> cargo buildwarning: field is never used: `head`--> src/fourth.rs:5:5|5| head:Link<T>,|^^^^^^^^^^^^^|= note: #[warn(dead_code)] on by defaultwarning: field is never used: `tail`--> src/fourth.rs:6:5|6| tail:Link<T>,|^^^^^^^^^^^^^warning: field is never used: `elem`--> src/fourth.rs:12:5|12| elem:T,|^^^^^^^warning: field is never used: `next`--> src/fourth.rs:13:5|13| next:Link<T>,|^^^^^^^^^^^^^warning: field is never used: `prev`--> src/fourth.rs:14:5|14| prev:Link<T>,|^^^^^^^^^^^^^