ERL-359: Enable debugging for Elixir sources with loader hack

[OTP-Maintainer]

Original reporter: bryanhunt
Affected version: OTP-20.0
Component: debugger
Migrated from: https://bugs.erlang.org/browse/ERL-359

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If a user tries to load debug Elixir source code via Module -> Interpret -> Interpret Modules 

The following error is generated:

"Error when interpreting: EchoProtocol.ex: No Beam file"

On the console the following is output:

"Invalid beam file or no abstract code: "/common/elixir_ranch/lib/EchoProtocol.ex"

The work around to get this working is:

{code}{{iex(15)> {:ok,beam_bin} = File.read("_build/dev/lib/elixir_ranch/ebin/Elixir.ElixirRanch.beam")

{:ok,
 <<70, 79, 82, 49, 0, 0, 7, 68, 66, 69, 65, 77, 69, 120, 68, 99, 0, 0, 0, 226,
   131, 104, 2, 100, 0, 14, 101, 108, 105, 120, 105, 114, 95, 100, 111, 99, 115,
   95, 118, 49, 108, 0, 0, 0, 4, 104, 2, 100, 0, ...>>}

iex(16)> :int.i({ElixirRanch, 'lib/elixir_ranch.ex', '_build/dev/lib/elixir_ranch/ebin/Elixir.ElixirRanch.beam', beam_bin})
{:module, ElixirRanch}}}{code}

This workaround has been known for a long time (since 2014) and is described in depth by Josh Burroughs (qhool) here: http://qhool.github.io/elixir/2014/02/06/elixir-debug.html

I have also attached a PDF of that page in case the site vanishes some day.

It would be nice if this worked a little better, the obviously comparison will be with Java tooling which can handle any source files it pleases, provided line number info has been generated.