[Development] CMake UNITY_BUILD ( QTBUG-109394 )

[Edward Welbourne]

On Monday, 16 January 2023 04:49:23 PST Friedemann Kleint via Development wrote:
>>> Summmarising: we stand to gain a speed-up of compilation; particularly
>>> for clean builds like in COIN; but it requires some work. We might do a
>>> step-by step approach process enabling modules one by one. What do you
>>> think?

On 17.01.23 05:09, Thiago Macieira wrote:
>> I was going to object to it being in COIN, but considering it may improve
>> build times in the CI, especially for constrained platforms (like macOS), I
>> withdraw my objection before I even make it. I think this effort is worth it.

Christian Tismer-Sperling (17 January 2023 11:55) asked:
> What was the reason that you first considered to object to it?

While I can't be sure what Thiago was thinking of, one obvious objection
to doing this in Coin would be that: if someone changes a source file in
a way that needs a #include addition to the changed source file, but
Coin's Unity build combines that file with one that has the #include,
Coin won't notice, the change will get integrated and those doing a
non-Unity builds, or for whom the Unity build splits files up
differently, may find themselves with broken builds.  I imagine there
are other mistakes that can likewise be concealed by a Unity build.

	Eddy.