Tracking issue for release notes of #134367: Stabilize feature(trait_upcasting)

[rustbot]

This issue tracks the release notes text for #134367.

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Release notes text

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# Language
- [Stabilize `feature(trait_upcasting)`](https://github.com/rust-lang/rust/pull/134367)

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# Trait upcasting

This release includes a long awaited feature — the ability to upcast trait objects.
If a trait has a [supertrait](https://doc.rust-lang.org/reference/items/traits.html#supertraits) you can coerce trait object of that trait to a trait object of the super trait:

```rust
trait Trait: Supertrait {}
trait Supertrait {}

fn upcast(x: &dyn Trait) -> &dyn Supertrait {
    x
}
```

The same would work with any other kind of (smart)-pointer, like `Arc<dyn Trait> -> Arc<dyn Supertrait>` or `*const dyn Trait -> *const dyn Supertrait`.

Note that this adds a new _safety invariant_  to raw pointers — leaking a raw pointer to a trait object with an invalid vtable for that trait into safe code may lead to undefined behavior, since trait upcasting works on raw pointers and requires a valid vtable.

Trait upcasting may be especially useful with the `Any` trait, as it allows upcasting your trait object to `dyn Any` to call the downcast methods. Whereas before you'd have to write workarounds for this or use external crates.

```rust
use std::any::Any;

trait MyAny: Any {}

impl dyn MyAny {
    fn downcast_ref<T>(&self) -> Option<&T> {
        (self as &dyn Any).downcast_ref()
    }
}
```

You can [learn more about trait upcasting in the Rust reference](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions).

cc @WaffleLapkin, @compiler-errors -- origin issue/PR authors and assignees for starting to draft text