#![allow(dead_code, deprecated, unused_variables, unused_mut)]
#![allow(dead_code)]
use std::mem;
struct Buffer<T> {
    buf: Vec<T>,
}
impl<T> Buffer<T> {
    fn replace_index(&mut self, i: usize, v: T) -> T {
        mem::replace(&mut self.buf[i], v)
    }
}
fn main() {
    let mut buffer = Buffer { buf: vec![0, 1] };
    assert_eq!(buffer.buf[0], 0);
    assert_eq!(buffer.replace_index(0, 2), 0);
    assert_eq!(buffer.buf[0], 2);
}