Missed layout optimisation with multiple niches

[GnomedDev]

I tried this code:

use std::num::NonZeroU8;

struct Test {
    niche_one: NonZeroU8,
    niche_two: NonZeroU8,
}

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

I expected this program to exit fine, and fit the tag of the second option in the available niche.
Instead, the program panics, as the second option wastes a byte of memory making it's own tag byte.

Meta

rustc --version --verbose:

rustc 1.77.0-nightly (e51e98dde 2023-12-31)
binary: rustc
commit-hash: e51e98dde6a60637b6a71b8105245b629ac3fe77
commit-date: 2023-12-31
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6

[saethlin]

I'm pretty sure we already have an issue for this, but I can't find one that's written clearly. #101567 was the first one that clearly looks like a duplicate when I was scrolling.

The problem is that the rustc layout representation (in this case of Test) only represents a single contiguous niche with a single contiuous (albeit wrapping) range of niche values. That should be (probably is somewhere) tracked in an issue.