Can non-nullable references be implemented as a library?

[Jonathan M Davis]

On Sunday, November 07, 2010 18:42:08 Adam Burton wrote:
> 1. Default struct constructor.
> This means the NN can be created without assigning a value. I have tried to
> get around this issue somewhat by adding a null check in the invariant but
> it seems the invariant is not called when using the default constructor.
> Should it be or should it not? If it is then atleast while contracts are
> enabled we get to know about uninitialised NNs.

There are no default constructors for structs. They use init, which is known at 
compile-time, so there is no constructor for the invariant to be called after. 
It's quite easy to create structs which pass their invariant when you construct 
them with a constructor but whose init fails the invariant. The fact that the 
invariant gets called before opAssign() doesn't help either: 
http://d.puremagic.com/issues/show_bug.cgi?id=5058

The result is that at this point, you pretty much either need to have init pass 
your invariant or have no invariant.

- Jonathan M Davis