The same error could technically occur in Rust if the API was designed that way but I think the "Rust way" would mandate using an `Option` instead of a special

The same error could technically occur in Rust if the API was designed that way but I think the "Rust way" would mandate using an `Option` instead of a special MODE_INVALID value. So it would become something like `mode.unwrap_or(0o644)` which doesn't leave a lot of room for error.