Shouldn't casting an object to void* be considered @safe?

[Steven Schveighoffer]

On 12/14/19 6:48 PM, Joseph Rushton Wakeling wrote:
> On Saturday, 14 December 2019 at 20:53:49 UTC, Dennis wrote:
>>> No, that won't do.  What if you cast from a `ulong` to a `void*`?
>>
>> That is `@safe`, unless there is a way to corrupt memory in `@safe` 
>> code by doing that.
> 
> No, it is not @safe, and for good reason.  When you cast an integral 
> value to a `void*` that value gets reinterpreted as a memory address.  
> But you have absolutely no right to assume that it is a valid memory 
> address.

It's not technically unsafe, but is prone to safety problems. It's true 
that a void * is completely unusable in safe code. But loads of code has 
both safe and unsafe parts.

Not to mention that the GC makes different decisions based on whether a 
type contains a pointer or not. I think we are reasonably fine leaving a 
rule in place to prevent casting from a non reference type to a void *.

However, a class reference is very similar to a pointer, we should allow 
that cast (to void * only).

> 
> Things like this are WHY the spec has the rule that one cannot cast from 
> a non-pointer type to `void*` in code marked @safe.

The spec rule is likely not focused on void * or classes, but really 
something like:

auto p = cast(int*) 8;

which then can be used in safe code to do damage:

*p = 5;

-Steve