1-basics: slice.get().unwrap_or() does not compile about gentle-intro HOT 3 CLOSED

    let maybe_last = slice.get(5);
    let mut last = -1;
    if maybe_last.is_some() {
        last = *maybe_last.unwrap();
    }

    let maybe_last = slice.get(5);
    let last = if maybe_last.is_some() {
        *maybe_last.unwrap()
    } else {
        -1
    }

    let maybe_last = slice.get(5);
    let last =  if slice.get(5).is_some() {
        slice[5]
    } else {
         -1
   }