Per reviewer request, cast the other side.

I don't think this matters.  As long as one or the other of these is
a std::string, I get an operator== that looks at content rather than
pointer equality.  I picked casting the constant over casting the dynamic
thing in hopes that the compiler would have a better chance at optimizing
away any runtime cost.

Deferring to reviewer.

nixos/modules/installer/tools/nixos-option/nixos-option.cc

@@ -140,7 +140,7 @@ bool isOption(Context * ctx, const Value & v)
         if (evaluated_type.type != tString) {
             return false;
         }
-        return evaluated_type.string.s == static_cast<std::string>("option");
+        return static_cast<std::string>(evaluated_type.string.s) == "option";
     } catch (Error &) {
         return false;
     }