Isn't the entire point of an optimizer to convert "bad code" into "good code"?
Your proposed solution is to have the user manually implement a hash table, but if you have a good optimizer, users can focus on writing clear code without bugs or logic errors and let the machine turn that into efficient code.
Your proposed solution is to have the user manually implement a hash table, but if you have a good optimizer, users can focus on writing clear code without bugs or logic errors and let the machine turn that into efficient code.