pallet-xcm: enhance reserve_transfer_assets to support remote reserves (#1672)

## Motivation

`pallet-xcm` is the main user-facing interface for XCM functionality,
including assets manipulation functions like `teleportAssets()` and
`reserve_transfer_assets()` calls.

While `teleportAsset()` works both ways, `reserve_transfer_assets()`
works only for sending reserve-based assets to a remote destination and
beneficiary when the reserve is the _local chain_.

## Solution

This PR enhances `pallet_xcm::(limited_)reserve_withdraw_assets` to
support transfers when reserves are other chains.
This will allow complete, **bi-directional** reserve-based asset
transfers user stories using `pallet-xcm`.

Enables following scenarios:
- transferring assets with local reserve (was previously supported iff
asset used as fee also had local reserve - now it works in all cases),
- transferring assets with reserve on destination,
- transferring assets with reserve on remote/third-party chain (iff
assets and fees have same remote reserve),
- transferring assets with reserve different than the reserve of the
asset to be used as fees - meaning can be used to transfer random asset
with local/dest reserve while using DOT for fees on all involved chains,
even if DOT local/dest reserve doesn't match asset reserve,
- transferring assets with any type of local/dest reserve while using
fees which can be teleported between involved chains.

All of the above is done by pallet inner logic without the user having
to specify which scenario/reserves/teleports/etc. The correct scenario
and corresponding XCM programs are identified, and respectively, built
automatically based on runtime configuration of trusted teleporters and
trusted reserves.

#### Current limitations:
- while `fees` and "non-fee" `assets` CAN have different reserves (or
fees CAN be teleported), the remaining "non-fee" `assets` CANNOT, among
themselves, have different reserve locations (this is also implicitly
enforced by `MAX_ASSETS_FOR_TRANSFER=2`, but this can be safely
increased in the future).
- `fees` and "non-fee" `assets` CANNOT have **different remote**
reserves (this could also be supported in the future, but adds even more
complexity while possibly not being worth it - we'll see what the future
holds).

Fixes https://github.com/paritytech/polkadot-sdk/issues/1584
Fixes https://github.com/paritytech/polkadot-sdk/issues/2055

---------

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

cumulus/parachains/runtimes/assets/asset-hub-kusama/src/lib.rs

@@ -962,7 +962,7 @@ mod benches {
 		[cumulus_pallet_xcmp_queue, XcmpQueue]
 		[cumulus_pallet_dmp_queue, DmpQueue]
 		// XCM
-		[pallet_xcm, PolkadotXcm]
+		[pallet_xcm, PalletXcmExtrinsiscsBenchmark::<Runtime>]
 		// NOTE: Make sure you point to the individual modules below.
 		[pallet_xcm_benchmarks::fungible, XcmBalances]
 		[pallet_xcm_benchmarks::generic, XcmGeneric]
@@ -1200,6 +1200,7 @@ impl_runtime_apis! {
 			use frame_support::traits::StorageInfoTrait;
 			use frame_system_benchmarking::Pallet as SystemBench;
 			use cumulus_pallet_session_benchmarking::Pallet as SessionBench;
+			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsiscsBenchmark;
 
 			// This is defined once again in dispatch_benchmark, because list_benchmarks!
 			// and add_benchmarks! are macros exported by define_benchmarks! macros and those types
@@ -1243,6 +1244,39 @@ impl_runtime_apis! {
 			use cumulus_pallet_session_benchmarking::Pallet as SessionBench;
 			impl cumulus_pallet_session_benchmarking::Config for Runtime {}
 
+			use pallet_xcm::benchmarking::Pallet as PalletXcmExtrinsiscsBenchmark;
+			impl pallet_xcm::benchmarking::Config for Runtime {
+				fn reachable_dest() -> Option<MultiLocation> {
+					Some(Parent.into())
+				}
+
+				fn teleportable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+					// Relay/native token can be teleported between AH and Relay.
+					Some((
+						MultiAsset {
+							fun: Fungible(EXISTENTIAL_DEPOSIT),
+							id: Concrete(Parent.into())
+						},
+						Parent.into(),
+					))
+				}
+
+				fn reserve_transferable_asset_and_dest() -> Option<(MultiAsset, MultiLocation)> {
+					// AH can reserve transfer native token to some random parachain.
+					let random_para_id = 43211234;
+					ParachainSystem::open_outbound_hrmp_channel_for_benchmarks_or_tests(
+						random_para_id.into()
+					);
+					Some((
+						MultiAsset {
+							fun: Fungible(EXISTENTIAL_DEPOSIT),
+							id: Concrete(Parent.into())
+						},
+						ParentThen(Parachain(random_para_id).into()).into(),
+					))
+				}
+			}
+
 			use xcm::latest::prelude::*;
 			use xcm_config::{KsmLocation, MaxAssetsIntoHolding};
 			use pallet_xcm_benchmarks::asset_instance_from;

cumulus/parachains/runtimes/assets/asset-hub-kusama/src/xcm_config.rs

@@ -552,11 +552,6 @@ pub type XcmRouter = WithUniqueTopic<(
 	XcmpQueue,
 )>;
 
