less-than ideal representation for enums that contain a large element with multiple niches #145128
Description
I tried this code:
use std::num::NonZero; enum Enum { BigWithNiches((NonZero<u32>, NonZero<u32>)), Small1(u16), Small2(u16), } fn main() { dbg!(size_of::<Enum>(), size_of::<Union>()); } #[repr(C)] union Union { big_with_niches: (NonZero<u32>, NonZero<u32>), small: (u32, u16, u16), } impl Union { fn as_enum(self) -> Enum { let (niche, tag, val) = unsafe { self.small }; if niche == 0 { if tag == 0 { Enum::Small1(val) } else { Enum::Small2(val) } } else { Enum::BigWithNiches(unsafe { self.big_with_niches }) } } }I expected to see this happen: Both datatypes are the same size
Instead, this happened: rustc is not able to find the optimization that the union implements manually
Meta
rustc --version --verbose:
1.91.0-nightly (2025-08-07 2fd855fbfc8239285aa2)