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

[Joseph Rushton Wakeling]

On Saturday, 14 December 2019 at 18:10:20 UTC, Ola Fosheim 
Grøstad wrote:
> Sometimes you just want a set of all object identities with a 
> specific property so that you can query for that property.
>
> It is the same a the "tag"  reference type in Pony lang. It is 
> safe for multi-threading too.
>
> Say, if a thread is waiting for a set of futures to complete 
> then it can remove object identities from the set based on 
> events.

Fair enough, but note that the safety of the `void*` in those 
use-cases is still down to the specifics of how they are used.

The obligation to put a `@trusted` on the cast to `void*` for 
use-cases like this seems minor compared to the value of 
explicitly nudging the developer to verify that the use-case is 
safe.

IOW, having casts to `void*` be unsafe seems a tactic, rather 
than a principle.