[Issue 24584] [phobos] `make unittest` should not rerun tests unnecessarily

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24584

--- Comment #5 from kinke <kinke at gmx.net> ---
(In reply to Nick Treleaven from comment #3)
> I have an old PC (I don't use -j as I only have 2 cores and 1 I keep for
> other things):
> 
> time make unittest
> ...
> make[1]: Leaving directory '/home/nick/git/phobos'
> 
> real	16m57.602s
> user	15m21.911s
> sys	1m0.822s

Oh wow. Then the issue most likely isn't about *running* a subset of unittests
only, but improving incremental builds of the unittest runners. Currently,
changing a single Phobos source module leads to a complete rebuild (recompiling
all Phobos unittest object files):
https://github.com/dlang/phobos/blob/ad9f87d1b816783bd8c13f461490e68f7fcae20a/Makefile#L400-L401

The Makefiles don't exploit the compiler `-makedeps` switch yet. That would
allow only recompiling object files which are dirty (directly or indirectly
importing a changed Phobos module).

--