A way to know what modules a module has used in Elixir.

1-uses_tracker.ex

# For more elaborate use cases, see the IndirectUsesTracker instead:
# https://gist.github.com/christhekeele/fc4e058ee7d117016b9b041b83c6546a

###
# A way to know, at runtime, what modules a module has used at compile time.
# In this case, you include `UsesTracker` into a module. When that module gets
# used in some other module, it registers itself with the other module.
##
defmodule UsesTracker do
  
  defmodule Registry do
    defmacro __before_compile__(_) do
      quote location: :keep do
        def uses do
          @uses
        end
      end
    end
  end
  
  defmacro __using__(module) do
    quote location: :keep do
      @before_compile Registry
      Module.register_attribute(__MODULE__, :uses, accumulate: true)
      Module.put_attribute(__MODULE__, :uses, unquote(module))
    end
  end
  
end

2-example.exs

defmodule A do
  
  defmacro __using__(_) do
    quote location: :keep do
    
      use UsesTracker, unquote(__MODULE__)
      
      # Your code here...
    end
  end
  
end

defmodule B do
  
  defmacro __using__(_) do
    quote location: :keep do
    
      use UsesTracker, unquote(__MODULE__)
      
      # Your code here...
    end
  end
  
end

defmodule C do
  use A
end

defmodule D do
  use A
  use B
end

C.uses #=> [A]
D.uses #=> [B, A]