Allocator-aware @safe reference counting is still not possible

[Timon Gehr]

On 1/28/23 16:56, Nick Treleaven wrote:
> 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.

Isolated is not sufficient, you also have to guarantee the pointer was 
allocated with `malloc`.