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[Marc Schütz via Digitalmars-d]

On Monday, 16 November 2015 at 14:45:35 UTC, Andrei Alexandrescu 
wrote:
> The challenge is proving that a mutation is not observable. Got 
> an attack on that? -- Andrei

Here's what I wrote a few days ago, without having read the 
current discussion, so it's not a perfect fit:
http://wiki.dlang.org/DIP85

For the case of lazy initialization, assigning a member exactly 
once is safe, if it hasn't been read and returned before. The 
compiler can check this by automatically inserting a few asserts. 
The rules in the DIP aren't completely water-tight, but they are 
a good approximation.

But for members that have to be mutated several times (e.g. 
refcount), this obviously isn't useful. I think we can make this 
@system, thereby forcing implementors to write @trusted code and 
(hopefully) consider the consequences thoroughly. But the 
compiler can also assist with that. For example, it could detect 
whether a "privately mutable" value, or something depending on 
it, is returned from the function, and reject that.

Maybe the implementation for the two kinds of use-cases (write 
once vs write multiple times) can be unified, too.