Add hook for customizing the debug representation of objects

lib/debug/variable_inspector.rb

@@ -17,6 +17,19 @@ def indexed_members_of(obj, start:, count:)
     def named_members_of(obj)
       return [] if NaiveString === obj
 
+      if M_RESPOND_TO.bind_call(obj, :debug_representation)
+        debug_representation = obj.debug_representation
+        members = named_members_of(debug_representation)
+
+        # Discard the "#class" member of the debug representation, if any.
+        members.delete_if { |m| m.name == '#class' }
+
+        # Add the real "#class" of the object being inspected.
+        members.unshift Variable.internal(name: '#class', value: M_CLASS.bind_call(obj))
+
+        return members
+      end
+
       members = case obj
       when Hash then obj.map { |k, v| Variable.new(name: value_inspect(k), value: v) }
       when Struct then obj.members.map { |name| Variable.new(name: name, value: obj[name]) }
@@ -69,6 +82,7 @@ def self.value_inspect(obj, short: true)
 
     # TODO: Replace with Reflection helpers once they are merged
     # https://github.com/ruby/debug/pull/1002
+    M_RESPOND_TO = method(:respond_to?).unbind
     M_INSTANCE_VARIABLES = method(:instance_variables).unbind
     M_INSTANCE_VARIABLE_GET = method(:instance_variable_get).unbind
     M_CLASS = method(:class).unbind

test/debug/variable_inspector_test.rb

@@ -196,6 +196,31 @@ def test_named_members_of_other_objects
       assert_equal expected, @inspector.named_members_of(point)
     end
 
+    def test_debug_representation_hook
+      object_with_simple_repr = ClassWithCustomDebugRepresentation.new({ a: 1, b: 2 })
+
+      expected = [
+        # We should always show the `#class` when using this hook, even if the
+        # debug_representation is a simple value.
+        Variable.internal(name: '#class', value: ClassWithCustomDebugRepresentation),
+        Variable.new(name: ':a', value: 1),
+        Variable.new(name: ':b', value: 2),
+      ]
+
+      assert_equal expected, @inspector.named_members_of(object_with_simple_repr)
+
+      object_with_complex_repr = ClassWithCustomDebugRepresentation.new(Point.new(x: 1, y: 2))
+
+      expected = [
+        # Make sure we don't add the '#class' twice for non-simple debug representations
+        Variable.internal(name: '#class', value: ClassWithCustomDebugRepresentation),
+        Variable.new(name: :@x, value: 1),
+        Variable.new(name: :@y, value: 2),
+      ]
+
+      assert_equal expected, @inspector.named_members_of(object_with_complex_repr)
+    end
+
     private
 
     class PointStruct < Struct.new(:x, :y, keyword_init: true)
@@ -211,5 +236,15 @@ def initialize(x:, y:)
         @y = y
       end
     end
+
+    class ClassWithCustomDebugRepresentation
+      def initialize(debug_representation)
+        @debug_representation = debug_representation
+      end
+
+      def debug_representation
+        @debug_representation
+      end
+    end
   end
 end