start to describe trait upcasting

Author: tshepang

src/changelog.rst

@@ -18,6 +18,13 @@ with the change that has been applied due to it.
    just the language changes that had an impact to the FLS. See the `release
    notes`_ for a full list of changes.
 
+Language changes in Rust 1.86.0
+-------------------------------
+
+- `Stabilize upcasting trait objects to supertraits. <https://github.com/rust-lang/rust/pull/134367>`_
+
+  - New paragraph: :p:`fls_QB4c6FNKxaPl`
+
 Language changes in Rust 1.85.0
 -------------------------------
 

src/types-and-traits.rst

@@ -2048,6 +2048,19 @@ occur when:
 * :dp:`fls_SYnFJBhi0IWj`
   The source :t:`type` is a :t:`trait object type` and the target :t:`type` is a :t:`trait object type` with the same or no :t:`[principal trait bound]`, and the target :t:`type` has the same or less non-:t:`principal trait` :t:`[trait bound]s`.
 
+* :dp:`fls_QB4c6FNKxaPl`
+  The source :t:`type` is ``dyn T`` and the target :t:`type` is ``dyn U``,
+  where ``U`` is a :t:`supertrait` of ``T``.
+
+  .. code-block:: rust
+
+     trait Sub: Super {}
+     trait Super {}
+
+     fn upcast(x: &dyn Sub) -> &dyn Super {
+         x // implicit coercion
+     }
+
 :dp:`fls_iiiu2q7pym4p`
 An :t:`unsized coercion` is a :t:`type coercion` that converts a :t:`sized type`
 into an :t:`unsized type`. :t:`Unsized coercion` from a source :t:`type` to a