Enhancements can enable memory-safe reference counting

[vitoroak]

On Wednesday, 26 May 2021 at 22:06:27 UTC, tsbockman wrote:
> On Wednesday, 26 May 2021 at 21:48:40 UTC, Paul Backus wrote:
>> On Wednesday, 26 May 2021 at 18:53:21 UTC, vitoroak wrote:
>>> Every time I tried to do something similar in D I stumbled 
>>> across the same problems and as far as I know it's not 
>>> possible to implement it completely @safe today. I think one 
>>> of the problems is that you can manually destroy/move any 
>>> struct while there are still references/pointers to it or its 
>>> internals like in the example below (I used your borrow mixin 
>>> template).
>>
>> In theory, these examples are fine, since they result in a 
>> null dereference,
>
> No. That's what I thought at first, too, but if you walk 
> through the code more carefully you will see that `x1` never 
> gets set to `null`, and still points to the old target of `u1`. 
> So, he is correct.
>
> I've opened [issue 
> #21981](https://issues.dlang.org/show_bug.cgi?id=21981) 
> requesting a fix.

I saw you mentioning breaking things in @safe code. This example 
let you access an invalid pointer without no @trusted code and 
heap allocation, only @safe code.

```d
struct IntRef {
	int* ptr = void;

     this(return scope int* p) @safe {
     	ptr = p;
     }

     int* borrow() return scope @safe {
		return ptr;
     }
}

void main() @safe {
     import std.stdio: writeln;

     auto x = 1;
     auto r = IntRef(&x);

     writeln(*r.borrow);

	destroy!true(r);

     writeln(*r.borrow);
}
```