🔍 Improve Config#pretty_print (for Kernel::pp)

Viewing config objects in Irb was _much_ too messy to easily inspect: it
printed every attribute for each ancestor, and most of them would simply
be the Net::IMAP::Config::AttrInheritance::INHERITED const, repeated
over and over.

So, just like `Config#inspect` was improved by #546, this simplifies the
`Kernel#pp` output to make it more useful for inspection on the console.

Unlike `Config#inspect`, `Config#pretty_print` prints _every_ config
attribute, including attributes that have been defined by a named
default config.  Like `Config#inspect`, each attributes is printed only
once, nested in the config which assigned it.

Author: nevans

lib/net/imap/config.rb

@@ -523,12 +523,42 @@ def to_h; data.members.to_h { [_1, send(_1)] } end
       #     #      inherits from Net::IMAP::Config.global debug=true
       #     #      inherits from Net::IMAP::Config.default>"
       #
+      # Use +pp+ (see #pretty_print) to inspect _all_ config attributes,
+      # including default values.
+      #
       # Use #to_h to inspect all config attributes ignoring inheritance.
       def inspect;
         "#<#{inspect_recursive}>"
       end
       alias to_s inspect
 
+      # Used by PP[https://docs.ruby-lang.org/en/master/PP.html] to create a
+      # string representation of all config attributes and the inheritance
+      # chain.  Inherited attributes are listed with the ancestor config from
+      # which they are inherited.
+      #
+      #     pp Config.new[0.4].new(open_timeout: 10, idle_response_timeout: 10)
+      #     # #<Net::IMAP::Config:0x0000745871125410
+      #     #   open_timeout=10
+      #     #   idle_response_timeout=10
+      #     #   inherits from Net::IMAP::Config[0.4]
+      #     #     responses_without_block=:silence_deprecation_warning
+      #     #     max_response_size=nil
+      #     #     sasl_ir=true
+      #     #     enforce_logindisabled=false
+      #     #     parser_use_deprecated_uidplus_data=true
+      #     #     parser_max_deprecated_uidplus_data_size=1000
+      #     #     inherits from Net::IMAP::Config.global
+      #     #       inherits from Net::IMAP::Config.default
+      #     #         debug=false>
+      #
+      # Related: #inspect, #to_h.
+      def pretty_print(pp)
+        pp.group(2, "#<", ">") do
+          pretty_print_recursive(pp)
+        end
+      end
+
       # :stopdoc:
 
       protected
@@ -566,6 +596,26 @@ def inspect_recursive(attrs = AttrAccessors.struct.members)
         strings.join " "
       end
 
+      def pretty_print_recursive(pp, attrs = AttrAccessors.struct.members)
+        pp.text name
+        assigned = assigned_attrs_hash(attrs)
+        pp.breakable
+        pp.seplist(assigned, ->{pp.breakable}) do |key, val|
+          pp.text key.to_s
+          pp.text "="
+          pp.pp val
+        end
+        if parent
+          pp.breakable if assigned.any?
+          pp.nest(2) do
+            pp.text "inherits from "
+            parent.pretty_print_recursive(pp, attrs - assigned.keys)
+          end
+        elsif assigned.empty?
+          pp.text "(overridden)"
+        end
+      end
+
       def assigned_attrs_hash(attrs)
         own_attrs = attrs.reject { inherited?(_1) }
         own_attrs.to_h { [_1, data[_1]] }

test/net/imap/test_config.rb

@@ -484,4 +484,33 @@ def config_id(config)
     )
   end
 
+  test "#pretty_print" do
+    default_attrs = Config.global.to_h.map { "%s=%p" % _1 }
+    width = 80
+    output = String.new
+    PP.pp Config.global, output, width
+    assert_equal(<<~PP, output)
+      #<#{Config}.global
+        inherits from #{Config}.default
+          #{default_attrs.join("\n    ")}>
+    PP
+
+    Config.global.debug = true
+    config = Config[0.5].new(enforce_logindisabled: :when_capabilities_cached)
+    default_attrs = Config[0.5].to_h.except(:debug, :enforce_logindisabled)
+      .map { "%s=%p" % _1 }
+
+    output = String.new
+    PP.pp config, output, width
+    assert_equal(<<~PP, output)
+      #<#{Config}:0x#{config_id(config)}
+        enforce_logindisabled=:when_capabilities_cached
+        inherits from #{Config}[0.5]
+          #{default_attrs.join("\n    ")}
+          inherits from #{Config}.global
+            debug=true
+            inherits from #{Config}.default
+              (overridden)>
+    PP
+  end
 end