I've tried to debug another issue and added a https_proxy config setting.
it was written to the hex.config but I got an error that my global rebar3.config was existing but unreadable as a consequence
the proxy setting is passed to http_uri:parse which fails if you pass it a binary because it is expecting a string. You should add a type conversion there. rebar3_http_hex.erl:123
I've tried to debug another issue and added a https_proxy config setting.