"You could imagine some hypothetical language in which all objects are immutable, as far as the programmer is concerned - however the compiler is allowed to reu

"You could imagine some hypothetical language in which all objects are immutable, as far as the programmer is concerned - however the compiler is allowed to reuse objects behind the scenes if there is provably no way that a side-effect could be introduced." There are functional languages that do this. Like many other optimizations, you get less than your intuition might naturally expect, but it does work. A trivial case is "tail call optimization", which most immediately involves not creating a literal in-memory stack record for recursive invocations but also can easily involve the compiler re-using the memory for the arguments. IIRC Erlang will do that as much as it can in tight recursive calls.