Challenge: write a reference counted slice that works as much as possible like a built-in slice

[Atila Neves]

On Monday, 8 November 2021 at 21:42:12 UTC, Andrei Alexandrescu 
wrote:
> This was prompted by this exchange with Paul Backus that went 
> like this:
>
> Me: "We never got reference counting to work in safe pure nogc 
> code. And we don't know how. If we did, we could write a 
> slice-like type that does everything a slice does PLUS manages 
> its own memory. This is the real problem with containers."
>
> Paul: "I'm pretty sure at least some of us (including Atila) 
> know how."

I'll hijack this thread with a slide from my DConf Online 2020 
talk, because with the current language it's impossible for a 
library writer to write a useful @safe vector struct:

```d
scope v = vector(1, 2, 3, 4);
scope s = v[];
v ~= 5;  // could reallocate here
s[3] = 42;  // oops
```

I initialy "solved" this with @trusted, but fortunately somebody 
on the forums pointed out I was being an idiot.

It might just be my biases but I think this is a more useful nut 
to crack. And the only way to crack it is to some sort of 
ownership system, since there's no way currently in D to have a 
compile-time guarantee that there's only one alias left.

"We can have @safe vectors, unless you actually want to append" 
isn't a great slogan. And vectors are clearly simpler than ref 
counted arrays.