Want reasonable reference counting? Disable automatic sharing of immutable

[tsbockman]

On Friday, 12 November 2021 at 18:12:14 UTC, Andrei Alexandrescu 
wrote:
> On 2021-11-12 8:03, Timon Gehr wrote:
>> On 12.11.21 13:31, Steven Schveighoffer wrote:
>>>
>>> I've come to the conclusion, in order to fix this situation, 
>>> so `__mutable` really means mutable and `shared` is an 
>>> orthogonal piece, you need to remove the implicit sharing of 
>>> `immutable`.
>> 
>> I agree, that would be much better.
>
> We discussed this a couple of times. It's interesting. Sadly at 
> this point implicit thread sharing of immutable is so baked 
> into the language, it would take a lot of care to extricate. It 
> would be very difficult even for Timon or Paul.

Just track whether an RC target needs atomic RC operations at 
compile time, using the template parameters of RC types. No 
changes to language semantics are necessary.

For example, reject attempts to assign an `RCPtr!(immutable T)` 
to an `RCPtr!(const T)`, while allowing assignment to a 
`RCPtr!(shared const T)`. Borrowed payloads can and should follow 
the normal D qualifier conversion rules.

Or, if more granularity is needed, use a separate template 
parameter:

```D
struct RCPtr(T, bool sharedMeta = is(T == shared))
```