`ExUnit` `assert` crashing when trying to format `AssertionError` under some wierd condition with "improper list"

[grzuy]

Elixir and Erlang/OTP versions

1.18.2
27.2.2

Operating system

Linux

Current behavior

Hi 👋

This is a very narrowed down of an issue with an assert of a pattern match of a map or struct that has in one of it fields value a :logger.log_event() map, that contains improper lists in the stacktrace list.

If having an ExUnit test case assertion:

    assert(
      %{
        a: nil
      } =
        %{
          a: %{
            b: [10 | "** (RuntimeError) an error"]
          }
        }
    )

** (FunctionClauseError) no function clause matching in Enum."-map/2-lists^map/1-1-"/2
    (elixir 1.18.2) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"(#Function<2.96213035/1 in ExUnit.Diff.container_to_algebra/5>, "** (RuntimeError) an error")
    (elixir 1.18.2) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (ex_unit 1.18.2) lib/ex_unit/diff.ex:1101: ExUnit.Diff.container_to_algebra/5
    (ex_unit 1.18.2) lib/ex_unit/diff.ex:1073: ExUnit.Diff.safe_keyword_to_algebra/2
    (elixir 1.18.2) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (ex_unit 1.18.2) lib/ex_unit/diff.ex:1101: ExUnit.Diff.container_to_algebra/5
    (ex_unit 1.18.2) lib/ex_unit/diff.ex:1141: ExUnit.Diff.wrap_on_diff/3
    (ex_unit 1.18.2) lib/ex_unit/diff.ex:1073: ExUnit.Diff.safe_keyword_to_algebra/2
    (elixir 1.18.2) lib/enum.ex:1714: Enum."-map/2-lists^map/1-1-"/2
    (ex_unit 1.18.2) lib/ex_unit/diff.ex:1101: ExUnit.Diff.container_to_algebra/5
    (ex_unit 1.18.2) lib/ex_unit/formatter.ex:489: ExUnit.Formatter.format_sides/6
    (ex_unit 1.18.2) lib/ex_unit/formatter.ex:471: ExUnit.Formatter.format_assertion_diff/4
    (ex_unit 1.18.2) lib/ex_unit/formatter.ex:382: ExUnit.Formatter.format_exception/6
    (ex_unit 1.18.2) lib/ex_unit/formatter.ex:278: anonymous fn/7 in ExUnit.Formatter.format_test_failure/5
    (elixir 1.18.2) lib/enum.ex:1815: anonymous fn/2 in Enum.map_join/3
    (elixir 1.18.2) lib/enum.ex:4493: Enum.map_intersperse_list/3
    (elixir 1.18.2) lib/enum.ex:1815: Enum.map_join/3
    (ex_unit 1.18.2) lib/ex_unit/formatter.ex:277: ExUnit.Formatter.format_test_failure/5
    (ex_unit 1.18.2) lib/ex_unit/cli_formatter.ex:122: ExUnit.CLIFormatter.handle_cast/2
    (stdlib 6.2) gen_server.erl:2371: :gen_server.try_handle_cast/3
Last message: {:"$gen_cast", {:test_finished, %ExUnit.Test{name: :"test test", case: TowerTest, module: TowerTest, state: {:failed, [{:error, %ExUnit.AssertionError{left: {:%{}, [line: 52, column: 7], [a: nil]}, right: %{a: %{b: [10 | "** (RuntimeError) an error"]}}, message: "match (=) failed", expr: {:assert, [line: 54], [{:=, [line: 54], [{:%{}, [line: 52], [a: nil]}, {:%{}, [line: 55], [a: {:%{}, [line: 56], [b: [{:|, [line: 57], [10, "** (RuntimeError) an error"]}]]}]}]}]}, args: :ex_unit_no_meaningful_value, doctest: :ex_unit_no_meaningful_value, context: {:match, []}}, [{TowerTest, :"test test", 1, [file: ~c"test/tower_test.exs", line: 51]}]}]}, time: 844, tags: %{async: false, line: 50, module: TowerTest, registered: %{}, file: "/home/gon/dev/tower/test/tower_test.exs", test: :"test test", describe_line: nil, test_type: :test, describe: nil}, logs: "", parameters: %{}}}}
State: %{trace: false, width: 190, colors: [enabled: true, diff_delete: :red, diff_delete_whitespace: "\e[48;5;88m", diff_insert: :green, diff_insert_whitespace: "\e[48;5;28m", success: :green, invalid: :yellow, skipped: :yellow, failure: :red, error_info: :red, extra_info: :cyan, location_info: [:bright, :black]], slowest: 0, slowest_modules: 0, failure_counter: 0, test_counter: %{test: 28, doctest: 5}, test_timings: [], skipped_counter: 0, invalid_counter: 0, excluded_counter: 33}

Expected behavior

What happens in elixir 1.17.3

     match (=) failed
     code:  assert %{a: nil} = %{a: %{b: [10 | "** (RuntimeError) an error"]}}
     left:  %{a: nil}
     right: %{a: %{b: [10 | "** (RuntimeError) an error"]}}
     stacktrace:

[grzuy]

For what is worth, running a git bisect start v1.18 v1.17 found that this seems to start happening after commit a8c9206.

[grzuy]

Note also that

    assert(
      %{
        a: %{}
      } =
        %{
          a: %{
            b: [10 | "** (RuntimeError) an error"]
          }
        }
    )

doesn't crash.

[grzuy]

Haven't figured it out yet but it seems for some reason the "improper list" ends up here trying to be Enum.maped:

elixir/lib/ex_unit/lib/ex_unit/diff.ex

Lines 1064 to 1065 in b823f9e

	list
	|> Enum.map(&item_to_algebra.(&1, diff_wrapper))

, which fails.

[sabiwara]

Thanks a lot for the report and the investigation @grzuy 💜

[grzuy]

Thanks @sabiwara!

Tested against v1.18 branch with your backport commit (#14292 (comment)) and everything working now ✅