Changing an alias isn't the kind of mutability that would allow high performance, though.

That is not mutability.

It is though. A mutable state layer written on an immutable backend still has all the pitfalls of a native mutable data structure.

Take the context into consideration: this whole thread is about performance and the comment on mutability was speaking of mutability in place, because that matters for performance.

> It is though.

No. That can trivially be inferred from SSA being a thing, mutable bindings can trivially and automatically be converted to immutable ones.