You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: smallbox.rs
    path of file: ./repos/smallbox/src
    
    the code of the file until where you have to start completion: use core::any::Any;
use core::cmp::Ordering;
use core::fmt;
use core::hash::Hash;
use core::hash::{self};
use core::marker::PhantomData;
#[cfg(feature = "coerce")]
use core::marker::Unsize;
use core::mem::MaybeUninit;
use core::mem::{self};
use core::ops;
#[cfg(feature = "coerce")]
use core::ops::CoerceUnsized;
use core::ptr;

use ::alloc::alloc::Layout;
use ::alloc::alloc;

#[cfg(feature = "coerce")]
impl<T: ?Sized + Unsize
    
    fn test_downcast() {
        let stacked: SmallBox<dyn Any, S1> = smallbox!(0x01u32);
        assert!(!stacked.is_heap());
        assert_eq!(SmallBox::new(0x01), stacked.downcast::<u32>().unwrap());

        let heaped: SmallBox<dyn Any, S1> = smallbox!([1usize, 2]);
        assert!(heaped.is_heap());
        assert_eq!(
            smallbox!([1usize, 2]),
            heaped.downcast::<[usize; 2]>().unwrap()
        );

        let stacked_send: SmallBox<dyn Any + Send, S1> = smallbox!(0x01u32);
        assert!(!stacked_send.is_heap());
        assert_eq!(SmallBox::new(0x01), stacked_send.downcast::<u32>().unwrap());

        let heaped_send: SmallBox<dyn Any + Send, S1> = smallbox!([1usize, 2]);
        assert!(heaped_send.is_heap());
        assert_eq!(
            SmallBox::new([1usize, 2]),
            heaped_send.downcast::<[usize; 2]>().unwrap()
        );

        let mismatched: SmallBox<dyn Any, S1> = smallbox!(0x01u32);
        assert!(mismatched.downcast::<u8>().is_err());
        let mismatched: SmallBox<dyn Any, S1> = smallbox!(0x01u32);
        assert!(mismatched.downcast::<u64>().is_err());
    }