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, as well as "import"
and "throw".

src/libexpr/eval.cc

@@ -92,6 +92,16 @@ RootValue allocRootValue(Value * v)
 #endif
 }
 
+// Returns `true' is a string is a reserved keyword which requires quotation
+// when printing attribute set field names.
+static inline bool isReservedKeyword(const std::string_view str)
+{
+    static const std::unordered_set<std::string_view> reservedKeywords = {
+        "if", "then", "else", "assert", "with", "let", "in", "rec", "inherit",
+        "or", "throw", "import"
+    };
+    return reservedKeywords.contains(str);
+}
 
 void Value::print(const SymbolTable & symbols, std::ostream & str,
     std::set<const void *> * seen) const
@@ -128,7 +138,13 @@ 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 << "; ";
             }