gistfile1.rs

use std::io::{BufferedReader,File};
use std::collections::BinaryHeap; //latest nightlies have switch renamed PriorityQueue to BinaryHeap

fn main() {
    let n = 3u;                                            // n longest lines.
    let path = Path::new("test.txt");
    let mut file = BufferedReader::new(File::open(&path));
    let mut pq = BinaryHeap::new();                     // note: max-heap. See above about BinaryHeap

    for (i, line) in file.lines().enumerate() {
        let contents = line.unwrap();
        let neg_line_length:i32 = -(contents.len() as i32);             // simulate min-heap. need to negate after casting, since contents.len
        let element = (neg_line_length, contents);
        if i <= n {                                        // push first n lines onto
            pq.push(element);                              // heap unconditionally.
        } else {
            let curr_min = pq.top().unwrap().clone();                  // thereafter, add line. The clone() call addresses your actual problem
            let (neg_curr_min_length, _) = curr_min;          // to heap only if its
            if neg_line_length < neg_curr_min_length {         // length greater than
                pq.push_pop(element);                          // current heap minimum.
            }
        }
    }
    for item in pq.into_sorted_vec().iter() {
        println!("{}:{}", -item.0, item.1); //undo your negation and gives your formatting flexbility by accessing the elements of your tuple separately
    }
}