Allow optional: :all when deriving Inspect (#14400)

* Allow optional: :all when deriving Inspect

* Improve error message when passing a non-list

lib/elixir/lib/inspect.ex

@@ -67,7 +67,8 @@ defprotocol Inspect do
     * `:optional` - (since v1.14.0) a list of fields that should not be
       included when they match their default value. This can be used to
       simplify the struct representation at the cost of hiding
-      information.
+      information. Since v1.19.0, the `:all` atom can be passed to
+      mark all fields as optional.
 
   Whenever `:only` or `:except` are used to restrict fields,
   the struct will be printed using the `#User<...>` notation,
@@ -159,11 +160,19 @@ defprotocol Inspect do
 
     only = Keyword.get(options, :only, fields)
     except = Keyword.get(options, :except, [])
-    optional = Keyword.get(options, :optional, [])
 
     :ok = validate_option(:only, only, fields, module)
     :ok = validate_option(:except, except, fields, module)
-    :ok = validate_option(:optional, optional, fields, module)
+
+    optional =
+      case Keyword.get(options, :optional, []) do
+        :all ->
+          fields
+
+        optional ->
+          :ok = validate_option(:optional, optional, fields, module)
+          optional
+      end
 
     inspect_module =
       if fields == Enum.sort(only) and except == [] do
@@ -226,6 +235,13 @@ defprotocol Inspect do
   end
 
   defp validate_option(option, option_list, fields, module) do
+    if not is_list(option_list) do
+      raise ArgumentError,
+            "invalid value #{Kernel.inspect(option_list)} in #{Kernel.inspect(option)} " <>
+              "when deriving the Inspect protocol for #{Kernel.inspect(module)} " <>
+              "(expected a list)"
+    end
+
     case option_list -- fields do
       [] ->
         :ok

lib/elixir/test/elixir/inspect_test.exs

@@ -715,6 +715,17 @@ defmodule Inspect.MapTest do
                  end
   end
 
+  test "passing a non-list to the :only option" do
+    assert_raise ArgumentError,
+                 "invalid value :not_a_list in :only when deriving the Inspect protocol for Inspect.MapTest.StructInvalidListInOnlyOption (expected a list)",
+                 fn ->
+                   defmodule StructInvalidListInOnlyOption do
+                     @derive {Inspect, only: :not_a_list}
+                     defstruct [:a, :b]
+                   end
+                 end
+  end
+
   defmodule StructWithExceptOption do
     @derive {Inspect, except: [:b, :c]}
     defstruct [:a, :b, :c, :d]
@@ -772,6 +783,20 @@ defmodule Inspect.MapTest do
     assert inspect(struct) ==
              "#Inspect.MapTest.StructWithExceptOptionalAndOrder<d: nil, a: nil, ...>"
   end
+
+  defmodule StructWithOptionalAll do
+    @derive {Inspect, optional: :all}
+    defstruct [:a, :b, :c, :d]
+  end
+
+  test "struct with :optional set to :all" do
+    struct = %StructWithOptionalAll{a: 1, b: 2}
+
+    assert inspect(struct) == "%Inspect.MapTest.StructWithOptionalAll{a: 1, b: 2}"
+
+    struct = %StructWithOptionalAll{}
+    assert inspect(struct) == "%Inspect.MapTest.StructWithOptionalAll{}"
+  end
 end
 
 defmodule Inspect.OthersTest do