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/xcm/xcm-builder/src/tests/pay/mock.rs

@@ -115,14 +115,14 @@ impl pallet_assets::Config for Test {
 }
 
 parameter_types! {
-	pub const RelayLocation: MultiLocation = Here.into_location();
+	pub const RelayLocation: Location = Here.into_location();
 	pub const AnyNetwork: Option<NetworkId> = None;
-	pub UniversalLocation: InteriorMultiLocation = (ByGenesis([0; 32]), Parachain(42)).into();
+	pub UniversalLocation: InteriorLocation = (ByGenesis([0; 32]), Parachain(42)).into();
 	pub UnitWeightCost: u64 = 1_000;
 	pub static AdvertisedXcmVersion: u32 = 3;
 	pub const BaseXcmWeight: Weight = Weight::from_parts(1_000, 1_000);
-	pub CurrencyPerSecondPerByte: (AssetId, u128, u128) = (Concrete(RelayLocation::get()), 1, 1);
-	pub TrustedAssets: (MultiAssetFilter, MultiLocation) = (All.into(), Here.into());
+	pub CurrencyPerSecondPerByte: (AssetId, u128, u128) = (AssetId(RelayLocation::get()), 1, 1);
+	pub TrustedAssets: (AssetFilter, Location) = (All.into(), Here.into());
 	pub const MaxInstructions: u32 = 100;
 	pub const MaxAssetsIntoHolding: u32 = 64;
 	pub CheckingAccount: AccountId = XcmPallet::check_account();
@@ -130,28 +130,25 @@ parameter_types! {
 
 type AssetIdForAssets = u128;
 
-pub struct FromMultiLocationToAsset<MultiLocation, AssetId>(
-	core::marker::PhantomData<(MultiLocation, AssetId)>,
-);
-impl MaybeEquivalence<MultiLocation, AssetIdForAssets>
-	for FromMultiLocationToAsset<MultiLocation, AssetIdForAssets>
+pub struct FromLocationToAsset<Location, AssetId>(core::marker::PhantomData<(Location, AssetId)>);
+impl MaybeEquivalence<Location, AssetIdForAssets>
+	for FromLocationToAsset<Location, AssetIdForAssets>
 {
-	fn convert(value: &MultiLocation) -> Option<AssetIdForAssets> {
-		match value {
-			MultiLocation { parents: 0, interior: Here } => Some(0 as AssetIdForAssets),
-			MultiLocation { parents: 1, interior: Here } => Some(1 as AssetIdForAssets),
-			MultiLocation { parents: 0, interior: X2(PalletInstance(1), GeneralIndex(index)) }
-				if ![0, 1].contains(index) =>
+	fn convert(value: &Location) -> Option<AssetIdForAssets> {
+		match value.unpack() {
+			(0, []) => Some(0 as AssetIdForAssets),
+			(1, []) => Some(1 as AssetIdForAssets),
+			(0, [PalletInstance(1), GeneralIndex(index)]) if ![0, 1].contains(index) =>
 				Some(*index as AssetIdForAssets),
 			_ => None,
 		}
 	}
 
-	fn convert_back(value: &AssetIdForAssets) -> Option<MultiLocation> {
+	fn convert_back(value: &AssetIdForAssets) -> Option<Location> {
 		match value {
-			0u128 => Some(MultiLocation { parents: 1, interior: Here }),
+			0u128 => Some(Location { parents: 1, interior: Here }),
 			para_id @ 1..=1000 =>
-				Some(MultiLocation { parents: 1, interior: X1(Parachain(*para_id as u32)) }),
+				Some(Location { parents: 1, interior: [Parachain(*para_id as u32)].into() }),
 			_ => None,
 		}
 	}
@@ -163,7 +160,7 @@ pub type LocalAssetsTransactor = FungiblesAdapter<
 	ConvertedConcreteId<
 		AssetIdForAssets,
 		Balance,
-		FromMultiLocationToAsset<MultiLocation, AssetIdForAssets>,
+		FromLocationToAsset<Location, AssetIdForAssets>,
 		JustTry,
 	>,
 	SovereignAccountOf,
@@ -187,10 +184,10 @@ impl WeightTrader for DummyWeightTrader {
 	fn buy_weight(
 		&mut self,
 		_weight: Weight,
-		_payment: xcm_executor::Assets,
+		_payment: xcm_executor::AssetsInHolding,
 		_context: &XcmContext,
-	) -> Result<xcm_executor::Assets, XcmError> {
-		Ok(xcm_executor::Assets::default())
+	) -> Result<xcm_executor::AssetsInHolding, XcmError> {
+		Ok(xcm_executor::AssetsInHolding::default())
 	}
 }
 
@@ -228,13 +225,10 @@ parameter_types! {
 
 pub struct TreasuryToAccount;
 impl ConvertLocation<AccountId> for TreasuryToAccount {
-	fn convert_location(location: &MultiLocation) -> Option<AccountId> {
-		match location {
-			MultiLocation {
-				parents: 1,
-				interior:
-					X2(Parachain(42), Plurality { id: BodyId::Treasury, part: BodyPart::Voice }),
-			} => Some(TreasuryAccountId::get()), // Hardcoded test treasury account id
+	fn convert_location(location: &Location) -> Option<AccountId> {
+		match location.unpack() {
+			(1, [Parachain(42), Plurality { id: BodyId::Treasury, part: BodyPart::Voice }]) =>
+				Some(TreasuryAccountId::get()), // Hardcoded test treasury account id
 			_ => None,
 		}
 	}
@@ -277,7 +271,7 @@ pub const INITIAL_BALANCE: Balance = 100 * UNITS;
 pub const MINIMUM_BALANCE: Balance = 1 * UNITS;
 
 pub fn sibling_chain_account_id(para_id: u32, account: [u8; 32]) -> AccountId {
-	let location: MultiLocation =
+	let location: Location =
 		(Parent, Parachain(para_id), Junction::AccountId32 { id: account, network: None }).into();
 	SovereignAccountOf::convert_location(&location).unwrap()
 }