Rust function that takes mut ref and returns FnMut

main.rs

fn f<'a>(i: &'a mut i32) -> impl FnMut() -> i32 + 'a {
    move|| {*i = *i + 1; *i}
}

fn main() {
    let mut i = 0;
    {
      let mut ff = f(&mut i);
      dbg!(ff());
      dbg!(ff());
    }
    dbg!(i);
}