Lib.rs

What if the dyn were something of known size?

With normal usage async (whether std or core) you can write steadily:

async fn async_things(...) -> T {}

// it is equivalent to:
fn async_things(...) -> impl Future<Output=T> {}

This is okay, because in place of impl can be any type, and we do not care where it is and how its memory is freed.
Everything stays fine until you want to `async trait'. Imagine that you have such a trait:

mod io {
    type Result<T> = ...;
}

pub trait AsyncRead {
    async fn read(&mut self, buf: &mut [u8]) -> io::Result<usize>;

    ...
}

Previously you would have had to rewrite `read' like this:

fn poll_read(
    self: Pin<&mut Self>,
    cx: &mut Context<'_>,
    buf: &mut [u8]
) -> Poll<io::Result<usize>>;

But now you can write it this way:

fn read(&mut self, buf: &mut [u8]) -> impl Future<Output=io::Result<usize>>;
// aka: async fn read(&mut self, buf: &mut [u8]) -> io::Result<usize>;

This creates a new type implementing Future for each individual implementation, so it is not object-safety:

error[E0038]: the trait `AsyncRead` cannot be made into an object
  --> src\main.rs:20:12
   |
20 |     let _: dyn AsyncRead = todo!();
   |            ^^^^^^^^^^^^^ `AsyncRead` cannot be made into an object
   |
note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
  --> src\main.rs:16:43
   |
15 | pub trait AsyncRead {
   |           --------- this trait cannot be made into an object...
16 |     fn read(&mut self, buf: &mut [u8]) -> impl Future<Output = io::Result<usize>>;
   |                                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...because method `read` references an `impl Trait` type in its return type
   = help: consider moving `read` to another trait

Problem: no memory allocator in core

The obvious solution is to use Box<dyn Future>, But in no-std systems (where only core and alloc are available) the Box hardcode seems redundant, since it needs allocators.

Problem: &dyn Traits can't be substituted for impl Traits

But what if there was a type that allows you to save a type whose size is the same as the pointer along with its Vtable, which stores the pointer to drop. Then we would get a kind of new fat pointer that lives as T: 'a' (as opposed to &'a T' of `&dyn Trait')

Basic Usage

In today's rast, there is no mechanism for pass dyn Trait to impl Trait:

fn print(x: impl Display) {
    println!(“{x}”);
}

and we can easily use it in 'static context:

fn print_later(x: impl Display + Send + 'static) {
    thread::spawn(move || println!("{x}"));
}

But how can we send here &dyn Trait not by reference, but by value? Of course Box.

fn print_later(x: Box<dyn Display + Send>) {
    thread::spawn(move || println!("{x}"));
}

However, this always requires a memory allocation, which often (and no_std almost always) does not make sense.

What if you just store dyn Trait?

But how much space does it take up? And how is it aligned? You have to know all this at compile time, and it's definitely not about dyn.
But how do you choose the size?
Let's remember one fact - Box<T> is transparent as pointer. Then, if you imagine our new type, it looks something like this:

struct DynPtr<Dyn: ?Sized> {
    repr: PtrRepr,
    // is alias to `*const ()` not to be confused 
    drop: unsafe fn(*const ()),
}

Remind you of anything? This is very similar to the most popular way to create vtables in std. This is what the raw creation of DynPtr for Box might look like:

let ptr = DynPtr::<dyn Display> {
    repr: Box::into_raw(x) as PtrRepr,
    drop: |repr| unsafe {
        let _ = Box::from_raw(repr as *mut i32);
    },
};
(ptr.drop)(ptr.repr); // drop original box 

You may notice that from_raw/into_raw can be generalized to the case of mem::transmute since the repr of Box similar pointer (*const ()).
Also in favor of this method would be the fact that in practice many futures consist of a very small state (which is equal to the pointer or even smaller) or its state is also boxed in Arc-like. This means that in the general case you can continue to use Box, which is also comparable to a pointer, and for sufficiently thin primitives use your own transformation.

This is how Dyn<dyn Trait> appears (dyn in this case is needed because of the requirements of the 2021 edition of Rust), which can be used almost as easily as impl Trait:

fn print_later(x: Dyn<dyn Display + Send + 'static>) {
    thread::spawn(move || println!("{}", &*x)); // `Dyn` is not automatically delegate traits like `dyn`
}

// use `.do_dyn()` to coerce type into `Dyn<dyn ...>`
let x = & 12;
print_later(x.do_dyn());
print_later(12_usize.do_dyn());
print_later(Box::new(x).do_dyn());