Add the XCM primitives crate. (#1760)

Co-authored-by: Gavin Wood <gavin@parity.io>

Co-authored-by: Gavin Wood <gavin@parity.io>

Cargo.lock

@@ -4340,9 +4340,9 @@ dependencies = [
 
 [[package]]
 name = "parity-scale-codec"
-version = "1.3.4"
+version = "1.3.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "34d38aeaffc032ec69faa476b3caaca8d4dd7f3f798137ff30359e5c7869ceb6"
+checksum = "7c740e5fbcb6847058b40ac7e5574766c6388f585e184d769910fe0d3a2ca861"
 dependencies = [
  "arrayvec 0.5.1",
  "bitvec",
@@ -4353,9 +4353,9 @@ dependencies = [
 
 [[package]]
 name = "parity-scale-codec-derive"
-version = "1.2.1"
+version = "1.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "cd20ff7e0399b274a5f5bb37b712fccb5b3a64b9128200d1c3cc40fe709cb073"
+checksum = "198db82bb1c18fc00176004462dd809b2a6d851669550aa17af6dacd21ae0c14"
 dependencies = [
  "proc-macro-crate",
  "proc-macro2 1.0.18",
@@ -10016,6 +10016,13 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "xcm"
+version = "0.8.22"
+dependencies = [
+ "parity-scale-codec",
+]
+
 [[package]]
 name = "yamux"
 version = "0.8.0"

Cargo.toml

@@ -40,6 +40,7 @@ members = [
 	"statement-table",
 	"service",
 	"validation",
+	"xcm",
 
 	"node/collation-generation",
 	"node/core/av-store",

xcm/Cargo.toml

+[package]
+name = "xcm"
+version = "0.8.22"
+authors = ["Parity Technologies x<admin@parity.io>"]
+description = "The basic XCM datastructures."
+edition = "2018"
+
+[dependencies]
+codec = { package = "parity-scale-codec", version = "1.3.5", default-features = false, features = [ "derive" ] }

xcm/src/lib.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Cross-Consensus Message format data structures.
+
+// NOTE, this crate is meant to be used in many different environments, notably wasm, but not
+// necessarily related to FRAME or even Substrate.
+//
+// Hence, `no_std` rather than sp-runtime.
+#![no_std]
+extern crate alloc;
+
+use codec::{Encode, Decode};
+
+pub mod v0;
+
+/// A single XCM message, together with its version code.
+#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
+pub enum VersionedXcm {
+	V0(v0::Xcm),
+}
+
+/// A versioned multi-location, a relative location of a cross-consensus system identifier.
+#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
+pub enum VersionedMultiLocation {
+	V0(v0::MultiLocation),
+}
+
+/// A versioned multi-asset, an identifier for an asset within a consensus system.
+#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
+pub enum VersionedMultiAsset {
+	V0(v0::MultiAsset),
+}

