Add frozen string literal comment into generated files in rails v5.1.0

add_frozen_string_literal_comment.rb

# frozen_string_literal: true

# Adds a `frozen_string_literal` comment to the top of files created by Rails
# generators.
#
# Originally inspired by:
#   https://gist.github.com/ta1kt0me/6a7058d16621785d4f7038bde6cd3b98
# Modified version for Rails 6 at:
#   https://gist.github.com/thornomad/4e2f0905e2a4a6eefbc4be5772dfd4f7

require 'rails/generators/migration'
require 'rails/generators/actions/create_migration'
require 'rails/generators/named_base'

module AddFrozenStringLiteralComment
  def add_frozen_string_literal_comment(dist)
    return unless File.exist?(dist) && File.extname(dist) == '.rb'

    File.open(dist, 'r') do |f|
      body = f.read

      File.open(dist, 'w') do |new_f|
        new_f.write("# frozen_string_literal: true\n\n" + body)
      end
    end
  end
end

module GeneratorPrepend
  include AddFrozenStringLiteralComment

  def invoke!
    res = super
    add_frozen_string_literal_comment(existing_migration)
    res
  end
end

module TemplatePrepend
  include AddFrozenStringLiteralComment

  def template(source, *args, &block)
    res = super
    add_frozen_string_literal_comment(args.first)
    res
  end
end

Rails::Generators::Actions::CreateMigration.prepend GeneratorPrepend
Rails::Generators::NamedBase.prepend TemplatePrepend

A simpler version of this might be

return unless defined?(::Rails::Generators)

module RailsGeneratorFrozenStringLiteralPrepend
  RUBY_EXTENSIONS = %w[.rb .rake]

  def render
    return super unless RUBY_EXTENSIONS.include? File.extname(self.destination)
    "# frozen_string_literal: true\n\n" + super
  end
end

Thor::Actions::CreateFile.prepend RailsGeneratorFrozenStringLiteralPrepend