In Rust, an `&Thing` (reference to thing), `Option<&Thing>` (optional reference to thing), `Box<Thing>` (thing on the heap), and `Option<Box<Thing>>` (optional

In Rust, an `&Thing` (reference to thing), `Option<&Thing>` (optional reference to thing), `Box<Thing>` (thing on the heap), and `Option<Box<Thing>>` (optional thing on the heap) all have the same in-memory representation of a single pointer, but prohibit accidentally dereferencing a null pointer, and make non-nullable pointers the default.