xcm/src/v0/junction.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Support datastructures for `MultiLocation`, primarily the `Junction` datatype.
+
+use alloc::vec::Vec;
+use codec::{self, Encode, Decode};
+
+/// A global identifier of an account-bearing consensus system.
+#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
+pub enum NetworkId {
+	/// Unidentified/any.
+	Any,
+	/// Some named network.
+	Named(Vec<u8>),
+	/// The Polkadot Relay chain
+	Polkadot,
+	/// Kusama.
+	Kusama,
+}
+
+/// A single item in a path to describe the relative location of a consensus system.
+///
+/// Each item assumes a pre-existing location as its context and is defined in terms of it.
+#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
+pub enum Junction {
+	/// The consensus system of which the context is a member and state-wise super-set.
+	///
+	/// NOTE: This item is *not* a sub-consensus item: a consensus system may not identify itself trustlessly as
+	/// a location that includes this junction.
+	Parent,
+	/// An indexed parachain belonging to and operated by the context.
+	///
+	/// Generally used when the context is a Polkadot Relay-chain.
+	Parachain { #[codec(compact)] id: u32 },
+	/// A 32-byte identifier for an account of a specific network that is respected as a sovereign endpoint within
+	/// the context.
+	///
+	/// Generally used when the context is a Substrate-based chain.
+	AccountId32 { network: NetworkId, id: [u8; 32] },
+	/// An 8-byte index for an account of a specific network that is respected as a sovereign endpoint within
+	/// the context.
+	///
+	/// May be used when the context is a Frame-based chain and includes e.g. an indices pallet.
+	AccountIndex64 { network: NetworkId, #[codec(compact)] index: u64 },
+	/// A 20-byte identifier for an account of a specific network that is respected as a sovereign endpoint within
+	/// the context.
+	///
+	/// May be used when the context is an Ethereum or Bitcoin chain or smart-contract.
+	AccountKey20 { network: NetworkId, key: [u8; 20] },
+	/// An instanced, indexed pallet that forms a constituent part of the context.
+	///
+	/// Generally used when the context is a Frame-based chain.
+	PalletInstance { id: u8 },
+	/// A non-descript index within the context location.
+	///
+	/// Usage will vary widely owing to its generality.
+	///
+	/// NOTE: Try to avoid using this and instead use a more specific item.
+	GeneralIndex { #[codec(compact)] id: u128 },
+	/// A nondescript datum acting as a key within the context location.
+	///
+	/// Usage will vary widely owing to its generality.
+	///
+	/// NOTE: Try to avoid using this and instead use a more specific item.
+	GeneralKey(Vec<u8>),
+	/// The unambiguous child.
+	///
+	/// Not currently used except as a fallback when deriving ancestry.
+	OnlyChild,
+}
+
+impl Junction {
+	pub fn is_sub_consensus(&self) -> bool {
+		match self {
+			Junction::Parent => false,
+
+			Junction::Parachain { .. } |
+			Junction::AccountId32 { .. } |
+			Junction::AccountIndex64 { .. } |
+			Junction::AccountKey20 { .. } |
+			Junction::PalletInstance { .. } |
+			Junction::GeneralIndex { .. } |
+			Junction::GeneralKey(..) |
+			Junction::OnlyChild => true,
+		}
+	}
+}

