feat(yard): add initial support of YARD tags

At this time, `@yield`, `@private` and `@api private` are supported.

Author: okuramasafumi

lib/rdoc/code_object.rb

@@ -1,4 +1,7 @@
 # frozen_string_literal: true
+
+require_relative 'yard'
+
 ##
 # Base class for the RDoc code tree.
 #

lib/rdoc/code_object/constant.rb

@@ -174,6 +174,20 @@ def store=(store)
     @file = @store.add_file @file.full_name if @file
   end
 
+  ##
+  # Process comment as YARD comment
+  def comment=(comment)
+    # Process YARD tags while we still have the RDoc::Comment object
+    if comment.is_a?(RDoc::Comment)
+      RDoc::YARD.process(comment, self)
+    end
+
+    # Then set the comment normally
+    super(comment)
+
+    @comment
+  end
+
   def to_s # :nodoc:
     parent_name = parent ? parent.full_name : '(unknown)'
     if is_alias_for

lib/rdoc/code_object/method_attr.rb

@@ -156,6 +156,20 @@ def store=(store)
     @file = @store.add_file @file.full_name if @file
   end
 
+  ##
+  # Process comment as YARD comment
+  def comment=(comment)
+    # Process YARD tags while we still have the RDoc::Comment object
+    if comment.is_a?(RDoc::Comment)
+      RDoc::YARD.process(comment, self)
+    end
+
+    # Then set the comment normally
+    super(comment)
+
+    @comment
+  end
+
   def find_see # :nodoc:
     return nil if singleton || is_alias_for
 

lib/rdoc/yard.rb

@@ -0,0 +1,134 @@
+# frozen_string_literal: true
+
+##
+# YARD compatibility module for RDoc
+#
+# This module provides support for parsing YARD tags in RDoc comments.
+# Currently supports:
+# - @yield [params] - Describes block parameters
+# - @private - Marks method/class as private
+# - @api private - Alternative private marking
+
+class RDoc::YARD
+
+  ##
+  # Processes YARD tags in the given comment text for a code object
+  #
+  # Returns modified comment text with YARD tags processed
+  def self.process(comment, code_object)
+    new(comment, code_object).process
+  end
+
+  attr_reader :comment, :code_object, :text
+
+  def initialize(comment, code_object)
+    @comment = comment
+    @code_object = code_object
+  end
+
+  def process
+    return comment unless comment && code_object
+
+    # Only process RDoc::Comment objects to avoid modifying plain strings
+    # that aren't actually documentation comments
+    return comment unless comment.is_a?(RDoc::Comment)
+
+    # Skip processing if comment is in TomDoc format
+    return comment if comment.format == 'tomdoc'
+
+    @text = comment.text.dup
+
+    # Process @yield tags
+    process_yield_tags
+
+    # Process @private tags
+    process_private_tags
+
+    # Process @api tags
+    process_api_tags
+
+    # Only update comment.text if we actually modified it
+    # This preserves the @document for comments created from parsed documents
+    if text != comment.text
+      comment.text = text
+    end
+
+    comment
+  end
+
+  private
+
+  ##
+  # Process @yield tags to extract block parameters
+  def process_yield_tags
+    return unless code_object.respond_to?(:block_params=)
+
+    # Skip if already has block_params from :yields: directive
+    return if code_object.block_params && !code_object.block_params.empty?
+
+    # Match @yield [params] description
+    if text =~ /^\s*#?\s*@yield\s*(?:\[([^\]]*)\])?\s*(.*?)$/
+      params = $1
+
+      if params && !params.empty?
+        # Clean up the params - remove types if present
+        clean_params = extract_param_names(params)
+        code_object.block_params = clean_params unless clean_params.empty?
+      end
+
+      # Remove the @yield line from comment
+      text.gsub!(/^\s*#?\s*@yield.*?$\n?/, '')
+    end
+  end
+
+  ##
+  # Process @private tags to set visibility
+  def process_private_tags
+    return unless code_object.respond_to?(:visibility=)
+
+    if text =~ /^\s*#?\s*@private\s*$/
+      code_object.visibility = :private
+
+      # Remove the @private line from comment
+      text.gsub!(/^\s*#?\s*@private\s*$\n?/, '')
+    end
+  end
+
+  ##
+  # Process @api tags (specifically @api private)
+  def process_api_tags
+    return unless code_object.respond_to?(:visibility=)
+
+    if text =~ /^\s*#?\s*@api\s+private\s*$/
+      code_object.visibility = :private
+
+      # Remove the @api private line from comment
+      text.gsub!(/^\s*#?\s*@api\s+private\s*$\n?/, '')
+    end
+  end
+
+  ##
+  # Extract parameter names from YARD type specification
+  # e.g., "[String, Integer]" -> "value1, value2"
+  # e.g., "[item, index]" -> "item, index"
+  def extract_param_names(params_string)
+    return '' if params_string.nil? || params_string.empty?
+
+    # If params look like variable names (lowercase start), use them
+    if params_string =~ /^[a-z_]/
+      params_string.split(',').map(&:strip).join(', ')
+    else
+      # If they look like types (uppercase start), generate generic names
+      types = params_string.split(',').map(&:strip)
+      types.each_with_index.map do |type, i|
+        if type =~ /^[A-Z]/
+          # It's a type, generate a generic param name
+          "arg#{i + 1}"
+        else
+          # It's already a param name
+          type
+        end
+      end.join(', ')
+    end
+  end
+end