XCMv4 (#1230)

# Note for reviewer

Most changes are just syntax changes necessary for the new version.
Most important files should be the ones under the `xcm` folder.

# Description 

Added XCMv4.

## Removed `Multi` prefix
The following types have been renamed:
- MultiLocation -> Location
- MultiAsset -> Asset
- MultiAssets -> Assets
- InteriorMultiLocation -> InteriorLocation
- MultiAssetFilter -> AssetFilter
- VersionedMultiAsset -> VersionedAsset
- WildMultiAsset -> WildAsset
- VersionedMultiLocation -> VersionedLocation

In order to fix a name conflict, the `Assets` in `xcm-executor` were
renamed to `HoldingAssets`, as they represent assets in holding.

## Removed `Abstract` asset id

It was not being used anywhere and this simplifies the code.

Now assets are just constructed as follows:

```rust
let asset: Asset = (AssetId(Location::new(1, Here)), 100u128).into();
```

No need for specifying `Concrete` anymore.

## Outcome is now a named fields struct

Instead of

```rust
pub enum Outcome {
  Complete(Weight),
  Incomplete(Weight, Error),
  Error(Error),
}
```

we now have

```rust
pub enum Outcome {
  Complete { used: Weight },
  Incomplete { used: Weight, error: Error },
  Error { error: Error },
}
```

## Added Reanchorable trait

Now both locations and assets implement this trait, making it easier to
reanchor both.

## New syntax for building locations and junctions

Now junctions are built using the following methods:

```rust
let location = Location {
    parents: 1,
    interior: [Parachain(1000), PalletInstance(50), GeneralIndex(1984)].into()
};
```

or

```rust
let location = Location::new(1, [Parachain(1000), PalletInstance(50), GeneralIndex(1984)]);
```

And they are matched like so:

```rust
match location.unpack() {
  (1, [Parachain(id)]) => ...
  (0, Here) => ...,
  (1, [_]) => ...,
}
```

This syntax is mandatory in v4, and has been also implemented for v2 and
v3 for easier migration.

This was needed to make all sizes smaller.

# TODO
- [x] Scaffold v4
- [x] Port github.com/paritytech/polkadot/pull/7236
- [x] Remove `Multi` prefix
- [x] Remove `Abstract` asset id

---------

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

polkadot/runtime/common/src/impls.rs

@@ -21,7 +21,7 @@ use frame_support::traits::{Currency, Imbalance, OnUnbalanced};
 use parity_scale_codec::{Decode, Encode, MaxEncodedLen};
 use primitives::Balance;
 use sp_runtime::{traits::TryConvert, Perquintill, RuntimeDebug};
-use xcm::VersionedMultiLocation;
+use xcm::VersionedLocation;
 
 /// Logic for the author to get a portion of fees.
 pub struct ToAuthor<R>(sp_std::marker::PhantomData<R>);
@@ -107,12 +107,9 @@ pub fn era_payout(
 )]
 pub enum VersionedLocatableAsset {
 	#[codec(index = 3)]
-	V3 {
-		/// The (relative) location in which the asset ID is meaningful.
-		location: xcm::v3::MultiLocation,
-		/// The asset's ID.
-		asset_id: xcm::v3::AssetId,
-	},
+	V3 { location: xcm::v3::MultiLocation, asset_id: xcm::v3::AssetId },
+	#[codec(index = 4)]
+	V4 { location: xcm::v4::Location, asset_id: xcm::v4::AssetId },
 }
 
 /// Converts the [`VersionedLocatableAsset`] to the [`xcm_builder::LocatableAssetId`].
@@ -125,22 +122,29 @@ impl TryConvert<VersionedLocatableAsset, xcm_builder::LocatableAssetId>
 	) -> Result<xcm_builder::LocatableAssetId, VersionedLocatableAsset> {
 		match asset {
 			VersionedLocatableAsset::V3 { location, asset_id } =>
-				Ok(xcm_builder::LocatableAssetId { asset_id, location }),
+				Ok(xcm_builder::LocatableAssetId {
+					location: location.try_into().map_err(|_| asset.clone())?,
+					asset_id: asset_id.try_into().map_err(|_| asset.clone())?,
+				}),
+			VersionedLocatableAsset::V4 { location, asset_id } =>
+				Ok(xcm_builder::LocatableAssetId { location, asset_id }),
 		}
 	}
 }
 