xcm/src/v0/mod.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Version 0 of the Cross-Consensus Message format data structures.
+
+use core::{result, convert::TryFrom};
+use alloc::{boxed::Box, vec::Vec};
+
+use codec::{self, Encode, Decode};
+use super::{VersionedXcm, VersionedMultiAsset};
+
+mod junction;
+mod multi_asset;
+mod multi_location;
+mod order;
+mod traits;
+pub use junction::{Junction, NetworkId};
+pub use multi_asset::{MultiAsset, AssetInstance};
+pub use multi_location::MultiLocation;
+pub use order::Order;
+pub use traits::{Error, Result, SendXcm, ExecuteXcm};
+
+// TODO: Efficient encodings for Vec<MultiAsset>, Vec<Order>, using initial byte values 128+ to encode the number of
+//   items in the vector.
+
+/// Basically just the XCM (more general) version of `ParachainDispatchOrigin`.
+#[derive(Copy, Clone, Eq, PartialEq, Encode, Decode, Debug)]
+pub enum OriginKind {
+	/// Origin should just be the native origin for the sender. For Cumulus/Frame chains this is
+	/// the `Parachain` origin.
+	Native,
+
+	/// Origin should just be the standard account-based origin with the sovereign account of
+	/// the sender. For Cumulus/Frame chains, this is the `Signed` origin.
+	SovereignAccount,
+
+	/// Origin should be the super-user. For Cumulus/Frame chains, this is the `Root` origin.
+	/// This will not usually be an available option.
+	Superuser,
+}
+
+/// Cross-Consensus Message: A message from one consensus system to another.
+///
+/// Consensus systems that may send and receive messages include blockchains and smart contracts.
+///
+/// All messages are delivered from a known *origin*, expressed as a `MultiLocation`.
+///
+/// This is the inner XCM format and is version-sensitive. Messages are typically passed using the outer
+/// XCM format, known as `VersionedXcm`.
+#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
+pub enum Xcm {
+	/// Withdraw asset(s) (`assets`) from the ownership of `origin` and place them into `holding`. Execute the
+	/// orders (`effects`).
+	///
+	/// - `assets`: The asset(s) to be withdrawn into holding.
+	/// - `effects`: The order(s) to execute on the holding account.
+	///
+	/// Kind: *Instruction*.
+	///
+	/// Errors:
+	WithdrawAsset { assets: Vec<MultiAsset>, effects: Vec<Order> },
+
+	/// Asset(s) (`assets`) have been received into the ownership of this system on the `origin` system.
+	///
+	/// Some orders are given (`effects`) which should be executed once the corresponding derivative assets have
+	/// been placed into `holding`.
+	///
+	/// - `assets`: The asset(s) that are minted into holding.
+	/// - `effects`: The order(s) to execute on the holding account.
+	///
+	/// Safety: `origin` must be trusted to have received and be storing `assets` such that they may later be
+	/// withdrawn should this system send a corresponding message.
+	///
+	/// Kind: *Trusted Indication*.
+	///
+	/// Errors:
+	ReserveAssetDeposit { assets: Vec<MultiAsset>, effects: Vec<Order> },
+
+	/// Asset(s) (`assets`) have been destroyed on the `origin` system and equivalent assets should be
+	/// created on this system.
+	///
+	/// Some orders are given (`effects`) which should be executed once the corresponding derivative assets have
+	/// been placed into `holding`.
+	///
+	/// - `assets`: The asset(s) that are minted into holding.
+	/// - `effects`: The order(s) to execute on the holding account.
+	///
+	/// Safety: `origin` must be trusted to have irrevocably destroyed the `assets` prior as a consequence of
+	/// sending this message.
+	///
+	/// Kind: *Trusted Indication*.
+	///
+	/// Errors:
+	TeleportAsset { assets: Vec<MultiAsset>, effects: Vec<Order> },
+
+	/// Indication of the contents of the holding account corresponding to the `QueryHolding` order of `query_id`.
+	///
+	/// - `query_id`: The identifier of the query that resulted in this message being sent.
+	/// - `assets`: The message content.
+	///
+	/// Safety: No concerns.
+	///
+	/// Kind: *Information*.
+	///
+	/// Errors:
+	Balances { #[codec(compact)] query_id: u64, assets: Vec<MultiAsset> },
+
+	/// Apply the encoded transaction `call`, whose dispatch-origin should be `origin` as expressed by the kind
+	/// of origin `origin_type`.
+	///
+	/// - `origin_type`: The means of expressing the message origin as a dispatch origin.
+	/// - `call`: The encoded transaction to be applied.
+	///
+	/// Safety: No concerns.
+	///
+	/// Kind: *Instruction*.
+	///
+	/// Errors:
+	Transact { origin_type: OriginKind, call: Vec<u8> },
+
+	/// Relay an inner message (`inner`) to a locally reachable destination ID `dest`.
+	///
+	/// The message sent to the destination will be wrapped into a `RelayedFrom` message, with the
+	/// `superorigin` being this location.
+	///
+	/// - `dest: MultiLocation`: The location of the to be relayed into. This may never contain `Parent`, and
+	///   it must be immediately reachable from the interpreting context.
+	/// - `inner: VersionedXcm`: The message to be wrapped and relayed.
+	///
+	/// Safety: No concerns.
+	///
+	/// Kind: *Instruction*.
+	///
+	/// Errors:
+	RelayTo { dest: MultiLocation, inner: Box<VersionedXcm> },
+
+	/// A message (`inner`) was sent to `origin` from `superorigin` with the intention of being relayed.
+	///
+	/// - `superorigin`: The location of the `inner` message origin, **relative to `origin`**.
+	/// - `inner`: The message sent by the super origin.
+	///
+	/// Safety: `superorigin` must express a sub-consensus only; it may *NEVER* contain a `Parent` junction.
+	///
+	/// Kind: *Trusted Indication*.
+	///
+	/// Errors:
+	RelayedFrom { superorigin: MultiLocation, inner: Box<VersionedXcm> },
+}
+
+impl From<Xcm> for VersionedXcm {
+	fn from(x: Xcm) -> Self {
+		VersionedXcm::V0(x)
+	}
+}
+
+impl TryFrom<VersionedXcm> for Xcm {
+	type Error = ();
+	fn try_from(x: VersionedXcm) -> result::Result<Self, ()> {
+		match x {
+			VersionedXcm::V0(x) => Ok(x),
+		}
+	}
+}

