Why is rehash not @safe?

[monarch_dodra via Digitalmars-d-learn]

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();

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.