Fix infinite loop when diffing functions, closes #12828

Author: josevalim

lib/ex_unit/lib/ex_unit/diff.ex

@@ -200,11 +200,15 @@ defmodule ExUnit.Diff do
 
       {diff, env}
     else
-      diff_value(left, right, env)
+      non_recursive_diff_value(left, right, env)
     end
   end
 
   defp diff_value(left, right, env) do
+    non_recursive_diff_value(left, right, env)
+  end
+
+  defp non_recursive_diff_value(left, right, env) do
     diff_left = escape(left) |> update_diff_meta(true)
     diff_right = escape(right) |> update_diff_meta(true)
     diff = %__MODULE__{equivalent?: false, left: diff_left, right: diff_right}

lib/ex_unit/test/ex_unit/diff_test.exs

@@ -1121,6 +1121,8 @@ defmodule ExUnit.DiffTest do
     )
   end
 
+  @compile {:no_warn_undefined, String}
+
   test "functions" do
     identity = & &1
     inspect = inspect(identity)
@@ -1130,11 +1132,15 @@ defmodule ExUnit.DiffTest do
 
     refute_diff(identity == :a, "-#{inspect}-", "+:a+")
     refute_diff({identity, identity} == :a, "-{#{inspect}, #{inspect}}", "+:a+")
-
     refute_diff({identity, :a} == {:a, identity}, "{-#{inspect}-, -:a-}", "{+:a+, +#{inspect}+}")
-
     refute_diff(%{identity => identity} == :a, "-%{#{inspect} => #{inspect}}", "+:a+")
 
+    refute_diff(
+      (&String.to_charlist/1) == (&String.unknown/1),
+      "-&String.to_charlist/1-",
+      "+&String.unknown/1"
+    )
+
     refute_diff(
       %Opaque{data: identity} == :a,
       "-#Opaque<???>-",