Gem::Ext::CargoBuilder#write_deffile

# File tmp/rubies/ruby-3.3.0/lib/rubygems/ext/cargo_builder.rb, line 304
def write_deffile(dest_dir, crate_name)
  deffile_path = File.join(dest_dir, "#{crate_name}-#{RbConfig::CONFIG["arch"]}.def")
  export_prefix = makefile_config("EXPORT_PREFIX") || ""

  File.open(deffile_path, "w") do |f|
    f.puts "EXPORTS"
    f.puts "#{export_prefix.strip}Init_#{crate_name}"
  end

  deffile_path
end