The general solution here is to, instead of having code like this:

if Mix.env == :prod do
  # prod stuff
else
  # other stuff
end

Do this:

if Application.get_env(:my_app, :some_relevant_flag) do
  # stuff related to that flag
else
 # other stuff
end

Then in your configuration you can choose to set that flag to true or false.

If you don’t call Mix.env() at runtime, you can set it up at compile time.

There are multiple ways to do that.

use module attribute

@env Mix.env()

Then, read @env as a normal module attribute.

use config

config :my_app, compile_env: Mix.env()

Then, get compile_env with Application.get_env(:my_app, :compile_env).

If you want to call Mix.env() at runtime, just do not do that, like @hauleth said.