@trace attribute rewriting ignored cons arguments

[nathanjohnson320]

Describe the bug

When adding @trace to a function with an ignored list tail the macro creates an improper list with :__ignored__ as the tail instead of creating an ignored tail variable.

Below is an example module that produces the issue

defmodule Test do
  use NewRelic.Tracer

  @trace :example
  def example([a | _ignored]) do
    IO.inspect([a])
  end

This code will raise warnings in dialyzer which say that the list is improper

And then below I logged out what the Tracer build_call_args/1 returns with and without the rewrite_call_term postwalk

with trace
"[[a | :__ignored__]]"

without trace
"[[a | _ignored]]"

Environment

• Elixir & Erlang version (elixir -v): Elixir 1.12.1 (compiled with Erlang/OTP 22)
• Agent version (mix deps | grep new_relic_agent): 1.27.4
• macOS 11.6

[binaryseed]

This one is interesting, we need to "re write" the arguments of the function so we can log the function call itself. The :__ignored__ atom is stuck in there to avoid compiler warnings

The code is here:

• https://github.com/newrelic/elixir_agent/blob/master/lib/new_relic/tracer/macro.ex#L251 Tests are here:
• https://github.com/newrelic/elixir_agent/blob/master/test/tracer_macro_test.exs#L18

I'm not sure what to re-write it to here, but it should be possible to craft a test for this, detect the situation & do something else.