The problem with that is that it would modify file twice. You can “hand craft” something like that by using entr(1), however I would say that you should use pre-save hook in your editor.

Here’s what I’m using (auto format and re-run affected tests):

watchexec -e 'ex,exs,eex,leex' -- 'mix format && mix test --stale'

You need to configure your editor to properly pick up external changes automatically and not nag you with a popup.

Your client need to support the code format action.

I know nothing about which features are supported by coc.vim. You need to consult its documentation.

Or perhaps @hauleth can help you. I know he uses vim + elixir-ls, though I do not know which client he uses.

if you have a program like say or speak, you can do really fun stuff like:

ls **/*.{ex,exs,heex} | entr sh -c "mix format; mix test && say 'tests all green' || say 'we have a problem, some test failed'"