# 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 <[email protected]>

This was discovered during a debugging session, and it only served to
mask the underlying error, which was not great.

* Document non-uniqueness of SetTopic IDs

* More comments on WithUniqueTopic

fix for:
```
error: using `clone` on type `[u8; 32]` which implements the `Copy`
trait
  --> xcm/xcm-builder/src/routing.rs:44:28
   |
44 |             message.0.push(SetTopic(unique_id.clone()));
   |                                     ^^^^^^^^^^^^^^^^^ help: try
removing the `clone` call: `unique_id`
   |
   = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#clone_on_copy
   = note: `-D clippy::clone-on-copy` implied by `-D clippy::complexity`
```

* Tools for unique topic references

* Formatting

* Naming

* Repot into routing.rs.

* More things done

* Universal Exporter supports topic-as-reference

* Some tests for the topic routing

* More tests

* Paid bridge tests

* Add message ID to sending events

* Formatting

* fix and integrate into test nets

* Move DenyThenTry and friend from Cumulus

* Append SetTopic rather than prepend

* Docs

* Docs

* Work with new ProcessMessage ID API

* Formatting

* Fix build

* Fixes

* Formatting

* Update xcm/xcm-builder/src/barriers.rs

Co-authored-by: Francisco Aguirre <[email protected]>

* Update xcm/xcm-builder/src/routing.rs

Co-authored-by: Francisco Aguirre <[email protected]>

* Docs

* Rename message_hash

* Formatting

* ".git/.scripts/commands/fmt/fmt.sh"

* Rename

* Another Rename

* ".git/.scripts/commands/fmt/fmt.sh"

* ".git/.scripts/commands/fmt/fmt.sh"

* Update xcm/xcm-builder/src/routing.rs

Co-authored-by: Keith Yeung <[email protected]>

---------

Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Keith Yeung <[email protected]>