@type t() :: %RustQ.Syn.Type.Fn{
  abi: String.t() | nil,
  arg_names: [String.t() | nil],
  args: [RustQ.Syn.type()],
  code: String.t(),
  external: boolean(),
  lifetimes: [String.t()],
  returns: RustQ.Syn.type() | nil,
  unsafe: boolean(),
  variadic: boolean(),
  variadic_name: String.t() | nil
}