DIP 1027---String Interpolation---Format Assessment

[aliak]

On Monday, 24 February 2020 at 21:23:43 UTC, Adam D. Ruppe wrote:
> On Monday, 24 February 2020 at 20:55:16 UTC, Walter Bright 
> wrote:
>> and proposed a lowering to:
>>
>> > i"your hex data is ${%02x}someByte"
>> >
>> > (_d_interpolated_string!("your hex data is ",
>> > _d_interpolated_format_spec("%02x"))(), someByte)
>
> Do you understand that `_d_interpolated_string` and 
> `_d_interpolated_format_spec` are to be defined EXCLUSIVELY 
> inside druntime?
>
> There's nothing user-defined about this.

Does that mean no betterC support if it's in druntime?

Actually, was it different with the pure tuple approach in 
DIP1027?