About structs and performant handling

[Namespace]

> Here is what I think. How the thing is actually passed don't 
> matter. What does matter is that the semantic is know : ie that 
> the program will behave in a known way.
I agree.

> As a consequence, if the compiler choose to pass by ref instead 
> of passing by value as an optimization, it must do so only if 
> it can prove that the resulting code will do the same thing.
And how could it be proved? IMO with const (not mutable) scope 
(no escaping). What are your thoughts?