[xcm] Small enhancements for NetworkExportTable and xcm-builder (#1848)

## Summary

This PR introduces several enhancements.

The current implementation of `NetworkExportTable` lacks remote location
filtering support beyond `NetworkId` lookup. To provide more control and
granularity, it's essential to allow configuration for bridging to
different consensus `NetworkId` while restricting access e.g. to
particular remote parachains.

Additionally, the `StartsWith` and `Equals` and
`StartsWithExplicitGlobalConsensus` helper functions, which are in
active use, are moved to the `xcm-builder` and `frame_support` modules
for better code organization.

Adds a new `LocationWithAssetFilters` filter to enable location-based
and asset-related filtering. This filter is useful for configuring the
`pallet_xcm` filter for
[XcmTeleportFilter](https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/pallet-xcm/src/lib.rs#L212)
and
[XcmReserveTransferFilter](https://github.com/paritytech/polkadot-sdk/blob/master/polkadot/xcm/pallet-xcm/src/lib.rs#L216)
to restrict specific assets.

Furthermore, the `BridgeMessage` fields are not accessible outside of
`xcm-builder`, limiting the ability to create custom logic dependent on
it.

---------

Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>

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

@@ -117,16 +117,54 @@ impl ExporterFor for Tuple {
 	}
 }
 
+/// Configuration item representing a single exporter in the `NetworkExportTable`.
+pub struct NetworkExportTableItem {
+	/// Supported remote network.
+	pub remote_network: NetworkId,
+	/// Remote location filter.
+	/// If `Some`, the requested remote location must be equal to one of the items in the vector.
+	/// These are locations in the remote network.
+	/// If `None`, then the check is skipped.
+	pub remote_location_filter: Option<Vec<InteriorMultiLocation>>,
+	/// Locally-routable bridge with bridging capabilities to the `remote_network` and
+	/// `remote_location`. See [`ExporterFor`] for more details.
+	pub bridge: MultiLocation,
+	/// The local payment.
+	/// See [`ExporterFor`] for more details.
+	pub payment: Option<MultiAsset>,
+}
+
+impl NetworkExportTableItem {
+	pub fn new(
+		remote_network: NetworkId,
+		remote_location_filter: Option<Vec<InteriorMultiLocation>>,
+		bridge: MultiLocation,
+		payment: Option<MultiAsset>,
+	) -> Self {
+		Self { remote_network, remote_location_filter, bridge, payment }
+	}
+}
+
+/// An adapter for the implementation of `ExporterFor`, which attempts to find the
+/// `(bridge_location, payment)` for the requested `network` and `remote_location` in the provided
+/// `T` table containing various exporters.
 pub struct NetworkExportTable<T>(sp_std::marker::PhantomData<T>);
-impl<T: Get<Vec<(NetworkId, MultiLocation, Option<MultiAsset>)>>> ExporterFor
-	for NetworkExportTable<T>
-{
+impl<T: Get<Vec<NetworkExportTableItem>>> ExporterFor for NetworkExportTable<T> {
 	fn exporter_for(
 		network: &NetworkId,
-		_: &InteriorMultiLocation,
+		remote_location: &InteriorMultiLocation,
 		_: &Xcm<()>,
 	) -> Option<(MultiLocation, Option<MultiAsset>)> {
-		T::get().into_iter().find(|(ref j, ..)| j == network).map(|(_, l, p)| (l, p))
+		T::get()
+			.into_iter()
+			.find(|item| {
+				&item.remote_network == network &&
+					item.remote_location_filter
+						.as_ref()
+						.map(|filters| filters.iter().any(|filter| filter == remote_location))
+						.unwrap_or(true)
+			})
+			.map(|item| (item.bridge, item.payment))
 	}
 }
 