xcm/src/v0/multi_asset.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Cross-Consensus Message format data structures.
+
+use core::{result, convert::TryFrom};
+use alloc::vec::Vec;
+
+use codec::{self, Encode, Decode};
+use super::{MultiLocation, VersionedMultiAsset};
+
+/// A general identifier for an instance of a non-fungible asset class.
+#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
+pub enum AssetInstance {
+	/// Undefined - used if the NFA class has only one instance.
+	Undefined,
+
+	/// A compact index. Technically this could be greater than u128, but this implementation supports only
+	/// values up to `2**128 - 1`.
+	Index { #[codec(compact)] id: u128 },
+
+	/// A 4-byte fixed-length datum.
+	Array4([u8; 4]),
+
+	/// An 8-byte fixed-length datum.
+	Array8([u8; 8]),
+
+	/// A 16-byte fixed-length datum.
+	Array16([u8; 16]),
+
+	/// A 32-byte fixed-length datum.
+	Array32([u8; 32]),
+
+	/// An arbitrary piece of data. Use only when necessary.
+	Blob(Vec<u8>),
+}
+
+/// A single general identifier for an asset.
+///
+/// Represents both fungible and non-fungible assets. May only be used to represent a single asset class.
+///
+/// Wildcards may or may not be allowed by the interpreting context.
+///
+/// Assets classes may be identified in one of two ways: either an abstract identifier or a concrete identifier.
+/// Implementations may support only one of these. A single asset may be referenced from multiple asset identifiers,
+/// though will tend to have only a single *preferred* identifier.
+///
+/// ### Abstract identifiers
+///
+/// Abstract identifiers are absolute identifiers that represent a notional asset which can exist within multiple
+/// consensus systems. These tend to be simpler to deal with since their broad meaning is unchanged regardless stay
+/// of the consensus system in which it is interpreted.
+///
+/// However, in the attempt to provide uniformity across consensus systems, they may conflate different instantiations
+/// of some notional asset (e.g. the reserve asset and a local reserve-backed derivative of it) under the same name,
+/// leading to confusion. It also implies that one notional asset is accounted for locally in only one way. This may
+/// not be the case, e.g. where there are multiple bridge instances each providing a bridged "BTC" token yet none
+/// being fungible between the others.
+///
+/// Since they are meant to be absolute and universal, a global registry is needed to ensure that name collisions
+/// do not occur.
+///
+/// An abstract identifier is represented as a simple variable-size byte string. As of writing, no global registry
+/// exists and no proposals have been put forth for asset labeling.
+///
+/// ### Concrete identifiers
+///
+/// Concrete identifiers are *relative identifiers* that specifically identify a single asset through its location in
+/// a consensus system relative to the context interpreting. Use of a `MultiLocation` ensures that similar but non
+/// fungible variants of the same underlying asset can be properly distinguished, and obviates the need for any kind
+/// of central registry.
+///
+/// The limitation is that the asset identifier cannot be trivially copied between consensus
+/// systems and must instead be "re-anchored" whenever being moved to a new consensus system, using the two systems'
+/// relative paths.
+///
+/// Throughout XCM, messages are authored such that *when interpreted from the receiver's point of view* they will
+/// have the desired meaning/effect. This means that relative paths should always by constructed to be read from the
+/// point of view of the receiving system, *which may be have a completely different meaning in the authoring system*.
+///
+/// Concrete identifiers are the preferred way of identifying an asset since they are entirely unambiguous.
+///
+/// A concrete identifier is represented by a `MultiLocation`. If a system has an unambiguous primary asset (such as
+/// Bitcoin with BTC or Ethereum with ETH), then it will conventionally be identified as the chain itself. Alternative
+/// and more specific ways of referring to an asset within a system include:
+///
+/// - `<chain>/PalletInstance(<id>)` for a Frame chain with a single-asset pallet instance (such as an instance of the
+///   Balances pallet).
+/// - `<chain>/PalletInstance(<id>)/GeneralIndex(<index>)` for a Frame chain with an indexed multi-asset pallet
+///   instance (such as an instance of the Assets pallet).
+/// - `<chain>/AccountId32` for an ERC-20-style single-asset smart-contract on a Frame-based contracts chain.
+/// - `<chain>/AccountKey20` for an ERC-20-style single-asset smart-contract on an Ethereum-like chain.
+///
+#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
+pub enum MultiAsset {
+	/// No assets. Rarely used.
+	None,
+
+	/// All assets. Typically used for the subset of assets to be used for an `Order`, and in that context means
+	/// "all assets currently in holding".
+	All,
+
+	/// All fungible assets. Typically used for the subset of assets to be used for an `Order`, and in that context
+	/// means "all fungible assets currently in holding".
+	AllFungible,
+
+	/// All non-fungible assets. Typically used for the subset of assets to be used for an `Order`, and in that
+	/// context means "all non-fungible assets currently in holding".
+	AllNonFungible,
+
+	/// All fungible assets of a given abstract asset `id`entifier.
+	AllAbstractFungible { id: Vec<u8> },
+
+	/// All non-fungible assets of a given abstract asset `class`.
+	AllAbstractNonFungible { class: Vec<u8> },
+
+	/// All fungible assets of a given concrete asset `id`entifier.
+	AllConcreteFungible { id: MultiLocation },
+
+	/// All non-fungible assets of a given concrete asset `class`.
+	AllConcreteNonFungible { class: MultiLocation },
+
+	/// Some specific `amount` of the fungible asset identified by an abstract `id`.
+	AbstractFungible { id: Vec<u8>, #[codec(compact)] amount: u128 },
+
+	/// Some specific `instance` of the non-fungible asset whose `class` is identified abstractly.
+	AbstractNonFungible { class: Vec<u8>, instance: AssetInstance },
+
+	/// Some specific `amount` of the fungible asset identified by an concrete `id`.
+	ConcreteFungible { id: MultiLocation, #[codec(compact)] amount: u128 },
+
+	/// Some specific `instance` of the non-fungible asset whose `class` is identified concretely.
+	ConcreteNonFungible { class: MultiLocation, instance: AssetInstance },
+}
+
+impl From<MultiAsset> for VersionedMultiAsset {
+	fn from(x: MultiAsset) -> Self {
+		VersionedMultiAsset::V0(x)
+	}
+}
+
+impl TryFrom<VersionedMultiAsset> for MultiAsset {
+	type Error = ();
+	fn try_from(x: VersionedMultiAsset) -> result::Result<Self, ()> {
+		match x {
+			VersionedMultiAsset::V0(x) => Ok(x),
+		}
+	}
+}

