Want reasonable reference counting? Disable automatic sharing of immutable

[Steven Schveighoffer]

On 11/14/21 2:16 AM, Walter Bright wrote:
> On 11/12/2021 4:31 AM, Steven Schveighoffer wrote:
>> One of the prerequisites to doing reference counting is to have a 
>> mutable piece of data inside an immutable piece of data.
> 
> Or maybe just give up on having immutable ref counted objects. Ref 
> counted objects are mutable.

First, I'll note that `__mutable` on its own is perfectly sound. The 
biggest problem is that `immutable(T)` no longer would mean "fully 
immutable", which removes a lot of optimizations (including strong-pure) 
-- even in the case of a type that doesn't have any `__mutable` in sight.

I'm fine with giving up on it. But... 
https://forum.dlang.org/post/smc5jb$2u8t$1@digitalmars.com

	- work with qualifiers like T[] does, both RCSlice!(qual T) and 
qual(RCSlice!T)

...

	2. Qualifiers compound problems with interlocking: mutable data is 
known to be single-threaded, so no need for interlocking. Immutable data 
may be multi-threaded, meaning reference counting needs atomic 
operations. Const data has unknown origin, which means the information 
of how data originated (mutable or not) must be saved at runtime.

We need a change of requirements if we want to give up on immutable 
reference counting.

-Steve