`warning: module attribute @my_value was set but never used` when used in a specific way in a macro

[dylan-chong]

Precheck

done

Environment

• Elixir & Erlang/OTP versions (elixir --version):

Erlang/OTP 22 [erts-10.7.2.1] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]

Elixir 1.11.0-rc.0 (8e9ece0) (compiled with Erlang/OTP 21)

• Operating system: mac os 10.14.6

Current behaviour

The following code produces a warning: module attribute @my_value was set but never used warning

defmodule Macros do
  defmacro save_this_somewhere(value) do
    Module.put_attribute(__CALLER__.module, :storage, value)
    loaded_value = Module.get_attribute(__CALLER__.module, :storage)
    loaded_value |> IO.inspect(label: "loaded_value")
    nil
  end
end

defmodule Test do
  import Macros

  @my_value 1
  save_this_somewhere(@my_value)
end

Expected behaviour

No warning

---

More complex version, but original version of the bug report

Not too important to look this section now that i think about it but it's a simplified architecture of my app, which triggered the warning. Just ignore this really

Current behavior

When we:

• use a module attribute in a macro
• save the value dynamically using Module.put_attribute
• load the value dynamically using Module.get_attribute
• but don't actually output any code into the compiled module that uses the module attribute
• we get an warning: module attribute @my_value was set but never used warning.

defmodule Scratch do
  def expand_ast(ast, env) do
    Macro.prewalk(ast, fn
      {_, _, _} = node ->
        Macro.expand(node, env)

      node ->
        node
    end)
  end

  defmacro save_this_somewhere(value) do
    expanded_value = expand_ast(value, __CALLER__) # Seemingly we need this for some reason
    Module.put_attribute(__CALLER__.module, :storage, {:unquote, [], [expanded_value]})
  end

  def __after_compile__(env, _) do
    value = Module.get_attribute(env.module, :storage)

    Module.create(
      Scratch.Storage,
      quote do
        def get_stored_value() do
          unquote(Macro.escape(value, unquote: true))
        end
      end,
      __ENV__
    )

    Scratch.Storage.get_stored_value
    |> IO.inspect(label: "Stored value is")
  end
end

defmodule Test do
  import Scratch

  @my_value 1
  save_this_somewhere(@my_value)  # WE ARE TOTALLY USING @my_value

  @after_compile Scratch
end

Expected behavior

We should not get a warning. We definitely used @my_value

---

[dylan-chong]

Thanks for doing this so quickly @josevalim !