あえて >>493 で頑張ると
https://paiza.io/projects/sk40IXUgPm0k_VgI1wsHBg

fn main() {
let cwln = |s| { println!("{}", s) };
let g = |(i, line): (usize, &str)| { format!("{:>2}:{}", i + 1, line) };
let f = |file_name: String| {
cwln(file_name.clone());
std::fs::read_to_string(file_name).map(|u8b| {
u8b.lines().enumerate().map(g).for_each(cwln)
}).unwrap()
};

std::env::args().skip(1).for_each(f)
}