Idiomatic Rust would probably avoid allocations, which means something more like

    fn main() {
        if let Some(arg) = std::env::args().nth(1) {
            println!("{}", {
                match arg.char_indices().nth(12) {
                    Some((idx, _)) => &arg[..idx],
                    None => &*arg
                }
            });
        }
    }

With the `unicode-segmentation` crate[1], you can just swap `char_indices()` with `grapheme_indices(true)`.

[1] https://crates.io/crates/unicode-segmentation