Why think unit tests should be in their own source code hierarchy instead of side-by-side

[Tony]

On Friday, 23 March 2018 at 22:32:50 UTC, H. S. Teoh wrote:
> On Fri, Mar 23, 2018 at 09:45:33PM +0000, Tony via 
> Digitalmars-d-announce wrote:
>> On Friday, 23 March 2018 at 20:43:15 UTC, H. S. Teoh wrote:
>> > On Friday, 23 March 2018 at 19:56:03 UTC, Steven 
>> > Schveighoffer wrote:
>> > > I've worked on a project where the testing was separated 
>> > > from the code, and it was a liability IMO. Things would 
>> > > get missed and not tested properly.
>> 
>> That's where Test Driven Development comes in.
>
> That's not an option when you have an existing codebase that 
> you have to work with.  You basically have to start out with 
> tons of code and no tests, and incrementally add them.  Having 
> to also maintain a separate test tree mirroring the source tree 
> is simply far too much overhead to be worth the effort.
>
I think that you could "Test Driven Develop" the code you are 
adding or changing.