You still have local reasoning with struct semantics. You are not giving that up in exchange for performance, you get both. func newStructValue(s: Struct) ->

You still have local reasoning with struct semantics. You are not giving that up in exchange for performance, you get both. func newStructValue(s: Struct) -> Struct { s.value = 5 return s } The reasoning for this block of code is totally local to the function body. Because the only reference to `s` is in the body of the function - it cannot be more local than that.