Psych::Visitors::YAMLTree#visit_String

# File tmp/rubies/ruby-4.0.0/ext/psych/lib/psych/visitors/yaml_tree.rb, line 290
def visit_String o
  plain = true
  quote = true
  style = Nodes::Scalar::PLAIN
  tag   = nil

  if binary?(o)
    o     = [o].pack('m0')
    tag   = '!binary' # FIXME: change to below when syck is removed
    #tag   = 'tag:yaml.org,2002:binary'
    style = Nodes::Scalar::LITERAL
    plain = false
    quote = false
  elsif o.match?(/\n(?!\Z)/)  # match \n except blank line at the end of string
    style = Nodes::Scalar::LITERAL
  elsif o == '<<'
    style = Nodes::Scalar::SINGLE_QUOTED
    tag   = 'tag:yaml.org,2002:str'
    plain = false
    quote = false
  elsif o == 'y' || o == 'Y' || o == 'n' || o == 'N'
    style = Nodes::Scalar::DOUBLE_QUOTED
  elsif @line_width && o.length > @line_width
    style = Nodes::Scalar::FOLDED
  elsif o.match?(/^[^[:word:]][^"]*$/)
    style = Nodes::Scalar::DOUBLE_QUOTED
  elsif not String === @ss.tokenize(o) or /\A0[0-7]*[89]/.match?(o)
    style = Nodes::Scalar::SINGLE_QUOTED
  end

  is_primitive = o.class == ::String
  ivars = is_primitive ? [] : o.instance_variables

  if ivars.empty?
    unless is_primitive
      tag = "!ruby/string:#{o.class}"
      plain = false
      quote = false
    end
    @emitter.scalar o, nil, tag, plain, quote, style
  else
    maptag = '!ruby/string'.dup
    maptag << ":#{o.class}" unless o.class == ::String

    register o, @emitter.start_mapping(nil, maptag, false, Nodes::Mapping::BLOCK)
    @emitter.scalar 'str', nil, nil, true, false, Nodes::Scalar::ANY
    @emitter.scalar o, nil, tag, plain, quote, style

    dump_ivars o

    @emitter.end_mapping
  end
end