@pf_moore Responding here, since it’s OT for that thread.
It’s a bit of a catch 22 – you need the installer to be installed in the environment (i.e. pip today) or be capable of installing things in an environment specified to it. There’s no way to know which it is and so, putting the constraint of “tool is in charge of setting up environment when building with tool” is OK. Non-pip tools can use pip for the setup if they want to.
FWIW, I don’t disagree that having flexibility in build environment installers is a bad idea – I just don’t know how things would work in that model, and I’m keeping things simple for now, especially on that thread.
I did use “should” instead of “must” for exactly this reason there. 