Why can't `&(?Sized + Trait)` be cast to `&dyn Trait`?

Question

In the code below it is not possible to obtain a reference to a trait object from a reference to a dynamically-sized type implementing the same trait. Why is this the case?

Answer 1

Not sure if that solves your concrete problem, but I did solve mine with the following trick:

I added the following method to FooTrait:

fn as_dyn(&self) -> &dyn FooTrait;

A default impl can not be provided (because it requires that Self be Sized, but constraining FooTrait to be Sized forbids creating trait objects for it...).

However, for all Sized implementations, it is trivially implemented as

fn as_dyn(&self) -> &dyn FooTrait { self }

So basically it constrains all implementations of FooTrait to be sized, except for dyn FooTrait.

Try it on the playground