Enhancements can enable memory-safe reference counting

[vitoroak]

On Friday, 14 May 2021 at 00:45:09 UTC, tsbockman wrote:
> [snip]
>
> I think D is very close to being able to sanely express `@safe` 
> reference counting APIs. I don't think `@live` is necessary; 
> rather, we just need to complete `scope` and `return` and fix 
> some RAII related bugs. For performance reasons, move operators 
> and some minor changes to the GC would also be good, but are 
> not actually required.
>
> Destroy?

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).

```d
void receiveByValue(Unique!(int*) u) @safe {
}

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

	auto u1 = Unique!(int*)(true);
     mixin borrow!(u1, "x1");
     writeln(*x1); // ok
     destroy(u1);
     writeln(*x1); // should not be possible

     import core.lifetime: move;

     auto u2 = Unique!(int*)(true);
     mixin borrow!(u2, "x2");
     writeln(*x2); // ok
     receiveByValue(move(u2));
     writeln(*x2); // should not be possible
}

```

I don't know how this could be solved but for me it's a blocker 
to do a @safe Unique or RC type. Maybe if I always return an 
RCRef or something like this but I think the overhead would be 
too big.