Consider macros and module attributes on the left side of ++ on ExUnit.Diff

Closes #10520

Author: josevalim

lib/ex_unit/lib/ex_unit/diff.ex

@@ -70,8 +70,14 @@ defmodule ExUnit.Diff do
     diff_quoted(number, right, env)
   end
 
-  defp diff_quoted({:++, _, _} = left, right, env) when is_list(right) do
-    diff_maybe_improper_list(left, right, env)
+  defp diff_quoted({:++, meta, [prefix, suffix]}, right, env) when is_list(right) do
+    case prefix do
+      {_, [expanded: expanded] ++ _, _} ->
+        diff_maybe_improper_list({:++, meta, [expanded, suffix]}, right, env)
+
+      _ ->
+        diff_maybe_improper_list({:++, meta, [prefix, suffix]}, right, env)
+    end
   end
 
   defp diff_quoted({:{}, _, left}, right, env) when is_tuple(right) do

lib/ex_unit/test/ex_unit/diff_test.exs

@@ -345,18 +345,23 @@ defmodule ExUnit.DiffTest do
 
     refute_diff([:a, :b] ++ [:c] = [:a, :b], "[:a, :b] ++ [-:c-]", "[:a, :b]")
     refute_diff([:a, :c] ++ [:b] = [:a, :b], "[:a, -:c-] ++ [:b]", "[:a, :b]")
-
     refute_diff([:a] ++ [:b] ++ [:c] = [:a, :b], "[:a] ++ [:b] ++ [-:c-]", "[:a, :b]")
 
     assert_diff([:a] ++ :b = [:a | :b], [])
     assert_diff([:a] ++ x = [:a, :b], x: [:b])
 
     refute_diff([:a, :b] ++ :c = [:a, :b, :c], "[:a, :b] ++ -:c-", "[:a, :b, +:c+]")
     refute_diff([:a] ++ [:b] ++ :c = [:a, :b, :c], "[:a] ++ [:b] ++ -:c-", "[:a, :b, +:c+]")
-
     refute_diff([:a] ++ [:b] = :a, "-[:a] ++ [:b]-", "+:a+")
   end
 
+  @a [:a]
+  test "concat lists with module attributes" do
+    assert_diff(@a ++ [:b] = [:a, :b], [])
+    refute_diff(@a ++ [:b] = [:a], "[:a] ++ [-:b-]", "[:a]")
+    refute_diff(@a ++ [:b] = [:b], "[-:a-] ++ [:b]", "[:b]")
+  end
+
   test "mixed lists" do
     refute_diff([:a | :b] = [:a, :b], "[:a | -:b-]", "[:a, +:b+]")
     refute_diff([:a, :b] = [:a | :b], "[:a, -:b-]", "[:a | +:b+]")