Compile-time global mutables

[bauss]

On Thursday, 30 June 2022 at 12:26:54 UTC, HuskyNator wrote:
> I've found several instances in which incrementation during 
> compile time is required.
> One example is generation of unique ID's (although there might 
> be some workarounds), another interesting idea the registration 
> of a function's call-count to some image loader;
>> Anyone know of a way in c++ to track static resource usage at 
>> compile time, basically i am trying to find a way so an 
>> routine can just do Image("foobar.jpeg2000"), at an arbitrary 
>> point in operation and I'll make sure the resources are 
>> resident before the Job gets run.
>
> Although this references c++, I have attempted to make this 
> work in D. It seems there are no workarounds that would make 
> this work sadly. (I've even tried writing to files instead but 
> this is not possible during ctfe)
>
> Essentially speaking, there are numerous instances in which 
> generating a list or incrementing a value is useful during 
> compile-time. I find this should be possible.
>
> One main argument against this seems to be the 
> non-deterministic nature of this, though this is not an issue 
> in virtually all cases in which doing this is useful or 
> necessary I find. Essentially speaking when order does not 
> matter and membership is only required during run-time.
>
> Are there other limiting factors preventing this possibility? 
> It would allow for some powerful things that currently seem 
> impossible to implement (save for some case-specific 
> workarounds).

You can already do that.

```d
enum counter(size_t x = [__traits(allMembers, 
mixin(__MODULE__))].length)=x;
char[] member(ulong x)
{
     char[] buf = "void[0] _X0000000000000000;".dup;
     enum mapping = "0123456789abcdef";
     foreach_reverse(i; buf.length-17 .. buf.length-1)
     {
         buf[i] = mapping[x & 0xf];
         x >>= 4;
     }
     return buf;
}
mixin template increment()
{
     mixin(member(counter!()));
}

pragma(msg, counter!());
mixin increment;
pragma(msg, counter!());
mixin increment;
pragma(msg, counter!());
```

Credits: p0nce