fn main() {

use std::io::{self, Write};
let mut line: Vec<u8> = Vec::new();
// ... fill line before slicing ...
line.extend_from_slice(b"hello");
let w = &line[..5];
let mut out = io::stdout().lock();
out.write_all(w).unwrap();
out.write_all(b"\n").unwrap();
line.clear(); // w's last use is behind the clear

assert!(line.is_empty());
}