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[Andrei Alexandrescu via Digitalmars-d]

On 11/16/2015 08:51 AM, Marc Schütz wrote:
> On Monday, 16 November 2015 at 02:26:29 UTC, Andrei Alexandrescu wrote:
>> Yah, I agree with that argument. Probably @mutable is a more
>> principled way to go about things.
>
> Glad to here that. I think the current transitive const system is really
> good and shouldn't be watered down beyond necessity. And a @mutable
> keyword, too, shouldn't just mean "immutability or const-ness end here",
> thus allowing any kind of mutation. What we actually need for
> immutable/const refcounting etc. is _non-observable mutation_, i.e.
> physical mutability, but without observable effects outside of the
> type's implementation (better yet, restricted to very short parts of it,
> just like @trusted).

The challenge is proving that a mutation is not observable. Got an 
attack on that? -- Andrei