Making RCSlice and DIP74 work with const and immutable

[Andrei Alexandrescu via Digitalmars-d]

Tracing garbage collection can afford the luxury of e.g. mutating data 
that was immutable during its lifetime.

Reference counting needs to make minute mutations to data while 
references to that data are created. In fact, it's not mutation of the 
"useful" data, the payload of a data structure; it's mutation of 
metadata, additional information about the data (i.e. a reference count 
integral).

The RCOs described in DIP74 and also RCSlice discussed in this forum 
need to work properly with const and immutable. Therefore, they need a 
way to reliably define and access metadata for a data structure.

One possible solution is to add a "@mutable" or "@metadata" attribute 
similar to C++'s keyword "mutable". Walter and I both dislike that 
solution because it's hamfisted and leaves too much opportunity for 
abuse - people can essentially create unbounded amounts of mutable 
payload for an object claimed to be immutable. That makes it impossible 
(or unsafe) to optimize code based on algebraic assumptions.

We have a few candidates for solutions, but wanted to open with a good 
discussion first. So, how do you envision a way to define and access 
mutable metadata for objects (including immutable ones)?


Andrei