Improvement in pure functions specification

[John Colvin via Digitalmars-d]

On Wednesday, 21 December 2016 at 15:40:42 UTC, Andrei 
Alexandrescu wrote:
> On 12/20/2016 05:49 PM, Andrei Alexandrescu wrote:
>> https://github.com/dlang/dlang.org/pull/1528 -- Andrei
>
> Dropped the void functions. On to the next scandal:
>
>>A function that accepts only parameters without mutable 
>>indirections and
>>returns a result that has mutable indirections is called a $(I 
>>pure factory
>>function). An implementation may assume that all mutable memory 
>>returned by
>>the call is not referenced by any other part of the program, 
>>i.e. it is
>>newly allocated by the function.
>
>
> Andrei

There are 3 levels:
1) no idea what's going on: e.g. the function returns a mutable 
reference and also reads from global mutable memory.
2) memory must be new: e.g. returns 2 mutable references, no 
accessing external mutable memory.
3) memory must be new and uniquely referenced: function returns 1 
mutable reference, does not access external mutable memory.

If I'm not mistaken only 3 enables anything useful like implicit 
casts to immutable.

Also, "returned references" should be extended to include "out" 
parameters, because there's no difference as far as memory 
uniqueness is concerned.