>> I have checked out a copy of the ports tree from CVS, and I am trying to install rust.
>> I am running OpenBSD 7.5 and not -current.
> 
> Try something simple and quick first. math/moo is a good one.
> 
>> However, I face the following error on running make install
>> pkg_add: pkg_add must be run as root
>>
>> Do I then run doas make install? Or is it a bug?
> 
> Neither. Setup sudo or doas so that your user can run things as root
> and pass environment variables through, and set the SUDO variable in
> /etc/mk.conf.
> 
> e.g.
> 
> for sudo, have an entry like "yourusername ALL = SETENV: ALL"
> and set "SUDO=sudo -E"
> 
> for doas, have an entry like "permit keepenv persist yourusername"
> and set "SUDO=doas"
> 

I find the documentation unclear on this point. The Porter's Handbook, 
in the section "Configuration of the Ports System" [1], briefly states:

"... ports can be built as a user, but must be installed by root or with 
doas(1)."

However, it does not provide further details on the matter.

[1]: https://www.openbsd.org/faq/ports/ports.html#PortsConfig