aliasing rules also apply inside private fields

Author: RalfJung

src/behavior-considered-undefined.md

@@ -38,6 +38,8 @@ r[undefined.alias]
   `&T` must point to memory that is not mutated while they are live (except for data inside an [`UnsafeCell<U>`]),
   and `&mut T` must point to memory that is not read or written by any pointer not derived from the reference and that no other reference points to while they are live.
   `Box<T>` is treated similar to `&'static mut T` for the purpose of these rules.
+  These rules apply to *all* references and `Box<T>`, including those inside private fields.
+
   The exact liveness duration is not specified, but some bounds exist:
   * For references, the liveness duration is upper-bounded by the syntactic
     lifetime assigned by the borrow checker; it cannot be live any *longer* than that lifetime.