So the implementation itself is for T: ?Sized, but the actual functions themselves are for T: Sized.

i.e.

#[lang = "const_ptr"]
impl<T: ?Sized> *const T {
    #[inline]
    pub fn is_null(self) -> bool where T: Sized {
        self == 0 as *const T
    }
}

This makes absolutely no sense. First: why would you impl<T: ?Sized> and then not allow you to call any functions on T: !Sized. Second, why can you only call is_null on T: Sized? Makes absolutely no sense.