Box with Recursive Data Structures

Recursive data types or data types with dynamic sizes need to use a Box:

#[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:?}");
}
הההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההההה
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
StackHeaplistTagConsTagConsTagNil010211

Speaker Notes

If the Box was not used here and we attempted to embed a List directly into the List, the compiler would not compute a fixed size of the struct in memory, it would look infinite.

Box solves this problem as it has the same size as a regular pointer and just points at the next element of the List in the heap.