Equivalent of NonZeroU8 but instead of 0, use u8::MAX? - rust

Question

I want to be able to use Option<NonZeroU8> for its compact repr, but still be able to use 0.
Is there a way to use u8::MAX as the non-representable value instead of 0?

Answer 1

To truly get the niche optimization directly, you'd have to use the super unstable rustc_ attributes.
#![feature(rustc_attrs)]
#[rustc_layout_scalar_valid_range_start(0)]
#[rustc_layout_scalar_valid_range_end(254)]
struct NonMaxU8(u8);
fn main() {
dbg!(std::mem::size_of::<Option<NonMaxU8>>());
}
[src/main.rs:8] std::mem::size_of::<Option<NonMaxU8>>() = 1
I say "super unstable" because these attributes will never be stabilized, at least not in their current form. Using them without feature(rustc_attrs) will emit the following message:
error[E0658]: the `#[rustc_layout_scalar_valid_range_start]` attribute is just used to enable niche optimizations in libcore and libstd and will never be stable
--> src/main.rs:3:1
|
3 | #[rustc_layout_scalar_valid_range_start(0)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
error[E0658]: the `#[rustc_layout_scalar_valid_range_end]` attribute is just used to enable niche optimizations in libcore and libstd and will never be stable
--> src/main.rs:4:1
|
4 | #[rustc_layout_scalar_valid_range_end(254)]
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
There have been asks for compiler support for ranged integers, like the discussion here, but I don't think anything has been formally proposed as of yet.

Answer 2

There is the nonmax crate that provides this. See NonMaxU8 specifically. This works because it uses NonZeroU8 internally and does the appropriate conversion upon creation and the .get() method.
If you wanted a different variant, not just zero or max, you can easily make your own. The nonmax crate uses an XOR strategy to zero-out the non-representable value or use a modular arithmetic method like is suggested in the comments:
pub struct NonTenU8(NonZeroU8);
impl NonTenU8 {
const UNREPRESENTABLE: u8 = 10;
/// Creates a u8 value that cannot be ten.
pub fn new(value: u8) -> Option<Self> {
NonZeroU8::new(value ^ Self::UNREPRESENTABLE).map(Self)
// NonZeroU8::new(value.wrapping_sub(Self::UNREPRESENTABLE)).map(Self)
}
pub fn get(&self) -> u8 {
self.0.get() ^ Self::UNREPRESENTABLE
// self.0.get().wrapping_add(Self::UNREPRESENTABLE)
}
}