Add trait_upcasting related languages changes

Author: crlf0710

src/behavior-considered-undefined.md

@@ -49,6 +49,7 @@ undefined behavior, it is *unsound*.
 * Invoking undefined behavior via compiler intrinsics.
 * Executing code compiled with platform features that the current platform
   does not support (see [`target_feature`]), *except* if the platform explicitly documents this to be safe.
+* Performing non-nop coercion on a dangling or unaligned raw pointer.
 * Calling a function with the wrong call ABI or unwinding from a function with the wrong unwind ABI.
 * Producing an [invalid value][invalid-values]. "Producing" a
   value happens any time a value is assigned to or read from a place, passed to

src/type-coercions.md

@@ -159,7 +159,7 @@ Coercion is allowed between the following types:
 ### Unsized Coercions
 
 The following coercions are called `unsized coercions`, since they
-relate to converting sized types to unsized types, and are permitted in a few
+relate to converting types to unsized types, and are permitted in a few
 cases where other coercions are not, as described above. They can still happen
 anywhere else a coercion can occur.
 
@@ -172,6 +172,8 @@ an implementation of `Unsize<U>` for `T` will be provided:
 
 * `T` to `dyn U`, when `T` implements `U + Sized`, and `U` is [object safe].
 
+* `dyn T` to `dyn U`, when `T` has `U` as one of its ancestor trait.
+
 * `Foo<..., T, ...>` to `Foo<..., U, ...>`, when:
     * `Foo` is a struct.
     * `T` implements `Unsize<U>`.