Polkadot gets topic-based message IDs (#7301)

* Polkadot gets topics

* Formatting

* Fixes

---------

Co-authored-by: Keith Yeung <kungfukeith11@gmail.com>

runtime/polkadot/src/xcm_config.rs

@@ -43,8 +43,8 @@ use xcm_builder::{
 	AllowSubscriptionsFrom, AllowTopLevelPaidExecutionFrom, BackingToPlurality,
 	ChildParachainAsNative, ChildParachainConvertsVia, CurrencyAdapter as XcmCurrencyAdapter,
 	IsConcrete, MintLocation, OriginToPluralityVoice, SignedAccountId32AsNative,
-	SignedToAccountId32, SovereignSignedViaLocation, TakeWeightCredit, UsingComponents,
-	WeightInfoBounds, WithComputedOrigin,
+	SignedToAccountId32, SovereignSignedViaLocation, TakeWeightCredit, TrailingSetTopicAsId,
+	UsingComponents, WeightInfoBounds, WithComputedOrigin, WithUniqueTopic,
 };
 use xcm_executor::traits::WithOriginFilter;
 
@@ -120,14 +120,14 @@ parameter_types! {
 
 /// The XCM router. When we want to send an XCM message, we use this type. It amalgamates all of our
 /// individual routers.
-pub type XcmRouter = (
+pub type XcmRouter = WithUniqueTopic<(
 	// Only one router so far - use DMP to communicate with child parachains.
 	ChildParachainRouter<
 		Runtime,
 		XcmPallet,
 		ExponentialPrice<FeeAssetId, BaseDeliveryFee, TransactionByteFee, Dmp>,
 	>,
-);
+)>;
 
 parameter_types! {
 	pub const Dot: MultiAssetFilter = Wild(AllOf { fun: WildFungible, id: Concrete(TokenLocation::get()) });
@@ -153,7 +153,7 @@ match_types! {
 }
 
 /// The barriers one of which must be passed for an XCM message to be executed.
-pub type Barrier = (
+pub type Barrier = TrailingSetTopicAsId<(
 	// Weight that is paid for may be consumed.
 	TakeWeightCredit,
 	// Expected responses are OK.
@@ -170,7 +170,7 @@ pub type Barrier = (
 		UniversalLocation,
 		ConstU32<8>,
 	>,
-);
+)>;
 
 /// A call filter for the XCM Transact instruction. This is a temporary measure until we
 /// properly account for proof size weights.

xcm/src/v3/junction.rs

@@ -288,6 +288,7 @@ pub enum Junction {
 	/// An instanced, indexed pallet that forms a constituent part of the context.
 	///
 	/// Generally used when the context is a Frame-based chain.
+	// TODO XCMv4 inner should be `Compact<u32>`.
 	PalletInstance(u8),
 	/// A non-descript index within the context location.
 	///