rust - Can I use the "null pointer optimization" for my own non-pointer types?

Question

When you have an Option<&T>, the compiler knows that NULL is never a possible value for &T, and encodes the None variant as NULL instead. This allows for space-saving:

use std::mem;

fn main() {
    assert_eq!(mem::size_of::<&u8>(), mem::size_of::<Option<&u8>>());
}

However, if you do the same with a non-pointer type, there's no extra bits to store that value in and extra space is required:

use std::mem;

fn main() {
    // fails because left is 1 and right is 2
    assert_eq!(mem::size_of::<u8>(), mem::size_of::<Option<u8>>()); 
}

In general, this is correct. However, I'd like to opt-in to the optimization because I know that my type has certain impossible values. As a made-up-example, I might have a player character that has an age. The age may be unknown, but will never be as high as 255:

struct Age(u8);

struct Player {
    age: Option<Age>,
}

I'd like to be able to inform the optimizer of this constraint - Age can never be 255, so it's safe to use that bit pattern as None. Is this possible?

Answer 1

As of Rust 1.28, you can use std::num::NonZeroU8 (and friends). This acts as a wrapper that tells the compiler the contents of a number will never contain a literal zero. It's also why Option<Box<T>> is pointer-sized.

Here's an example showing how to create an Age and read its payload.

use std::num::NonZeroU8;

struct Age(NonZeroU8);

impl Age {
    pub fn new(age: u8) -> Age {
        let age = NonZeroU8::new(age).expect("Age cannot be zero!");
        Age(age)
    }

    pub fn age(&self) -> u8 {
        self.0.get()
    }
}

struct Player {
    age: Option<Age>,
}

fn main() {
    println!("size: {}", std::mem::size_of::<Player>());
    // Output: size: 1
}