As we discussed in Chapter 1, Hello Rust!, stack variables are preferred thanks to their low overhead and speed compared to heap-allocated data, which automatically introduces overhead thanks to the necessary heap pointer. For stack variables, Rust's types even allow for zero overhead structures, so no additional metadata is stored. The following snippet asserts that there are no additional bytes being used for arrays or user-defined types:

use std::mem;

struct MyStruct {
    a: u8,
    b: u8,
    c: u8
}

fn main() {
    assert_eq!(mem::size_of::<MyStruct>(), 3 * mem::size_of::<u8>());
    assert_eq!(mem::size_of::<[MyStruct; 2]>(), 3 * mem::size_of::<u8>() * 2);
}

Consequently, the size of an instance of the MyStruct type is always going to be three bytes—perfectly suitable for placing it on the stack. Why is that good? In short, data...