The discoverability of Elixir's Libraries would be tremendously improved if autocompletion would work - afaik inf-elixir is quite important, as it is the only Elixir REPL for Emacs. Maybe get the language server autocompletions provided by eglot and "pipe" them into completion-in-region, when inside a Mix project.

(I'm not quite sure if this counts as a duplicate, since that one is about getting the tab-completions from the Elixir shell).