/// Return the index where the key-value pair exists or will be inserted.
Return the index where the key-value pair exists or will be inserted.
Return the index where the key - value pair exists or will be inserted .
pub fn index(&self) -> usize {
        match *self {
            Entry::Occupied(ref entry) => entry.index(),
            Entry::Vacant(ref entry) => entry.index(),
        }
    }
pub fn index ( & self ) -> usize { match * self { Entry :: Occupied ( ref entry ) => entry . index ( ) , Entry :: Vacant ( ref entry ) => entry . index ( ) , } }
Return the index where the key-value pair exists or will be inserted.
Return the index where the key - value pair exists or will be inserted .
&self