You can do this:

warn.ex:

defmodule Foo do
  defmacro __using__(_opts) do
    :elixir_errors.warn __ENV__.line, __ENV__.file, "this is compilation warning"
  end
end

defmodule Bar do
  use Foo
end

and when compiling you get:

$ elixirc --warnings-as-errors warn.ex 
warn.ex:3: warning: this is compilation warning
Compilation failed due to warnings while using the --warnings-as-errors option
$ rm *.beam                           
$ elixirc warn.ex                     
warn.ex:3: warning: this is compilation warning

In both cases warning is printed to the stderr while compilation. In first case, however, compilation fails and appropriate shell exit code is set. In second case only warning is printed.

@Edit: https://github.com/search?utf8=✓&q=elixir_errors+language%3Aelixir&type=Code&ref=searchresults

Looks like it’s ok to do that, but better in this form:

%{file: file, line: line} = __CALLER__
:elixir_errors.warn(line, file, "error")

No problem! Although Discourse shows I replied to you, it was really to everyone that is using private APIs, here or on GitHub.

This is great. I am terribly sorry to post misleading message, it was the thing I saw somewhere and adopted.

➜  elixir  cat warn.ex                         
defmodule Foo do
  defmacro __using__(_opts) do
    msg = "this is compilation error"
    IO.warn(msg, Macro.Env.stacktrace(__ENV__))
  end
end

defmodule Bar do
  use Foo
end
➜  elixir  elixirc --warnings-as-errors warn.ex
warning: this is compilation error
  warn.ex:4: Foo.__using__/1

Compilation failed due to warnings while using the --warnings-as-errors option

This is way better, however!

Edit: ah yeah it’s plenty of wrong usage in different libs then:
https://github.com/search?utf8=✓&q=elixir_errors+language%3Aelixir&type=Code&ref=searchresults

low hanging fruits if someone wants to make some pull requests.

On a more general note, you could write a function decorator for this, like this:

  defmodule DeprecationDecorator do
    use Decorator.Define, deprecated: 0

    def deprecated(body, context) do
      :elixir_errors.warn __ENV__.line, __ENV__.file, "Function #{context.module}.#{context.name}/#{context.arity} is deprecated"
      body
    end
  end

Which you would use like this:

defmodule SomeModule do
  use DeprecationDecorator

  @decorate deprecated
  def old_function_do_not_use() do
    # ...
  end
end

[/end shameless library-plug]