docs: say what actually guards the dynamic_message publish/take casts

rclrs/src/dynamic_message/dynamic_publisher.rs

@@ -125,9 +125,19 @@ impl DynamicPublisherState {
         }
         let rcl_publisher = &mut *self.handle.rcl_publisher.lock().unwrap();
         unsafe {
-            // SAFETY: The message type is guaranteed to match the publisher type by the type system.
+            // SAFETY: Unlike `Publisher<T>`, nothing here is enforced by the type system --
+            // a `DynamicMessage`'s type is a runtime value. What guarantees the match is the
+            // `message_type` comparison at the top of this function.
+            //
+            // That comparison is by name (`MessageTypeName`), so it stands on the two type
+            // support libraries for a given package describing the same layout: the
+            // introspection one laid out `message.storage` (via `DynamicMessageMetadata`),
+            // while `rcl_publish` serialises those bytes through the `rosidl_typesupport_c`
+            // one. `DynamicPublisher::create` resolves both from the same package name, so
+            // they come from one install prefix.
+            //
             // The message does not need to be valid beyond the duration of this function call.
-            // The third argument is explictly allowed to be NULL.
+            // The third argument is explicitly allowed to be NULL.
             rcl_publish(
                 rcl_publisher,
                 message.storage.as_mut_ptr() as *mut _,

rclrs/src/dynamic_message/dynamic_subscription.rs

@@ -201,6 +201,12 @@ impl<Payload> DynamicSubscriptionExecutable<Payload> {
             // SAFETY: The first two pointers are valid/initialized, and do not need to be valid
             // beyond the function call.
             // The latter two pointers are explicitly allowed to be NULL.
+            //
+            // `rmw_message` points at storage laid out by the *introspection* type support
+            // (`self.metadata.create()`), while `rcl_take` writes it through the
+            // `rosidl_typesupport_c` one. Those must describe the same layout;
+            // `DynamicSubscription::create` resolves both from the same package name, so they
+            // come from one install prefix.
             rcl_take(
                 rcl_subscription,
                 rmw_message as *mut _,