Elixir: ExUnit `assert` using macros not retaining new variables

[meyercm]

Environment

• Erlang 19.1, Elixir 1.4.2
• Operating system: OSX 10.10

Background

The ~M sigil from ShorterMaps expands ~M{a, b} => %{a: a, b: b}.

Current behavior

The following ExUnit test fails:

  test "that fails" do
    assert {:a, ~M{b}} = {:a, %{b: 2}} # LHS expands to {:a, %{b: b}} 
    assert b == 2 # <= crashing line
  end

```

mix test
warning: variable "b" does not exist and is being expanded to "b()", please use parentheses to remove the ambiguity or change the variable name
test/shorter_maps_test.exs:7

warning: variable "b" is unused
test/shorter_maps_test.exs:6

** (CompileError) test/shorter_maps_test.exs:7: undefined function b/0
(stdlib) lists.erl:1338: :lists.foreach/2
(stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
(elixir) lib/code.ex:370: Code.require_file/2
(elixir) lib/kernel/parallel_require.ex:57: anonymous fn/2 in Kernel.ParallelRequire.spawn_requires/5

and these tests pass:

```elixir
  test "passes" do
    assert ~M{b} = %{b: 2} # here the sigil is just not wrapped in a tuple
    assert b == 2
  end
  test "another passing test" do
    assert {:a, %{b: b}} = {:a, %{b: 2}} # this is the manually expanded LHS
    assert b == 2
  end

In the failing test, the sigil has been wrapped inside a tuple; the sigil expands and binds properly, note the warning variable "b" is unused.

Expected behavior

assert/2 should preserve all created variables in the LHS code block, even if they are in a nested datastructure.

This is an elevation of meyercm/shorter_maps#11 as the issue seems to be in assert.

[josevalim]

Thank you @meyercm! Can you please isolate the issue a bit further by providing a minimal macro that introduces the mentioned problems?

[meyercm]

Sorry, should have thought to simplify the macro portion of the problem.

defmodule MinimalMacroTest do
  use ExUnit.Case

  defmodule MinimalMacro do
    defmacro sigil_l({:<<>>, _, [string]}, _), do: Code.string_to_quoted!(string, [])
  end

  test "smaller macro" do
    import MinimalMacro
    assert ~l(a) = 1     # just the macro
    assert a == 1        # this works
    assert {~l(b)} = {2} # add a tuple wrapper
    assert b == 2        # crashes here
  end
end