Consider the following code (play):

type T = (); struct S<X>(X); impl Clone for S<T> { fn clone(&self) -> Self { S(()) } } fn main() { let s = S(()); drop(s.clone()); }

Today this emits the following warning diagnostic:

warning: type alias is never used: `T` --> src/main.rs:1:1 | 1 | type T = (); | ^^^^^^^^^^^^ | = note: #[warn(dead_code)] on by default 

But that type alias is not dead code. It is used in the impl Clone for S<T> { ... }, as one can see by trying to recompile the code after commenting out the type alias.