Additional explanation on the rule

Signed-off-by: Xiangfei Ding <dingxiangfei2009@protonmail.ch>

Author: dingxiangfei2009

library/core/src/ops/unsize.rs

@@ -113,6 +113,18 @@ impl<T: PointeeSized + Unsize<U>, U: PointeeSized> CoerceUnsized<*const U> for *
 ///     `DispatchFromDyn<FTarget>` where `FTarget` is the type of the corresponding coerced field in
 ///     `T`.
 ///
+/// Note that we do not support the case where multiple pointer narrowings or downcasting of trait
+/// objects into concrete objects are involved in coercing one type to another through
+/// the `DispatchFromDyn` trait.
+/// For instance, we do not support dispatch from `&Arc<dyn Tr>` to `&Arc<T>`.
+/// `&Arc<dyn Tr>` is already behind an immutable reference while the coercion would require a
+/// downcasting of the trait object `Arc<dyn Tr>` by chipping of the virtual table.
+/// This will break the invariant of an immutable reference that the place behind shall remain
+/// not mutated during the access.
+/// Similarly, the dispatch from `Box<Box<dyn Tr>>` to `Box<Box<T>>` also requires a similar memory
+/// layout change.
+/// Therefore, this category of coercion is not supported.
+///
 /// An example implementation of the trait:
 ///
 /// ```