In vibe.d's diet templates it is possible to use

"result: #{funktion_call(variable)} " 

inside,
so that in the end

"result: "~function_call(variable)!toString~" " 

or something similar is evaluated.

How to use this in functionality in an other context?
Where in the vibe.d sources I can find this part of evaluation?

Any hint for me?
Regards mt.
ps. First I thought it is an D feature...