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

[Adam D. Ruppe]

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.