Rust Box

pub enum List {
    Cons(i32, Box<List>),
    Nil,
}
 
List::Cons(1, Box::new(List::Nil))

Boxes don’t have performance overhead, other than storing their data on the heap instead of on the stack.

• Use cases
  • When you have a type whose size can’t be known at compile time and you want to use a value of that type in a context that requires an exact size
  • When you have a large amount of data and you want to transfer ownership but ensure the data won’t be copied when you do so
  • When you want to own a value and you care only that it’s a type that implements a particular trait rather than being of a specific type

Box<dyn Error>

• Boxing errors - Rust By Example

A way to write simple code while preserving the original errors is to Box them. The drawback is that the underlying error type is only known at runtime and not statically determined.