Proof of concept for v2 - NO duplication, NO `static if` hell, NO difficulty with interoperability

[Paul Backus]

On Friday, 5 November 2021 at 14:08:58 UTC, Andrei Alexandrescu 
wrote:
> 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.

I'm pretty sure at least some of us (including Atila) know how.

In order to manually free memory in @safe code, you need a way to 
guarantee that the memory is not aliased. To do that, you either 
need runtime checks or compile-time checks. Runtime checks are 
possible in the current D language, but compile-time checks are 
not. So, if we want @safe @nogc containers without runtime 
overhead, a new language feature is required.

Unfortunately @live will not help with this, because it is 
possible (by design!) for @safe code to violate the invariants of 
@live.

One idea that's come up recently in the community Discord is a 
`unique(T)` qualifier, which would guarantee that any memory 
reachable via indirections in a given `T` value is not aliased. 
Exactly how to enforce this is an open question.