Rebooting the __metada/__mutable discussion

[vit]

On Friday, 8 April 2022 at 18:00:14 UTC, rikki cattermole wrote:
> Here is my test code that I used to determine what I needed to 
> do to make const RC types useless (and hopefully safe).
>
> ```d
> ...
> ```

I implemented shared_ptr/rc_ptr with this functionality (non 
copyable const rc ptr):
https://github.com/submada/btl/blob/master/source/btl/autoptr/shared_ptr.d
https://github.com/submada/btl/blob/master/source/btl/autoptr/rc_ptr.d

It is in package btl:autoptr 
(https://code.dlang.org/packages/btl).

Example:
```d
SharedPtr!(const int) sp = SharedPtr!int.make(42);

const SharedPtr!(const int) csp1 = sp;  //OK
const SharedPtr!(int) csp2 = sp;  //OK

sp = csp1;   //ERROR
sp = csp2;   //ERROR

//RcPtr is same.

```
Ref counted pointers in D have problem:

RAII in D is bad: move ctor are not implemented, overloading copy 
ctors sometimes crash dmd, copy ctors are not called from GC 
slices, dtors have bad interaction with stack tuples/value 
sequences and opCast, emplace...


Simple ref counted pointers have also this problem:
```d
Rc!(const int) rc1 = Rc!(int).make(42);
Rc!(const int) rc2 = Rc!(immutable int).make(42);

```

Has `Rc!(const int)` atomic counting or not? :)