pallet-xcm: enhance reserve_transfer_assets to support remote reser…

…ves (paritytech#1672)

`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_.

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.

- 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 paritytech#1584
Fixes paritytech#2055

---------

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

cumulus/parachain-template/runtime/src/xcm_config.rs

@@ -150,11 +150,6 @@ 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;
 	type SendXcmOrigin = EnsureXcmOrigin<RuntimeOrigin, LocalOriginToLocation>;
@@ -180,8 +175,6 @@ impl pallet_xcm::Config for Runtime {
 	type SovereignAccountOf = LocationToAccountId;
 	type MaxLockers = ConstU32<8>;
 	type WeightInfo = pallet_xcm::TestWeightInfo;
-	#[cfg(feature = "runtime-benchmarks")]
-	type ReachableDest = ReachableDest;
 	type AdminOrigin = EnsureRoot<AccountId>;
 	type MaxRemoteLockConsumers = ConstU32<0>;
 	type RemoteLockConsumerIdentifier = ();

cumulus/parachains/integration-tests/emulated/assets/asset-hub-rococo/src/lib.rs

@@ -49,18 +49,20 @@ pub const ASSET_MIN_BALANCE: u128 = 1000;
 pub const ASSETS_PALLET_ID: u8 = 50;
 
 pub type RelayToSystemParaTest = Test<Rococo, AssetHubRococo>;
+pub type RelayToParaTest = Test<Rococo, PenpalA>;
 pub type SystemParaToRelayTest = Test<AssetHubRococo, Rococo>;
-pub type SystemParaToParaTest = Test<AssetHubRococo, PenpalRococoA>;
+pub type SystemParaToParaTest = Test<AssetHubRococo, PenpalA>;
+pub type ParaToSystemParaTest = Test<PenpalA, AssetHubRococo>;
 
-/// Returns a `TestArgs` instance to de used for the Relay Chain accross integraton tests
-pub fn relay_test_args(amount: Balance) -> TestArgs {
+/// Returns a `TestArgs` instance to be used for the Relay Chain across integration tests
+pub fn relay_test_args(
+	dest: MultiLocation,
+	beneficiary_id: AccountId32,
+	amount: Balance,
+) -> TestArgs {
 	TestArgs {
-		dest: Rococo::child_location_of(AssetHubRococo::para_id()),
-		beneficiary: AccountId32Junction {
-			network: None,
-			id: AssetHubRococoReceiver::get().into(),
-		}
-		.into(),
+		dest,
+		beneficiary: AccountId32Junction { network: None, id: beneficiary_id.into() }.into(),
 		amount,
 		assets: (Here, amount).into(),
 		asset_id: None,
@@ -69,21 +71,22 @@ pub fn relay_test_args(amount: Balance) -> TestArgs {
 	}
 }
 
-/// Returns a `TestArgs` instance to de used for the System Parachain accross integraton tests
-pub fn system_para_test_args(
+/// Returns a `TestArgs` instance to be used by parachains across integration tests
+pub fn para_test_args(
 	dest: MultiLocation,
 	beneficiary_id: AccountId32,
 	amount: Balance,
 	assets: MultiAssets,
 	asset_id: Option<u32>,
+	fee_asset_item: u32,
 ) -> TestArgs {
 	TestArgs {
 		dest,
 		beneficiary: AccountId32Junction { network: None, id: beneficiary_id.into() }.into(),
 		amount,
 		assets,
 		asset_id,
-		fee_asset_item: 0,
+		fee_asset_item,
 		weight_limit: WeightLimit::Unlimited,
 	}
 }