Use inspect in assertion diff for structs if possible (#7043)

2 files changed, 46 insertions, 3 deletions

diff --git a/lib/ex_unit/lib/ex_unit/diff.ex b/lib/ex_unit/lib/ex_unit/diff.ex
index 0e8bd2228..833e7273c 100644
--- a/lib/ex_unit/lib/ex_unit/diff.ex
+++ b/lib/ex_unit/lib/ex_unit/diff.ex
@@ -24,9 +24,18 @@ defmodule ExUnit.Diff do
 
   # Structs
   def script(%name{} = left, %name{} = right) do
-    left = Map.from_struct(left)
-    right = Map.from_struct(right)
-    script_map(left, right, inspect(name))
+    if Inspect.impl_for(left) != Inspect.Any do
+      inspect_left = inspect(left)
+      inspect_right = inspect(right)
+
+      if inspect_left != inspect_right do
+        script_string(inspect_left, inspect_right)
+      else
+        script_struct(left, right)
+      end
+    else
+      script_struct(left, right)
+    end
   end
 
   # Maps
@@ -435,6 +444,12 @@ defmodule ExUnit.Diff do
     [{:eq, "%" <> name <> "{"}, [elem_diff | rest], {:eq, "}"}]
   end
 
+  defp script_struct(%name{} = left, %name{} = right) do
+    left = Map.from_struct(left)
+    right = Map.from_struct(right)
+    script_map(left, right, inspect(name))
+  end
+
   defp map_difference(map1, map2) do
     {surplus, altered, same} =
       Enum.reduce(map1, {[], [], []}, fn {key, val1}, {surplus, altered, same} ->
diff --git a/lib/ex_unit/test/ex_unit/diff_test.exs b/lib/ex_unit/test/ex_unit/diff_test.exs
index da8e133a6..25bf01a3a 100644
--- a/lib/ex_unit/test/ex_unit/diff_test.exs
+++ b/lib/ex_unit/test/ex_unit/diff_test.exs
@@ -9,6 +9,16 @@ defmodule ExUnit.DiffTest do
     defstruct [:age]
   end
 
+  defmodule Opaque do
+    defstruct [:x]
+
+    defimpl Inspect do
+      def inspect(_, _) do
+        "#Opaque<???>"
+      end
+    end
+  end
+
   test "numbers" do
     int1 = 491_512_235
     int2 = 490_512_035
@@ -424,6 +434,24 @@ defmodule ExUnit.DiffTest do
     assert script(%User{}, %ExUnit.Test{}) == nil
   end
 
+  test "structs with inspect" do
+    date1 = ~D[2017-10-01]
+    date2 = ~D[2017-10-02]
+
+    assert script(date1, date2) == [eq: "~D[2017-10-0", del: "1", ins: "2", eq: "]"]
+  end
+
+  test "structs with no inspect difference" do
+    opaque1 = %Opaque{x: 1}
+    opaque2 = %Opaque{x: 2}
+
+    assert script(opaque1, opaque2) == [
+             {:eq, "%ExUnit.DiffTest.Opaque{"},
+             [[{:eq, "x: "}, [del: "1", ins: "2"]]],
+             {:eq, "}"}
+           ]
+  end
+
   test "not supported" do
     bin1 = <<147, 1, 2, 31>>
     bin2 = <<193, 1, 31>>