xcm/src/v0/multi_location.rs

+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Cumulus.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Cumulus.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Cross-Consensus Message format data structures.
+
+use core::{result, mem, convert::TryFrom};
+
+use codec::{self, Encode, Decode};
+use super::Junction;
+use crate::VersionedMultiLocation;
+
+/// A relative path between state-bearing consensus systems.
+///
+/// A location in a consensus system is defined as an *isolatable state machine* held within global consensus. The
+/// location in question need not have a sophisticated consensus algorithm of its own; a single account within
+/// Ethereum, for example, could be considered a location.
+///
+/// A very-much non-exhaustive list of types of location include:
+/// - A (normal, layer-1) block chain, e.g. the Bitcoin mainnet or a parachain.
+/// - A layer-0 super-chain, e.g. the Polkadot Relay chain.
+/// - A layer-2 smart contract, e.g. an ERC-20 on Ethereum.
+/// - A logical functional component of a chain, e.g. a single instance of a pallet on a Frame-based Substrate chain.
+/// - An account.
+///
+/// A `MultiLocation` is a *relative identifier*, meaning that it can only be used to define the relative path
+/// between two locations, and cannot generally be used to refer to a location universally. It is comprised of a
+/// number of *junctions*, each morphing the previous location, either diving down into one of its internal locations,
+/// called a *sub-consensus*, or going up into its parent location. Correct `MultiLocation` values must have all
+/// `Parent` junctions as a prefix to all *sub-consensus* junctions.
+///
+/// This specific `MultiLocation` implementation uses a Rust `enum` in order to make pattern matching easier.
+///
+/// The `MultiLocation` value of `Null` simply refers to the interpreting consensus system.
+#[derive(Clone, Eq, PartialEq, Ord, PartialOrd, Encode, Decode, Debug)]
+pub enum MultiLocation {
+	/// The interpreting consensus system.
+	Null,
+	/// A relative path comprising one junction.
+	X1(Junction),
+	/// A relative path comprising two junctions.
+	X2(Junction, Junction),
+	/// A relative path comprising three junctions.
+	X3(Junction, Junction, Junction),
+	/// A relative path comprising four junctions.
+	X4(Junction, Junction, Junction, Junction),
+}
+
+impl From<Junction> for MultiLocation {
+	fn from(x: Junction) -> Self {
+		MultiLocation::X1(x)
+	}
+}
+
+impl From<()> for MultiLocation {
+	fn from(_: ()) -> Self {
+		MultiLocation::Null
+	}
+}
+impl From<(Junction,)> for MultiLocation {
+	fn from(x: (Junction,)) -> Self {
+		MultiLocation::X1(x.0)
+	}
+}
+impl From<(Junction, Junction)> for MultiLocation {
+	fn from(x: (Junction, Junction)) -> Self {
+		MultiLocation::X2(x.0, x.1)
+	}
+}
+impl From<(Junction, Junction, Junction)> for MultiLocation {
+	fn from(x: (Junction, Junction, Junction)) -> Self {
+		MultiLocation::X3(x.0, x.1, x.2)