Proposal: Relax rules for 'pure'

[Don]

Walter Bright wrote:
> Steven Schveighoffer wrote:
>>> A pure function also cannot modify any data via its parameters. In 
>>> other words, its parameters must be transitively const.
>>
>> Yes, a strongly pure function must have those traits.
> 
> No, that would be a weakly pure one.

You're operating with a different definition. There are actually three 
levels. For extra clarity, let's split strongly-pure into two:

immutably pure == all parameters are immutable
const pure == all parameters are const
weakly pure == no access to globals

The interestingly thing is that an immutably pure function can safely 
call a weakly-pure function. The only things the weakly pure function 
will be able to change, are variables which are local to the immutably 
pure functions.

The *only* difference between weakly pure and immutably pure is the 
guarantees which are made about the parameters. And this is already 
covered by the const modifiers in the function signature. So that 
concept doesn't actually need to be part of pure.

Note that this is possible only because immutable and const are 
transitive. It wouldn't be possible with head-const, or logical const.
Transitive const gives us an absolutely huge win.