To implement its safety guarantees, the Rust compiler keeps careful track of ownership and references throughout a program. This makes writing certain kinds of data structures challenging; in particular, data structures that have circular references. Let's start with a simple binary tree: struct Tree { root: Option<Node>, } struct Node { data: i32, left: Option<Box<Node>>, right: Option<Box<Node>>, } Since the Rust compiler should be able to calculate the size of a struct at compile-time, left a...