Rust program for checking performance of Iterators

iterator.rs

extern crate rand;

use rand::Rng;
use std::io::BufWriter;
use std::io::Write;
use std::fs::OpenOptions;
use std::fs::File;
use std::str::FromStr;
use std::io::BufReader;
use std::io::BufRead;
use std::string::String;
use std::str::from_utf8;
use std::time::Instant;

fn main() {
    let filename : String = String::from_str("/tmp/rand.txt")
        .expect("unable to create string");
    let mut buffer : [i32; 10000] = read_from_file(filename);
    let coefficients: [i64; 12] = [34, 45, 56, 78, 90, 32, 12, 13, 45, 56, 89, 67];
    let qlp_shift: i16 = 0;
    /**
    * **********************************************
    * Only measuring the performance of below loop.
    * **********************************************
    */
    let now = Instant::now();
    for i in 12..buffer.len() {
        let prediction = coefficients.iter()
            .zip(&buffer[i - 12..i])
            .map(|(&c, &s)| c * s as i64)
            .sum::<i64>() >> 32;
        let delta = buffer[i];
        buffer[i] = prediction as i32 + delta;
    }
    println!("Elapsed time: {}", now.elapsed().as_nanos())
}

fn read_from_file(fname : String) -> [i32; 10000] {
    let file = File::open(fname)
        .expect("Could not open file");
    let mut array= [0; 10000];
    let mut reader = BufReader::new(file);
    let mut buf = vec![];
    let mut num_bytes = reader.read_until(b' ', &mut buf)
        .expect("Cursor read should not fail");
    let mut i = 0;
    while num_bytes != 0 {
        let int_str: &str = from_utf8(buf.as_slice())
            .expect("unable to create string")
            .trim();
        if !int_str.is_empty() {
            array[i] = int_str.parse::<i32>().expect(&format!("int_str cannot be parsed into digit {} ", int_str));
            i = i+1;
        }
        buf.clear();
        num_bytes = reader.read_until(b' ', &mut buf)
            .expect("Cursor read should not fail");
    }
    assert_eq!(array.len(), 10000);
    array
}