Comment by rlpb

Placing various artifacts (eg. build artifacts) inside the source tree always seemed like a historical mistake to me. It leads to various accidents such as people checking in their credentials and accidentally bundling such files in source distributions, for example. These consequences are real.

Debian build tooling places build artifacts in the parent directory on the assumption that this is acceptable, but it then surprises people since it's not the norm anywhere else.

Perhaps this ship has sailed. But I think it's worth pointing out that if you have an option, don't design things that place things inside the source tree if you can avoid it.