Why is rehash not @safe?

[via Digitalmars-d-learn]

On Saturday, 30 August 2014 at 17:31:54 UTC, monarch_dodra wrote:
> On Saturday, 30 August 2014 at 14:27:04 UTC, Nordlöw wrote:
>> I just noticed that AA rehash is @system. Is there a reason 
>> for this? Is it system because bad things can happen or simply 
>> because it's a low level function? Should I always tag 
>> functions calling rehash as @trusted?
>
> AFAIK, the whole problem is one of attributes, and run-time 
> inference.
>
> AA's are mostly run-time implemented. When you have a U[T], and 
> you want to rehash, then the AA will make a run-time call to 
> typeinfo(T).hash();

AFAICS, it doesn't:
https://github.com/D-Programming-Language/druntime/blob/master/src/rt/aaA.d#L355-L412

The computed hash is cached in the buckets. It doesn't even 
access the typeid that it gets passed from the user-facing 
rehash().

This means that _aaRehash() can probably marked as @trusted; 
rehash() will then be automatically inferred as @safe, because 
it's a set of templates.

>
> The issue is that here, you need to support *all* of the hash 
> function for *all* of the T types.
>
> If you make rehash @trusted, then you may end up calling 
> @system hash functions in a @safe context.
>
> If you make it @safe, then you either break code, or make it 
> impossible for end users to provide their @system hash 
> functions.
>
> Really, it's lose-lose. The only (AFAIK) solution is to migrate 
> AA's to a template-library that individually infers the correct 
> safety for every types.