pallet-xcm: add new flexible transfer_assets() call/extrinsic (#2388)

# Motivation (+testing)

### Enable easy `ForeignAssets` transfers using `pallet-xcm` 

We had just previously added capabilities to teleport fees during
reserve-based transfers, but what about reserve-transferring fees when
needing to teleport some non-fee asset?

This PR aligns everything under either explicit reserve-transfer,
explicit teleport, or this new flexible `transfer_assets()` which can
mix and match as needed with fewer artificial constraints imposed to the
user.

This will enable, for example, a (non-system) parachain to teleport
their `ForeignAssets` assets to AssetHub while using DOT to pay fees.
(the assets are teleported - as foreign assets should from their owner
chain - while DOT used for fees can only be reserve-based transferred
between said parachain and AssetHub).

Added `xcm-emulator` tests for this scenario ^.

# Description

Reverts `(limited_)reserve_transfer_assets` to only allow reserve-based
transfers for all `assets` including fees.

Similarly `(limited_)teleport_assets` only allows teleports for all
`assets` including fees.
    
For complex combinations of asset transfers where assets and fees may
have different reserves or different reserve/teleport trust
configurations, users can use the newly added `transfer_assets()`
extrinsic which is more flexible in allowing more complex scenarios.

`assets` (excluding `fees`) must have same reserve location or otherwise
be teleportable to `dest`.
No limitations imposed on `fees`.

- for local reserve: transfer assets to sovereign account of destination
chain and forward a notification XCM to `dest` to mint and deposit
reserve-based assets to `beneficiary`.
- for destination reserve: burn local assets and forward a notification
to `dest` chain to withdraw the reserve assets from this chain's
sovereign account and deposit them to `beneficiary`.
- for remote reserve: burn local assets, forward XCM to reserve chain to
move reserves from this chain's SA to `dest` chain's SA, and forward
another XCM to `dest` to mint and deposit reserve-based assets to
`beneficiary`.
- for teleports: burn local assets and forward XCM to `dest` chain to
mint/teleport assets and deposit them to `beneficiary`.

## Review notes

Only around 500 lines are prod code (see `pallet_xcm/src/lib.rs`), the
rest of the PR is new tests and improving existing tests.

---------

Co-authored-by: command-bot <>

cumulus/parachains/runtimes/testing/penpal/src/xcm_config.rs

@@ -94,12 +94,24 @@ pub type FungiblesTransactor = FungiblesAdapter<
 	// Use this fungibles implementation:
 	Assets,
 	// Use this currency when it is a fungible asset matching the given location or name:
-	ConvertedConcreteId<
-		AssetIdPalletAssets,
-		Balance,
-		AsPrefixedGeneralIndex<SystemAssetHubAssetsPalletLocation, AssetIdPalletAssets, JustTry>,
-		JustTry,
-	>,
+	(
+		ConvertedConcreteId<
+			AssetIdPalletAssets,
+			Balance,
+			AsPrefixedGeneralIndex<AssetsPalletLocation, AssetIdPalletAssets, JustTry>,
+			JustTry,
+		>,
+		ConvertedConcreteId<
+			AssetIdPalletAssets,
+			Balance,
+			AsPrefixedGeneralIndex<
+				SystemAssetHubAssetsPalletLocation,
+				AssetIdPalletAssets,
+				JustTry,
+			>,
+			JustTry,
+		>,
+	),
 	// Convert an XCM MultiLocation into a local account id:
 	LocationToAccountId,
 	// Our chain's account ID type (we can't get away without mentioning it explicitly):
@@ -237,6 +249,8 @@ where
 	}
 }
 
+// This asset can be added to AH as ForeignAsset and teleported between Penpal and AH
+pub const TELEPORTABLE_ASSET_ID: u32 = 2;
 parameter_types! {
 	/// The location that this chain recognizes as the Relay network's Asset Hub.
 	pub SystemAssetHubLocation: MultiLocation = MultiLocation::new(1, X1(Parachain(1000)));
@@ -244,11 +258,30 @@ parameter_types! {
 	// the Relay Chain's Asset Hub's Assets pallet index
 	pub SystemAssetHubAssetsPalletLocation: MultiLocation =
 		MultiLocation::new(1, X2(Parachain(1000), PalletInstance(50)));
+	pub AssetsPalletLocation: MultiLocation =
+		MultiLocation::new(0, X1(PalletInstance(50)));
 	pub CheckingAccount: AccountId = PolkadotXcm::check_account();
+	pub LocalTeleportableToAssetHub: MultiLocation = MultiLocation::new(
+		0,
+		X2(PalletInstance(50), GeneralIndex(TELEPORTABLE_ASSET_ID.into()))
+	);
+}
+
+/// Accepts asset with ID `AssetLocation` and is coming from `Origin` chain.
+pub struct AssetFromChain<AssetLocation, Origin>(PhantomData<(AssetLocation, Origin)>);
+impl<AssetLocation: Get<MultiLocation>, Origin: Get<MultiLocation>>
+	ContainsPair<MultiAsset, MultiLocation> for AssetFromChain<AssetLocation, Origin>
+{
+	fn contains(asset: &MultiAsset, origin: &MultiLocation) -> bool {
+		log::trace!(target: "xcm::contains", "AssetFromChain asset: {:?}, origin: {:?}", asset, origin);
+		*origin == Origin::get() && matches!(asset.id, Concrete(id) if id == AssetLocation::get())
+	}
 }
 
 pub type Reserves =
 	(NativeAsset, AssetsFrom<SystemAssetHubLocation>, NativeAssetFrom<SystemAssetHubLocation>);
+pub type TrustedTeleporters =
+	(AssetFromChain<LocalTeleportableToAssetHub, SystemAssetHubLocation>,);
 
 pub struct XcmConfig;
 impl xcm_executor::Config for XcmConfig {
@@ -259,7 +292,7 @@ impl xcm_executor::Config for XcmConfig {
 	type OriginConverter = XcmOriginToTransactDispatchOrigin;
 	type IsReserve = Reserves;
 	// no teleport trust established with other chains
-	type IsTeleporter = NativeAsset;
+	type IsTeleporter = TrustedTeleporters;
 	type UniversalLocation = UniversalLocation;
 	type Barrier = Barrier;
 	type Weigher = FixedWeightBounds<UnitWeightCost, RuntimeCall, MaxInstructions>;