Enabling Only Top-Level Unittests

[Nordlöw via Digitalmars-d-learn]

On Monday, 21 March 2016 at 15:25:29 UTC, Adam D. Ruppe wrote:
> Yes, compiling 33,000 lines from my libs happened in about one 
> second.
>
> My experience with slow D builds tends to be that it is caused 
> by CTFE, not by scale.

These kinds of modules are very different from the ones I'm 
working on. Especially the ones in

https://github.com/nordlow/phobos-next

This because many of my algorithms/ranges/containers are 
unittested with lots of combinations of template parameters 
(types) for robustness. This inevitable requires lots of CT-logic 
evaluation and of course takes long to compile.

A compiler switch would significantly lower the iteration time in 
my workflow. 5-10 seconds => ~ 1 second.

Could somebody at least outline where in the DMD source I should 
start digging for this?