Missed niche optimization (Box<impl Sized> & Box<impl Unsized>)

[CAD97]

playground

enum WithBox {
    Sized(Box<Sized>),
    Unsized(Box<Unsized>),
}

enum WithUnitPtr {
    Sized(  /*   0usize    */ ptr::NonNull<()>),
    Unsized(ptr::NonNull<()>, usize           ),
}

fn main() {
    dbg!(size_of::<WithBox>());        // = 24
    dbg!(size_of::<WithUnitPtr>());    // = 24
    dbg!(size_of::<ManuallyNiched>()); // = 16
}

struct ManuallyNiched {
    a: usize,
    b: usize,
}

impl From<WithBox> for ManuallyNiched {
    fn from(this: WithBox) -> ManuallyNiched {
        match this {
            WithBox::Sized(this) => ManuallyNiched {
                a: 0,
                b: Box::into_raw(this) as usize,
            },
            WithBox::Unsized(this) => ManuallyNiched {
                b: this.len(),
                a: Box::into_raw(this) as *mut Sized as usize,
            },
        }
    }
}

impl From<ManuallyNiched> for WithBox {
    fn from(this: ManuallyNiched) -> WithBox {
        unsafe {
            match this.a {
                0 => WithBox::Sized(Box::from_raw(this.b as *mut _)),
                _ => WithBox::Unsized(Box::from_raw(slice::from_raw_parts_mut(
                    this.a as *mut _,
                    this.b,
                ))),
            }
        }
    }
}

Specifically, the tagged union of ptr::NonNull<{Sized type}> and ptr::NonNull<{!Sized type}> can niche together to be the size of ptr::NonNull<{!Sized type}> (for unsized types with non-zero sized metadata) by storing the discriminant for the sized pointer in the 0 niche of the non-null pointer to the unsized type (and the pointer to the sized type in the pointer metadata of the unsized type).

[moshg]

I think this is a special case of #46213.

[petertodd]

Maybe I'm misunderstanding what you're trying to demonstrate here, but the type:

type Unsized = [usize];

doesn't lead to Box<[usize]> niche other than the NonNull pointer, as a slice's metadata can have any value, including zero¹.

Changing that to, say:

type Unsized = dyn Drop;

is a better example, as the vtable should never be null.

1. ...actually a slice length should be < isize::MAX. But IIUC the current niche scheme can't handle niches where the valid range ends before a certain point; it only handles cases where the valid range starts at a certain point.

[CAD97]

@petertodd that one niche is enough. Box<Sized> is one usize big, and Box<Unsized> is two usize big. The niched representation is to represent Box<Unsized> as is, and to represent Box<Sized> by exploiting the 0 niche in the nonnull pointer of Box<Unsized> and then putting the Box<Sized> in the other remaining usize of space.

[petertodd]

@CAD97 Thanks! I see what you mean now.

[CAD97]

This niche is now used and the playground prints 16 (on 64-bit) for the tested enums. Going ahead and closing as completed, as I don't think this needs a regression test (it's covered by other niche tests and isn't a guaranteed behavior).