Error code E0072

A recursive type has infinite size because it doesn't have an indirection.

Erroneous code example:

#![allow(unused)] fn main() { struct ListNode { head: u8, tail: Option<ListNode>, // error: no indirection here so impossible to // compute the type's size } }

When defining a recursive struct or enum, any use of the type being defined from inside the definition must occur behind a pointer (like Box, & or Rc). This is because structs and enums must have a well-defined size, and without the pointer, the size of the type would need to be unbounded.

In the example, the type cannot have a well-defined size, because it needs to be arbitrarily large (since we would be able to nest ListNodes to any depth). Specifically,

size of `ListNode` = 1 byte for `head` + 1 byte for the discriminant of the `Option` + size of `ListNode`

One way to fix this is by wrapping ListNode in a Box, like so:

#![allow(unused)] fn main() { struct ListNode { head: u8, tail: Option<Box<ListNode>>, } }

This works because Box is a pointer, so its size is well-known.