Copy structs when they exist

lib/mimic/module.ex

@@ -114,11 +114,20 @@ defmodule Mimic.Module do
     mimic_info = module_mimic_info()
     mimic_behaviours = generate_mimic_behaviours(module)
     mimic_functions = generate_mimic_functions(module)
-    quoted = [mimic_info | [mimic_behaviours ++ mimic_functions]]
+    mimic_struct = generate_mimic_struct(module)
+    quoted = [mimic_info, mimic_struct | mimic_behaviours ++ mimic_functions]
     Module.create(module, quoted, Macro.Env.location(__ENV__))
     module
   end
 
+  defp generate_mimic_struct(module) do
+    if module.__info__(:struct) != nil do
+      quote do
+        defstruct unquote(for %{field: field} <- module.__info__(:struct), do: field)
+      end
+    end
+  end
+
   defp module_mimic_info do
     quote do: def(__mimic_info__, do: :ok)
   end

test/mimic_test.exs

@@ -945,4 +945,19 @@ defmodule Mimic.Test do
       assert_receive {:add, 1, 2}
     end
   end
+
+  describe "structs" do
+    test "copies struct fields" do
+      struct_fields =
+        Structs.__info__(:struct)
+        |> Enum.map(&Map.get(&1, :field))
+
+      Mimic.copy(Structs)
+
+      Structs
+      |> stub(:foo, fn -> :stubbed end)
+
+      assert Structs.__info__(:struct) |> Enum.map(&Map.get(&1, :field)) == struct_fields
+    end
+  end
 end

test/support/test_modules.ex

@@ -38,3 +38,9 @@ defmodule NoStubs do
   @moduledoc false
   def add(x, y), do: x + y
 end
+
+defmodule Structs do
+  @moduledoc false
+  defstruct [:foo, :bar]
+  def foo, do: nil
+end