What is the Status of __mutable ?

[Vijay Nayar]

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.