Document non-uniqueness of SetTopic IDs (#7579)

* Document non-uniqueness of SetTopic IDs

* More comments on WithUniqueTopic

polkadot/xcm/src/v3/mod.rs

@@ -981,6 +981,10 @@ pub enum Instruction<Call> {
 
 	/// Set the Topic Register.
 	///
+	/// The 32-byte array identifier in the parameter is not guaranteed to be
+	/// unique; if such a property is desired, it is up to the code author to
+	/// enforce uniqueness.
+	///
 	/// Safety: No concerns.
 	///
 	/// Kind: *Instruction*

polkadot/xcm/xcm-builder/src/barriers.rs

@@ -207,6 +207,9 @@ impl<
 
 /// Sets the message ID to `t` using a `SetTopic(t)` in the last position if present.
 ///
+/// Note that the message ID does not necessarily have to be unique; it is the
+/// sender's responsibility to ensure uniqueness.
+///
 /// Requires some inner barrier to pass on the rest of the message.
 pub struct TrailingSetTopicAsId<InnerBarrier>(PhantomData<InnerBarrier>);
 impl<InnerBarrier: ShouldExecute> ShouldExecute for TrailingSetTopicAsId<InnerBarrier> {

polkadot/xcm/xcm-builder/src/routing.rs

@@ -25,6 +25,11 @@ use xcm::prelude::*;
 /// appends one to the message filled with a universally unique ID. This ID is returned from a
 /// successful `deliver`.
 ///
+/// If the message does already end with a `SetTopic` instruction, then it is the responsibility
+/// of the code author to ensure that the ID supplied to `SetTopic` is universally unique. Due to
+/// this property, consumers of the topic ID must be aware that a user-supplied ID may not be
+/// unique.
+///
 /// This is designed to be at the top-level of any routers, since it will always mutate the
 /// passed `message` reference into a `None`. Don't try to combine it within a tuple except as the
 /// last element.