What is the Status of __mutable ?

[Nicholas Wilson]

On Thursday, 20 December 2018 at 10:47:18 UTC, Vijay Nayar wrote:
> During the DConf of 2018, there was a talk about possibly 
> introducing __mutable into the D Language, which has utility in 
> allowing synchronization objects like locks, reference counts, 
> and lazy initialization to be used for objects that are 
> otherwise const or immutable. It was also stated that reference 
> counts are already being maintained by the "monitor", which I 
> know little about other than that it's built into the language 
> and that user code does not have the same privileges.
>
> However, the __mutable feature could open up problems in terms 
> of the compiler's ability to optimize generated code and 
> knowing which CPU L1/L2 caches are valid or not.
>
> What is the current status of this work? Have there been any 
> significant or unexpected hurdles that have come up? Any major 
> accomplishments?
>
> The most significant reason for my question is my own ignorance 
> of the normal communication channels in use. Apologies for the 
> spam if this information has already been shared elsewhere, but 
> a link to that information would be appreciated.

There is an implementation laying about in the PR queue: 
https://github.com/dlang/dmd/pull/8315

IIRC the motivating examples for __mutable were:
* Monitor for immutable classes
* Refcount fields for RC objects
* References to allocators in immutable containers

Immutable monitors can use a singleton mutex.
For RC objects I'd rather see opHeadMutable implemented (const 
T!B <-> T!(const B) ).
I'm not convinced of the utility of immutable containers (not to 
be confused with containers with immutable elements which are 
useful), see also opHeadMutable.