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/pallet-xcm-benchmarks/src/lib.rs

@@ -41,18 +41,18 @@ pub trait Config: frame_system::Config {
 	/// `TransactAsset` is implemented.
 	type XcmConfig: XcmConfig;
 
-	/// A converter between a multi-location to a sovereign account.
+	/// A converter between a location to a sovereign account.
 	type AccountIdConverter: ConvertLocation<Self::AccountId>;
 
 	/// Helper that ensures successful delivery for XCM instructions which need `SendXcm`.
 	type DeliveryHelper: EnsureDelivery;
 
 	/// Does any necessary setup to create a valid destination for XCM messages.
-	/// Returns that destination's multi-location to be used in benchmarks.
-	fn valid_destination() -> Result<MultiLocation, BenchmarkError>;
+	/// Returns that destination's location to be used in benchmarks.
+	fn valid_destination() -> Result<Location, BenchmarkError>;
 
 	/// Worst case scenario for a holding account in this runtime.
-	fn worst_case_holding(depositable_count: u32) -> MultiAssets;
+	fn worst_case_holding(depositable_count: u32) -> Assets;
 }
 
 const SEED: u32 = 0;
@@ -66,21 +66,21 @@ pub type AssetTransactorOf<T> = <<T as Config>::XcmConfig as XcmConfig>::AssetTr
 /// The call type of executor's config. Should eventually resolve to the same overarching call type.
 pub type XcmCallOf<T> = <<T as Config>::XcmConfig as XcmConfig>::RuntimeCall;
 
-pub fn mock_worst_case_holding(depositable_count: u32, max_assets: u32) -> MultiAssets {
+pub fn mock_worst_case_holding(depositable_count: u32, max_assets: u32) -> Assets {
 	let fungibles_amount: u128 = 100;
 	let holding_fungibles = max_assets / 2 - depositable_count;
 	let holding_non_fungibles = holding_fungibles;
 	(0..holding_fungibles)
 		.map(|i| {
-			MultiAsset {
-				id: Concrete(GeneralIndex(i as u128).into()),
+			Asset {
+				id: AssetId(GeneralIndex(i as u128).into()),
 				fun: Fungible(fungibles_amount * i as u128),
 			}
 			.into()
 		})
-		.chain(core::iter::once(MultiAsset { id: Concrete(Here.into()), fun: Fungible(u128::MAX) }))
-		.chain((0..holding_non_fungibles).map(|i| MultiAsset {
-			id: Concrete(GeneralIndex(i as u128).into()),
+		.chain(core::iter::once(Asset { id: AssetId(Here.into()), fun: Fungible(u128::MAX) }))
+		.chain((0..holding_non_fungibles).map(|i| Asset {
+			id: AssetId(GeneralIndex(i as u128).into()),
 			fun: NonFungible(asset_instance_from(i)),
 		}))
 		.collect::<Vec<_>>()
@@ -94,11 +94,11 @@ pub fn asset_instance_from(x: u32) -> AssetInstance {
 	AssetInstance::Array4(instance)
 }
 
-pub fn new_executor<T: Config>(origin: MultiLocation) -> ExecutorOf<T> {
+pub fn new_executor<T: Config>(origin: Location) -> ExecutorOf<T> {
 	ExecutorOf::<T>::new(origin, [0; 32])
 }
 
-/// Build a multi-location from an account id.
+/// Build a location from an account id.
 fn account_id_junction<T: frame_system::Config>(index: u32) -> Junction {
 	let account: T::AccountId = account("account", index, SEED);
 	let mut encoded = account.encode();
@@ -108,8 +108,8 @@ fn account_id_junction<T: frame_system::Config>(index: u32) -> Junction {
 	Junction::AccountId32 { network: None, id }
 }
 
-pub fn account_and_location<T: Config>(index: u32) -> (T::AccountId, MultiLocation) {
-	let location: MultiLocation = account_id_junction::<T>(index).into();
+pub fn account_and_location<T: Config>(index: u32) -> (T::AccountId, Location) {
+	let location: Location = account_id_junction::<T>(index).into();
 	let account = T::AccountIdConverter::convert_location(&location).unwrap();
 
 	(account, location)
@@ -121,21 +121,21 @@ pub trait EnsureDelivery {
 	/// Prepare all requirements for successful `XcmSender: SendXcm` passing (accounts, balances,
 	/// channels ...). Returns:
 	/// - possible `FeesMode` which is expected to be set to executor
-	/// - possible `MultiAssets` which are expected to be subsume to the Holding Register
+	/// - possible `Assets` which are expected to be subsume to the Holding Register
 	fn ensure_successful_delivery(
-		origin_ref: &MultiLocation,
-		dest: &MultiLocation,
+		origin_ref: &Location,
+		dest: &Location,
 		fee_reason: FeeReason,
-	) -> (Option<FeesMode>, Option<MultiAssets>);
+	) -> (Option<FeesMode>, Option<Assets>);
 }
 
 /// `()` implementation does nothing which means no special requirements for environment.
 impl EnsureDelivery for () {
 	fn ensure_successful_delivery(
-		_origin_ref: &MultiLocation,
-		_dest: &MultiLocation,
+		_origin_ref: &Location,
+		_dest: &Location,
 		_fee_reason: FeeReason,
-	) -> (Option<FeesMode>, Option<MultiAssets>) {
+	) -> (Option<FeesMode>, Option<Assets>) {
 		// doing nothing
 		(None, None)
 	}