Why does this mixin fail to compile?

[ryuukk_]

On Monday, 1 July 2024 at 21:43:02 UTC, Dennis wrote:
> On Monday, 1 July 2024 at 13:00:55 UTC, ryuukk_ wrote:
>> please stick to what i wrote, i don't want string 
>> concatenation, i provide a reduced example from my project, 
>> everything should be a single template block, no extra 
>> functions other than the append() one
>
> Mixin templates are a declaration scope, not a function scope, 
> so they aren't necessarily analyzed top to bottom. This is why 
> allowing partial string mixins would be complicated, consider 
> this example:
>
> ```D
> mixin template implement()
> {
>     mixin("struct " ~ str);
>
>     mixin("{");
>
>     immutable string str = "T";
>
>     mixin("}");
> }
> ```
>
> It's not obvious how this should be compiled, and this is 
> before throwing `static if` and `static foreach` in the mix! If 
> you want to procedurally build a type in sequential steps, 
> you'll have to do that in a function scope.
>
> If your concern is that such a function would add needless code 
> generation, you can use an immediately invoked anonymous 
> function like so:
>
> ```D
> mixin template implement(string typeName, string[] members)
> {
>     mixin(() {
>         string result = "struct " ~ typeName ~ " {";
>         foreach (name; members)
>         {
>             result ~= "int " ~ name ~ ";";
>         }
>         result ~= "}";
>         return result;
>     } ());
> }
>
> mixin implement!("S", ["x", "y", "z"]);
>
> immutable s = S(x: 3, y: 5, z: 7);
> ```
>
> You can use your fixed size array append function to try and 
> improve CTFE performance, but I'd start with straightforward 
> concatenation, and see if it's actually too slow. In that case, 
> maybe see if you can reduce it to a self-contained example and 
> post it on bugzilla as a performance bug.

I said it 2 times already, i don't want string concatenation, 
i'll benchmark later, but not right now, right now i'm looking 
for a functioning code without string concatenation