Multiline rust input with known and unknown number of lines

knownlines.rs

//https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9cb93f0342793c225be94aa46897c25a
use std::io::{Cursor, BufReader};
// for console input replace the above line with this
// use std::io::stdin;

fn main() {
    // simulate stdin in rust playground
    let f = Cursor::new("3\n40 3\n30 4\n20 5".to_string());
    let input = BufReader::new(f);
    let mut vec = input.lines();
    // for console input replace the above lines with these
    // let input = stdin();
    // let mut vec = input.lock().lines();
    let n = vec.next().unwrap().unwrap().parse::<usize>().unwrap();
    let mut ans = String::with_capacity(n * 5);
    for _ in 0..n {
        let v = vec.next().unwrap().unwrap();
        let mut i = v.split_whitespace();
        let x = i.next().unwrap().parse::<i64>().unwrap();
        let y = i.next().unwrap().parse::<i64>().unwrap();
        writeln!(&mut ans, "{} {}", x, y).ok();
    }
    println!("{}", ans);
}

unknownlines.rs

//https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=9b0bc18d0a49960bbfc6149dfc708142
// read multiple lines where the number of lines are not known upto <EOF>
//output the input as is

use std::fmt::Write;
use std::io::BufRead;
use std::io::{Cursor, BufReader};
// for console input replace the above line with this
// use std::io::stdin;

fn main() {
    // simulate stdin in rust playground
    let f = Cursor::new("3\n40 3\n30 4\n20 5".to_string());
    let input = BufReader::new(f);
    let input = input.lines();
    // for console input replace the above lines with these
    // let input = stdin();
    // let mut vec = input.lock().lines();
    let mut buf = String::new();
    for line in input {
        writeln!(&mut buf, "{}", line.unwrap()).unwrap();
    }
    print!("{}", buf);
}