How mutable is immutable?

[Timon Gehr]

On 01/01/2012 10:40 AM, Denis Shelomovskij wrote:
> So, I'm a function `f`, I have an `immutable(type)[]` argument and I
> want to store it for my friend `g` in an TLS variable `v`:
> ---
> string v;
> debug string sure;
>
> void f(string s) { v = s; debug sure = s.idup; }
> void g() { assert(v == sure); }
> ---
> I also store a copy of `s` into `sure` for my friend to ensure immutable
> date hasn't been mutated.
> Can my friend's assertion ever fail without breaking a type-system?
> Sure. Just consider this:
> ---
> void main() {
> auto s = "abba".idup;
> f(s);
> delete s;
> g();
> }
> ---
> Is it by-design? Looks like deleting immutable (and const because of
> implicit conversion) data should be prohibited.
> OK. Let `delete` be fixed. Can we still fail?
> ---
> void h() {
> immutable(char)[4] s = "abba";
> f(s);
> }
> void main() {
> h();
> g();
> }
> ---
> Damn! So, what can we do with it? Not sure, but I have a proposal.
>
> Fix it in language:
> * disallow `delete` of const/immutable data
> * disallow immutable data on the stack
>
> This makes data really immutable if I don't miss something. Anyway, I
> want `immutable` qualified data to be immutable without breaking a
> type-system (if one do it, its his own responsibility), so some changes
> should be made (IMHO).

You are using unsafe language features to break the type system. That is 
not the fault of the type system.

'@safe:' at the top of the program should stop both examples from 
working, it is a bug that it does not.