Better way to append to array than ~= ?

[Vladimirs Nordholm]

On Tuesday, 3 April 2018 at 19:53:11 UTC, Meta wrote:
> On Tuesday, 3 April 2018 at 19:02:25 UTC, Vladimirs Nordholm 
> wrote:
>> [...]
>
> In this specific case, since you know the length of `Args`, you 
> can pre-allocate an array of that size and loop through it 
> doing your initialization.
>
> However, if you want really performant code, you should 
> allocate a static array on the stack outside of the function 
> and pass it in as a buffer.

I don't think I know the size of the arguments.

If I pass in "123" and MySpecialType('a'), the result should be:

     assert(foo("123", MySpecialType('a')) == [MySpecialType('1'), 
MySpecialType('2'), MySpecialType('3'), MySpecialType('a')]);

What should the length of the pre-allocated array be?