Want reasonable reference counting? Disable automatic sharing of immutable

[Elronnd]

On Sunday, 14 November 2021 at 21:59:23 UTC, Timon Gehr wrote:
>> I assumed that 'outside of the type system' meant 
>> 'language-level support for reference counting'; a a type with 
>> opaque representation, like a hash table.
>> ...
>
> Yes, I guess that's another way to go about it, but I think 
> less language magic is better. Especially with reference 
> counting, you may want to have detailed control over the 
> allocator and/or the strategy for storing reference counts.

Definitely agree!

That said I think there may be a middle ground, involving some 
magic but also some control.  In particular, compared with 
__mutable, language-level support for intrusive rc can be sound 
without leading to surprising results, as long as you're not 
allowed to read from the reference count.  Saying 'build me a 
reference-counted object from this chunk of memory' should be 
pretty easy, and ditto specifying how the memory is disposed of 
once the reference count reaches 0.  Being polymorphic over 
intrusive/extrusive representation is harder, though.