Why does this mixin fail to compile?

[Lance Bachmeier]

On Wednesday, 3 July 2024 at 03:52:41 UTC, Steven Schveighoffer 
wrote:

> ```d
> mixin template implement()
> {
>     mixin(() {
>         // ctfe new array is basically the same as static array
>         char[] buffer = new char[4096];
>         int pos = 0;
>
>         void append(string str)
>         {
>             buffer[pos .. pos + str.length] = str[];
>             pos += str.length;
>         }
>
>         append("void* ctx;");
>         return buffer[0 .. pos];
>     }());
> }
> ```
>
> And yes, it is faster to do this than appending. But you have 
> to do a *lot* of it to make a huge difference.

What about creating an array of strings and then using join? One 
obvious problem with this solution is the requirement to specify 
the size of `buffer`.