols, str, seen);
                 str << "; ";
             }