Added page to Unstable Book.

src/doc/unstable-book/src/language-features/trait-upcasting.md

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+# `trait_upcasting`
+
+The tracking issue for this feature is: [#31436]
+
+[#65991]: https://github.com/rust-lang/rust/issues/65991
+
+------------------------
+
+The `trait_upcasting` feature adds support for trait upcasting. This allows a
+trait object of type `dyn Foo` to be cast to a trait object of type `dyn Bar`
+so long as `Foo: Bar`.
+
+```rust,edition2018
+#![feature(trait_upcasting)]
+
+trait Foo {}
+
+trait Bar: Foo {}
+
+impl Foo for i32 {}
+
+impl<T: Foo + ?Sized> Bar for T {}
+
+let foo: &dyn Foo = &123;
+let bar: &dyn Bar = foo;
+```