use std::io::{Cursor, Read};

fn main() {
    let v = [0, 1, 2, 3];

    let mut cur = Cursor::<&[u8]>::new(&v);
    let mut buf = Vec::with_capacity(4); // Length is known only at runtime
    unsafe { buf.set_len(4); }
    cur.read_exact(&mut buf).unwrap();
    println!("Buffer: {:?}", buf);
}