@@ -329,8 +367,8 @@ pub struct BridgeMessage {
 	/// The message destination as a *Universal Location*. This means it begins with a
 	/// `GlobalConsensus` junction describing the network under which global consensus happens.
 	/// If this does not match our global consensus then it's a fatal error.
-	universal_dest: VersionedInteriorMultiLocation,
-	message: VersionedXcm<()>,
+	pub universal_dest: VersionedInteriorMultiLocation,
+	pub message: VersionedXcm<()>,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
@@ -504,7 +542,7 @@ mod tests {
 			pub UniversalLocation: InteriorMultiLocation = X2(GlobalConsensus(Local::get()), Parachain(1234));
 			pub DifferentRemote: NetworkId = ByGenesis([22; 32]);
 			// no routers
-			pub BridgeTable: Vec<(NetworkId, MultiLocation, Option<MultiAsset>)> = vec![];
+			pub BridgeTable: Vec<NetworkExportTableItem> = vec![];
 		}
 
 		// check with local destination (should be remote)
@@ -539,4 +577,74 @@ mod tests {
 			>,
 		>(remote_dest, |result| assert_eq!(Err(NotApplicable), result));
 	}
+
+	#[test]
+	fn network_export_table_works() {
+		frame_support::parameter_types! {
+			pub NetworkA: NetworkId = ByGenesis([0; 32]);
+			pub Parachain1000InNetworkA: InteriorMultiLocation = X1(Parachain(1000));
+			pub Parachain2000InNetworkA: InteriorMultiLocation = X1(Parachain(2000));
+
+			pub NetworkB: NetworkId = ByGenesis([1; 32]);
+
+			pub BridgeToALocation: MultiLocation = MultiLocation::new(1, X1(Parachain(1234)));
+			pub BridgeToBLocation: MultiLocation = MultiLocation::new(1, X1(Parachain(4321)));
+
+			pub PaymentForNetworkAAndParachain2000: MultiAsset = (MultiLocation::parent(), 150).into();
+
+			pub BridgeTable: sp_std::vec::Vec<NetworkExportTableItem> = sp_std::vec![
+				// NetworkA allows `Parachain(1000)` as remote location WITHOUT payment.
+				NetworkExportTableItem::new(
+					NetworkA::get(),
+					Some(vec![Parachain1000InNetworkA::get()]),
+					BridgeToALocation::get(),
+					None
+				),
+				// NetworkA allows `Parachain(2000)` as remote location WITH payment.
+				NetworkExportTableItem::new(
+					NetworkA::get(),
+					Some(vec![Parachain2000InNetworkA::get()]),
+					BridgeToALocation::get(),
+					Some(PaymentForNetworkAAndParachain2000::get())
+				),
+				// NetworkB allows all remote location.
+				NetworkExportTableItem::new(
+					NetworkB::get(),
+					None,
+					BridgeToBLocation::get(),
+					None
+				)
+			];
+		}
+
+		let test_data = vec![
+			(NetworkA::get(), X1(Parachain(1000)), Some((BridgeToALocation::get(), None))),
+			(NetworkA::get(), X2(Parachain(1000), GeneralIndex(1)), None),
+			(
+				NetworkA::get(),
+				X1(Parachain(2000)),
+				Some((BridgeToALocation::get(), Some(PaymentForNetworkAAndParachain2000::get()))),
+			),
+			(NetworkA::get(), X2(Parachain(2000), GeneralIndex(1)), None),
+			(NetworkA::get(), X1(Parachain(3000)), None),
+			(NetworkB::get(), X1(Parachain(1000)), Some((BridgeToBLocation::get(), None))),
+			(NetworkB::get(), X1(Parachain(2000)), Some((BridgeToBLocation::get(), None))),
+			(NetworkB::get(), X1(Parachain(3000)), Some((BridgeToBLocation::get(), None))),
+		];
+
+		for (network, remote_location, expected_result) in test_data {
+			assert_eq!(
+				NetworkExportTable::<BridgeTable>::exporter_for(
+					&network,
+					&remote_location,
+					&Xcm::default()
+				),
+				expected_result,
+				"expected_result: {:?} not matched for network: {:?} and remote_location: {:?}",
+				expected_result,
+				network,
+				remote_location,
+			)
+		}
+	}
 }