Introducing Nullable Reference Types in C#. Is there hope for D, too?

[Jonathan M Davis]

On Friday, November 17, 2017 15:05:48 Timon Gehr via Digitalmars-d wrote:
> On 17.11.2017 12:22, Jonathan M Davis wrote:
> > On Friday, November 17, 2017 09:44:01 rumbu via Digitalmars-d wrote:
> >> I know your aversion towards C#, but this not about C#, it's
> >> about safety. And safety is one of the D taglines.
> >
> > Completely aside from whether having the compile-time checks would be
> > good or not, I would point out that this isn't actually a memory safety
> > issue.
> Memory safety is not the only kind of safety. Also, memory safety is
> usually formalized as (type) preservation which basically says that
> every memory location actually contains a value of the correct type.
> Hence, as soon as you have non-nullable pointers in the type system,
> this _becomes_ a memory safety issue.

This is definitely not how it is viewed in D. Walter has repeatedly stated
that dereferencing a null pointer is considered @safe, because doing so will
not corrupt memory or access memory that it should not access - and that's
all that @safe cares about. Whether there's an object of the correct type at
that location or not is irrelevant, because it's null. You do have a memory
safety issue if you somehow make the pointer or reference refer to an object
of a different type than the reference or pointer is allowed to point to,
but doing that requires getting around the type system via casting, which
would not be allowed in @safe code, and badly written @trusted code can
always screw up @safe code. Regardless, given that dereferencing null will
segfault, it does not present an @safety problem.

The only issue with dereferencing a null pointer in @safe code is that if
the type is sufficiently large (larger than a page of memory IIRC), you
don't actually get a segfault, and that hole does need to be plugged by
having the compiler add runtime checks where needed. But most null
pointers/references do not have that problem.

- Jonathan M Davis