Support struct default values

The required fields must appear first in the defstruct, followed by
2-tuples with the default value protected by `Macro.escape/1`

lib/mimic/module.ex

@@ -123,11 +123,27 @@ defmodule Mimic.Module do
 
   defp generate_mimic_struct(module) do
     if function_exported?(module, :__info__, 1) && module.__info__(:struct) != nil do
-      required_fields = for %{field: field, required: true} <- module.__info__(:struct), do: field
+      struct_info =
+        module.__info__(:struct)
+        |> Enum.split_with(& &1.required)
+        |> Tuple.to_list()
+        |> List.flatten()
+
+      required_fields = for %{field: field, required: true} <- struct_info, do: field
+      struct_template = Map.from_struct(module.__struct__())
+
+      struct_params =
+        for %{field: field, required: required} <- struct_info do
+          if required do
+            field
+          else
+            {field, Macro.escape(struct_template[field])}
+          end
+        end
 
       quote do
         @enforce_keys unquote(required_fields)
-        defstruct unquote(for %{field: field} <- module.__info__(:struct), do: field)
+        defstruct unquote(struct_params)
       end
     end
   end

test/mimic_test.exs

@@ -994,10 +994,23 @@ defmodule Mimic.Test do
       assert Structs.__info__(:struct) == [
                %{field: :foo, required: true},
                %{field: :bar, required: true},
-               %{field: :default, required: false}
+               %{field: :default, required: false},
+               %{field: :map_default, required: false}
              ]
     end
 
+    test "copies struct fields with default values" do
+      Structs
+      |> stub(:foo, fn -> :stubbed end)
+
+      assert Structs.__struct__() == %Structs{
+               foo: nil,
+               bar: nil,
+               default: "123",
+               map_default: %{}
+             }
+    end
+
     test "copies struct fields" do
       StructNoEnforceKeys
       |> stub(:bar, fn -> :stubbed end)

test/support/test_modules.ex

@@ -47,7 +47,7 @@ end
 defmodule Structs do
   @moduledoc false
   @enforce_keys [:foo, :bar]
-  defstruct [:foo, :bar, default: "123"]
+  defstruct [:foo, :bar, default: "123", map_default: %{}]
   def foo, do: nil
 end