My Reference Safety System (DIP???)

[Zach the Mystic via Digitalmars-d]

On Thursday, 26 February 2015 at 20:46:07 UTC, deadalnix wrote:
> Consider :
>
> void foo(T** a) {
>     T** b = a; // OK
>     T*  = ...;
>     *b = c; // Legal because of your transitive clause,
>             // but not safe as a can have an
>             // arbitrary large lifetime.
> }

This example's incomplete, but I can guess you meant something 
like this:

void foo(T** a) {
     T** b = a; // OK
     T d;
     T* c = &d;
     *b = c; // Legal because of your transitive clause,
             // but not safe as a can have an
             // arbitrary large lifetime.
}

> This show that anything you reach through an indirection can 
> have from the same lifetime as the indirection up to an 
> infinite lifetime (and anything in between). When using it as 
> an lvalue, you should consider the largest possible lifetime, 
> when using it as an rvalue, you should consider the smallest 
> (this is the only way to be safe).

I'm starting to see what you mean. I guess it's only applicable 
to variables with double (or more) indirections (e.g. T**, T***, 
etc.), since only they can lose information with transitive 
scopes. Looks like we need a new rule: variables assigning to one 
of their double indirections cannot acquire a scope-depth greater 
than (or lifetime less than) their current one. Does that fix the 
problem?