ICE: `Size::sub: 0 - 1 would result in negative size` about rust HOT 4 OPEN

#![feature(transmutability)]

mod assert {
    use std::mem::{Assume, BikeshedIntrinsicFrom};

    pub fn is_transmutable<Src, Dst>()
    where
        Dst: BikeshedIntrinsicFrom<Src, { Assume::NOTHING }>,
    {
    }
}

struct Zst;

#[repr(u8)]
enum V0 { V = 0 }

enum Lopsided {
    Smol(Zst),
    Lorg(V0),
}

fn should_pad_variants() {
    assert::is_transmutable::<Lopsided, ()>();
}