It's about clarity of intent at the call site. Passing by mutable ref looks like `foo`, same as passing by value, but passing mutability of a value by pointer i

It's about clarity of intent at the call site. Passing by mutable ref looks like `foo`, same as passing by value, but passing mutability of a value by pointer is textually readably different: `&foo`. That's the purpose of the pass by pointer style. You could choose to textually "tag" passing by mutable ref by passing `&foo` but this can rub people the wrong way, just like chaining pointer outvars with `&out_foo`.