Store mutable indirections in immutable data with this one weird trick!

[Paul Backus]

On Saturday, 13 November 2021 at 09:06:23 UTC, tsbockman wrote:
> On Saturday, 13 November 2021 at 08:32:42 UTC, Timon Gehr wrote:
>> On 13.11.21 09:30, tsbockman wrote:
>>>>
>>> 
>>> That's a compiler bug.
>>
>> Nonsense. Results of strongly pure function calls implicitly 
>> convert to immutable. Your "fix" just avoided purity inference.
>
> I see. This does not appear to be documented anywhere in the 
> spec at the moment.

The conversion itself is not documented, but the spec does 
explain why the original code is not allowed:

> A pure factory function is a strongly pure function that 
> returns a result that has mutable indirections. All mutable 
> memory returned by the call may not be referenced by any other 
> part of the program, i.e. it is newly allocated by the 
> function. Nor may the mutable references of the result refer to 
> any object that existed before the function call.

So, having a strongly-pure function return a mutable pointer to 
an existing object is forbidden.

I guess this is what we get for trying to make memory allocation 
count as "pure."