-/// Converts the [`VersionedMultiLocation`] to the [`xcm::latest::MultiLocation`].
-pub struct VersionedMultiLocationConverter;
-impl TryConvert<&VersionedMultiLocation, xcm::latest::MultiLocation>
-	for VersionedMultiLocationConverter
-{
+/// Converts the [`VersionedLocation`] to the [`xcm::latest::Location`].
+pub struct VersionedLocationConverter;
+impl TryConvert<&VersionedLocation, xcm::latest::Location> for VersionedLocationConverter {
 	fn try_convert(
-		location: &VersionedMultiLocation,
-	) -> Result<xcm::latest::MultiLocation, &VersionedMultiLocation> {
+		location: &VersionedLocation,
+	) -> Result<xcm::latest::Location, &VersionedLocation> {
 		let latest = match location.clone() {
-			VersionedMultiLocation::V2(l) => l.try_into().map_err(|_| location)?,
-			VersionedMultiLocation::V3(l) => l,
+			VersionedLocation::V2(l) => {
+				let v3: xcm::v3::MultiLocation = l.try_into().map_err(|_| location)?;
+				v3.try_into().map_err(|_| location)?
+			},
+			VersionedLocation::V3(l) => l.try_into().map_err(|_| location)?,
+			VersionedLocation::V4(l) => l,
 		};
 		Ok(latest)
 	}
@@ -161,11 +165,14 @@ pub mod benchmarks {
 	pub struct AssetRateArguments;
 	impl AssetKindFactory<VersionedLocatableAsset> for AssetRateArguments {
 		fn create_asset_kind(seed: u32) -> VersionedLocatableAsset {
-			VersionedLocatableAsset::V3 {
-				location: xcm::v3::MultiLocation::new(0, X1(Parachain(seed))),
-				asset_id: xcm::v3::MultiLocation::new(
+			VersionedLocatableAsset::V4 {
+				location: xcm::v4::Location::new(0, [xcm::v4::Junction::Parachain(seed)]),
+				asset_id: xcm::v4::Location::new(
 					0,
-					X2(PalletInstance(seed.try_into().unwrap()), GeneralIndex(seed.into())),
+					[
+						xcm::v4::Junction::PalletInstance(seed.try_into().unwrap()),
+						xcm::v4::Junction::GeneralIndex(seed.into()),
+					],
 				)
 				.into(),
 			}
@@ -173,29 +180,35 @@ pub mod benchmarks {
 	}
 
 	/// Provide factory methods for the [`VersionedLocatableAsset`] and the `Beneficiary` of the
-	/// [`VersionedMultiLocation`]. The location of the asset is determined as a Parachain with an
+	/// [`VersionedLocation`]. The location of the asset is determined as a Parachain with an
 	/// ID equal to the passed seed.
 	pub struct TreasuryArguments<Parents = ConstU8<0>, ParaId = ConstU32<0>>(
 		PhantomData<(Parents, ParaId)>,
 	);
 	impl<Parents: Get<u8>, ParaId: Get<u32>>
-		TreasuryArgumentsFactory<VersionedLocatableAsset, VersionedMultiLocation>
+		TreasuryArgumentsFactory<VersionedLocatableAsset, VersionedLocation>
 		for TreasuryArguments<Parents, ParaId>
 	{
 		fn create_asset_kind(seed: u32) -> VersionedLocatableAsset {
 			VersionedLocatableAsset::V3 {
-				location: xcm::v3::MultiLocation::new(Parents::get(), X1(Parachain(ParaId::get()))),
+				location: xcm::v3::MultiLocation::new(
+					Parents::get(),
+					[xcm::v3::Junction::Parachain(ParaId::get())],
+				),
 				asset_id: xcm::v3::MultiLocation::new(
 					0,
-					X2(PalletInstance(seed.try_into().unwrap()), GeneralIndex(seed.into())),
+					[
+						xcm::v3::Junction::PalletInstance(seed.try_into().unwrap()),
+						xcm::v3::Junction::GeneralIndex(seed.into()),
+					],
 				)
 				.into(),
 			}
 		}
-		fn create_beneficiary(seed: [u8; 32]) -> VersionedMultiLocation {
-			VersionedMultiLocation::V3(xcm::v3::MultiLocation::new(
+		fn create_beneficiary(seed: [u8; 32]) -> VersionedLocation {
+			VersionedLocation::V4(xcm::v4::Location::new(
 				0,
-				X1(AccountId32 { network: None, id: seed }),
+				[xcm::v4::Junction::AccountId32 { network: None, id: seed }],
 			))
 		}
 	}

polkadot/runtime/common/src/xcm_sender.rs

@@ -35,13 +35,13 @@ pub trait PriceForMessageDelivery {
 	/// Type used for charging different prices to different destinations
 	type Id;
 	/// Return the assets required to deliver `message` to the given `para` destination.
-	fn price_for_delivery(id: Self::Id, message: &Xcm<()>) -> MultiAssets;
+	fn price_for_delivery(id: Self::Id, message: &Xcm<()>) -> Assets;
 }
 impl PriceForMessageDelivery for () {
 	type Id = ();
 
