Function identity_write_iterator_fn 

pub fn identity_write_iterator_fn(_: &WriteContextArgs<'_>) -> TokenStream

Helper to write the write_iterator portion of OperatorConstraints::write_fn output for unary identity operators.

When no explicit type argument is given and AsCodeOptions::exclude_type_guards is set, the identity check_input/check_output type guard is omitted and the input/output is simply rebound.