Allocator-aware @safe reference counting is still not possible

[Nick Treleaven]

On Sunday, 22 January 2023 at 15:28:53 UTC, Atila Neves wrote:
> I'm pretty much convinced we need isolated. This is very 
> similar to why the language as it exists today doesn't allow a 
> library author to write a vector type that can be appended to, 
> which... is the main reason one would use a vector to begin 
> with.
>
> Some allocators (GC?) might have a @safe deallocate function 
> but most (all except the GC?) can't due to aliasing, and that 
> requires isolated.

`isolated` would be nice, but for now we can model it with a 
struct so that this works:
```d
class Mallocator : IAllocator
{
     import core.stdc.stdlib : free, malloc;

     void* safeAllocate(size_t n) @trusted
     {
         return malloc(n);
     }

     void safeDeallocate(Isolated!(void*) ip) @trusted
     {
         ip.unwrap.free;
     }
}

void main()
{
     IAllocator a = new Mallocator;
     scope m = a.safeAllocate(4);
     auto ip = (() @trusted => 
assumeIsolated(a.safeAllocate(4)))();
     a.safeDeallocate(ip.move);
     assert(ip.unwrap == null);
}
```
Working code:
https://github.com/ntrel/stuff/blob/master/typecons/isolated.d

Isolated could go in std.typecons.