-#[cfg(feature = "runtime-benchmarks")]
-parameter_types! {
-	pub ReachableDest: Option<MultiLocation> = Some(Parent.into());
-}
-
 impl pallet_xcm::Config for Runtime {
 	type RuntimeEvent = RuntimeEvent;
 	// We want to disallow users sending (arbitrary) XCMs from this chain.
@@ -586,8 +581,6 @@ impl pallet_xcm::Config for Runtime {
 	type SovereignAccountOf = LocationToAccountId;
 	type MaxLockers = ConstU32<8>;
 	type WeightInfo = crate::weights::pallet_xcm::WeightInfo<Runtime>;
-	#[cfg(feature = "runtime-benchmarks")]
-	type ReachableDest = ReachableDest;
 	type AdminOrigin = EnsureRoot<AccountId>;
 	type MaxRemoteLockConsumers = ConstU32<0>;
 	type RemoteLockConsumerIdentifier = ();

cumulus/parachains/runtimes/assets/asset-hub-kusama/tests/tests.rs

@@ -18,13 +18,14 @@
 //! Tests for the Statemine (Kusama Assets Hub) chain.
 
 use asset_hub_kusama_runtime::xcm_config::{
-	AssetFeeAsExistentialDepositMultiplierFeeCharger, KsmLocation, TrustBackedAssetsPalletLocation,
+	AssetFeeAsExistentialDepositMultiplierFeeCharger, KsmLocation, LocationToAccountId,
+	TrustBackedAssetsPalletLocation,
 };
 pub use asset_hub_kusama_runtime::{
 	xcm_config::{CheckingAccount, ForeignCreatorsSovereignAccountOf, XcmConfig},
 	AllPalletsWithoutSystem, AssetDeposit, Assets, Balances, ExistentialDeposit, ForeignAssets,
 	ForeignAssetsInstance, MetadataDepositBase, MetadataDepositPerByte, ParachainSystem, Runtime,
-	RuntimeCall, RuntimeEvent, SessionKeys, System, TrustBackedAssetsInstance,
+	RuntimeCall, RuntimeEvent, SessionKeys, System, TrustBackedAssetsInstance, XcmpQueue,
 };
 use asset_test_utils::{CollatorSessionKeys, ExtBuilder};
 use codec::{Decode, Encode};
@@ -518,12 +519,6 @@ asset_test_utils::include_teleports_for_native_asset_works!(
 			_ => None,
 		}
 	}),
-	Box::new(|runtime_event_encoded: Vec<u8>| {
-		match RuntimeEvent::decode(&mut &runtime_event_encoded[..]) {
-			Ok(RuntimeEvent::XcmpQueue(event)) => Some(event),
-			_ => None,
-		}
-	}),
 	1000
 );
 
@@ -632,3 +627,32 @@ asset_test_utils::include_create_and_manage_foreign_assets_for_local_consensus_p
 		assert_eq!(ForeignAssets::asset_ids().collect::<Vec<_>>().len(), 1);
 	})
 );
+
+#[test]
+fn reserve_transfer_native_asset_to_non_teleport_para_works() {
+	asset_test_utils::test_cases::reserve_transfer_native_asset_to_non_teleport_para_works::<
+		Runtime,
+		AllPalletsWithoutSystem,
+		XcmConfig,
+		ParachainSystem,
+		XcmpQueue,
+		LocationToAccountId,
+	>(
+		collator_session_keys(),
+		ExistentialDeposit::get(),
+		AccountId::from(ALICE),
+		Box::new(|runtime_event_encoded: Vec<u8>| {
+			match RuntimeEvent::decode(&mut &runtime_event_encoded[..]) {
+				Ok(RuntimeEvent::PolkadotXcm(event)) => Some(event),
+				_ => None,
+			}
+		}),
+		Box::new(|runtime_event_encoded: Vec<u8>| {
+			match RuntimeEvent::decode(&mut &runtime_event_encoded[..]) {
+				Ok(RuntimeEvent::XcmpQueue(event)) => Some(event),
+				_ => None,
+			}
+		}),
+		WeightLimit::Unlimited,
+	);
+}