Niche Optimization

#[derive(Debug)]
enum List<T> {
    Cons(T, Box<List<T>>),
    Nil,
}

fn main() {
    let list: List<i32> = List::Cons(1, Box::new(List::Cons(2, Box::new(List::Nil))));
    println!("{list:?}");
}

A Box cannot be empty, so the pointer is always valid and non-null. This allows the compiler to optimize the memory layout: