Is it hard to make pointee data mutable?
E.g. if have:
------
struct RCString
{
    private char[] data;
    private @mutable int* counter;
}
------
So for optimiser (in case of immutable) this looks like
------
struct RCString
{
    private char[] data;
    private @mutable void* counter; // pointer to garbage
}
------

On Sunday, 7 February 2016 at 14:00:24 UTC, Iakh wrote:

Explanations:
As far as "immutable" transitive:
------
immutable RCString str;
*str.counter++; // Impossible/error/undefined behavior(with const cast)
------
Language defines immutable to do some optimizations based on true
constness of str, fields, and variables pointed by fields. But if
pointer would be treated by optimizer(not by GC) as void* (or size_t)
pointee "true constness" does not matter.

The only drawback is the immutable function that reads @mutable field
can't be @pure because it reads "global variable".