-	fn price_for_delivery(_: Self::Id, _: &Xcm<()>) -> MultiAssets {
-		MultiAssets::new()
+	fn price_for_delivery(_: Self::Id, _: &Xcm<()>) -> Assets {
+		Assets::new()
 	}
 }
 
@@ -49,17 +49,17 @@ pub struct NoPriceForMessageDelivery<Id>(PhantomData<Id>);
 impl<Id> PriceForMessageDelivery for NoPriceForMessageDelivery<Id> {
 	type Id = Id;
 
-	fn price_for_delivery(_: Self::Id, _: &Xcm<()>) -> MultiAssets {
-		MultiAssets::new()
+	fn price_for_delivery(_: Self::Id, _: &Xcm<()>) -> Assets {
+		Assets::new()
 	}
 }
 
 /// Implementation of [`PriceForMessageDelivery`] which returns a fixed price.
 pub struct ConstantPrice<T>(sp_std::marker::PhantomData<T>);
-impl<T: Get<MultiAssets>> PriceForMessageDelivery for ConstantPrice<T> {
+impl<T: Get<Assets>> PriceForMessageDelivery for ConstantPrice<T> {
 	type Id = ();
 
-	fn price_for_delivery(_: Self::Id, _: &Xcm<()>) -> MultiAssets {
+	fn price_for_delivery(_: Self::Id, _: &Xcm<()>) -> Assets {
 		T::get()
 	}
 }
@@ -84,7 +84,7 @@ impl<A: Get<AssetId>, B: Get<u128>, M: Get<u128>, F: FeeTracker> PriceForMessage
 {
 	type Id = F::Id;
 
-	fn price_for_delivery(id: Self::Id, msg: &Xcm<()>) -> MultiAssets {
+	fn price_for_delivery(id: Self::Id, msg: &Xcm<()>) -> Assets {
 		let msg_fee = (msg.encoded_size() as u128).saturating_mul(M::get());
 		let fee_sum = B::get().saturating_add(msg_fee);
 		let amount = F::get_fee_factor(id).saturating_mul_int(fee_sum);
@@ -103,11 +103,11 @@ where
 	type Ticket = (HostConfiguration<BlockNumberFor<T>>, ParaId, Vec<u8>);
 
 	fn validate(
-		dest: &mut Option<MultiLocation>,
+		dest: &mut Option<Location>,
 		msg: &mut Option<Xcm<()>>,
 	) -> SendResult<(HostConfiguration<BlockNumberFor<T>>, ParaId, Vec<u8>)> {
 		let d = dest.take().ok_or(MissingArgument)?;
-		let id = if let MultiLocation { parents: 0, interior: X1(Parachain(id)) } = &d {
+		let id = if let (0, [Parachain(id)]) = d.unpack() {
 			*id
 		} else {
 			*dest = Some(d);
@@ -160,7 +160,7 @@ pub struct ToParachainDeliveryHelper<
 #[cfg(feature = "runtime-benchmarks")]
 impl<
 		XcmConfig: xcm_executor::Config,
-		ExistentialDeposit: Get<Option<MultiAsset>>,
+		ExistentialDeposit: Get<Option<Asset>>,
 		PriceForDelivery: PriceForMessageDelivery<Id = ParaId>,
 		Parachain: Get<ParaId>,
 		ToParachainHelper: EnsureForParachain,
@@ -174,10 +174,10 @@ impl<
 	>
 {
 	fn ensure_successful_delivery(
-		origin_ref: &MultiLocation,
-		_dest: &MultiLocation,
+		origin_ref: &Location,
+		_dest: &Location,
 		fee_reason: xcm_executor::traits::FeeReason,
-	) -> (Option<xcm_executor::FeesMode>, Option<MultiAssets>) {
+	) -> (Option<xcm_executor::FeesMode>, Option<Assets>) {
 		use xcm_executor::{
 			traits::{FeeManager, TransactAsset},
 			FeesMode,
@@ -234,7 +234,7 @@ mod tests {
 	parameter_types! {
 		pub const BaseDeliveryFee: u128 = 300_000_000;
 		pub const TransactionByteFee: u128 = 1_000_000;
-		pub FeeAssetId: AssetId = Concrete(Here.into());
+		pub FeeAssetId: AssetId = AssetId(Here.into());
 	}
 
 	struct TestFeeTracker;