https://github.com/rust-lang/rust/issues/81513#issuecomment-1109515624

thin_slice.rs

// pseudocode

pub struct ThinSlice<T> {
    len: usize,
    slice: [T],
}

impl Pointee for ThinSlice<T> {
    type Metadata = ThinSliceMetadata<T>;

    // ???????
    unsafe fn size_of(ptr: *const Self) -> usize {
        unsafe { (*ptr).len * size_of::<T>() }
    }
}

#[derive(all_the_stuff)]
pub struct ThinSliceMetadata<T>(PhantomData<T>);
  
pub struct ThinArray<T, const N: usize> {
    len: usize, // always = N to support unsizing
    array: [T; N],
}

// ???
impl<T, const N: usize> Unsize<ThinSlice<T>> for ThinArray<T, N> {}

fn main() {
    // Noop, ptr doesn't need any adjustments
    let _: *const ThinSlice<u8> =  ptr::null::<ThinArray<u8, 17>>();
}

thin_trait_obj.rs

// pseudocode

pub struct ThinAny {
    meta: DynMetadata<dyn Any>,
    obj: dyn Any,
}

impl Pointee for ThinAny {
    type Metadata = ThinAnyMetadata;

    // ???????
    unsafe fn align_of(ptr: *const Self) -> usize {
        unsafe { (*ptr).meta.align_of() }
    }
      
    // ???????
    unsafe fn size_of(ptr: *const Self) -> usize {
        unsafe { (*ptr).meta.size_of() }
    }
}

#[derive(all_the_stuff)]
#[non_exhaustive]
pub struct ThinAnyMetadata{};
  
pub struct ThinObj<T> {
    meta: DynMetadata<dyn Any>, // to support unsizing
    obj: T,
}

// ???
impl<T> Unsize<ThinAny> for ThinObj<T> {}

fn main() {
    // Noop, ptr doesn't need any adjustments
    let _: *const ThinAny =  ptr::null::<ThinObj<u8>>();
}