Missing niche optimization in non-trivial enum

[dragostis]

In the following example:

use std::{mem, num::NonZero};

struct Foo {
    a: NonZero<u32>,
    b: u32,
}

enum Bar {
    Foo([Foo; 2]),
    Bar {
        a: u32,
        b: *mut Foo,
    }
}

fn main() {
    println!("{}", mem::size_of::<Bar>()); // 24
}

Bar is 24 bytes when it could be optimized to 16 by aliasing Bar::Foo first element's a field with the 32-bit padding from Bar::Bar by offsetting a to 4 instead of 0. Thus, the 0 bit pattern of a/padding would be the discriminant for Bar::Bar and any other pattern would be the discriminant for Bar::Foo.

Maybe this optimization is not possible because a discriminant needs to be unique?

EDIT: a better way to triage this

use std::{mem, num::NonZero};

struct Foo {
    a: NonZero<u32>,
    b: u32,
}

enum BarSmall {
    Foo([Foo; 2]),
    Bar((u32, [u8; 8])),
}

enum BarBig {
    Foo([Foo; 2]),
    Bar((u32, u64)),
}

fn main() {
    println!("{}", mem::size_of::<BarSmall>()); // 16
    println!("{}", mem::size_of::<BarBig>()); // 24
}