Inspect.Opts.custom_options (#9096)

Author: Aleksei Matiushkin

lib/elixir/lib/inspect/algebra.ex

@@ -57,6 +57,10 @@ defmodule Inspect.Opts do
     * `:inspect_fun` (since v1.9.0) - a function to build algebra documents,
       defaults to `Inspect.inspect/2`
 
+    * `:custom_options` (since v1.9.0) - a keyword list storing custom user-defined
+      options. Useful when implementing the `Inspect` protocol for nested structs
+      to pass the custom options through.
+
   """
 
   # TODO: Remove :char_lists key on v2.0
@@ -71,7 +75,8 @@ defmodule Inspect.Opts do
             pretty: false,
             safe: true,
             syntax_colors: [],
-            inspect_fun: &Inspect.inspect/2
+            inspect_fun: &Inspect.inspect/2,
+            custom_options: []
 
   @type color_key :: atom
 
@@ -88,7 +93,8 @@ defmodule Inspect.Opts do
           pretty: boolean,
           safe: boolean,
           syntax_colors: [{color_key, IO.ANSI.ansidata()}],
-          inspect_fun: (any, t -> Inspect.Algebra.t())
+          inspect_fun: (any, t -> Inspect.Algebra.t()),
+          custom_options: keyword
         }
 end
 

lib/elixir/test/elixir/inspect_test.exs

@@ -679,4 +679,30 @@ defmodule Inspect.OthersTest do
     assert inspect(uri, opts) == "#URI<https://elixir-lang.org>"
     assert inspect([uri], opts) == "[#URI<https://elixir-lang.org>]"
   end
+
+  defmodule Nested do
+    defstruct nested: nil
+
+    defimpl Inspect do
+      import Inspect.Algebra
+
+      def inspect(%Nested{nested: nested}, opts) do
+        indent = Keyword.get(opts.custom_options, :indent, 2)
+        level = Keyword.get(opts.custom_options, :level, 1)
+
+        nested_str =
+          Kernel.inspect(nested, custom_options: [level: level + 1, indent: indent + 2])
+
+        concat(
+          nest(line("#Nested[##{level}/#{indent}]<", nested_str), indent),
+          nest(line("", ">"), indent - 2)
+        )
+      end
+    end
+  end
+
+  test "custom_options" do
+    assert inspect(%Nested{nested: %Nested{nested: 42}}) ==
+             "#Nested[#1/2]<\n  #Nested[#2/4]<\n    42\n  >\n>"
+  end
 end