XCM revamp (#2836)

* Remove unused relaying XCM * Aggregate HRMP (XCMP/HMP) messages. Payloads for spambot. * Revert lock * Fix * Broken example * Introduce fee payment mechanics into XCM. * Weight limitations on XCM execution * Mock environment for tests and the first test * Tests for XCM and a few refactors. * Remove code that's not ready * Fix for an XCM and an additional test * Query response system * XCMP message dispatch system reimagining - Moved most of the logic into xcm-handler pallet - Altered the outgoing XCMP API from push to pull - Changed underlying outgoing queue data structures to avoid multi-page read/writes - Introduced queuing for incoming messages - Introduced signal messages as a flow-control sub-stream - Introduced flow-control with basic threshold back-pressure - Introduced overall weight limitation on messages executed - Additonal alterations to XCM APIs for the new system * Some build fixes * Remove the Encode bounds sprayed around * More faff * Fix bounds amek use latest scale codec. * remove println * fixes * Fix XcmExecutor Tests * Fix XCM bounds using derivative crate * Refactor names of XcmGeneric &c into Xcm * Repot the xcm-executor into xcm-builder * Docs * Docs * Fixes * Update xcm/src/lib.rs Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com> * Fixes * Docs * Update runtime/parachains/src/ump.rs Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com> * Docs * Fixes * Fixes * Fixes * Docs * Fixes * Fixes * Introduce transfer_asset specialisation. * Fixes * Fixes Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
2026-05-30 15:11:02 +00:00 · 2021-04-07 22:38:29 +02:00
parent 8eae0fa443
commit adc238ad86
37 changed files with 2436 additions and 574 deletions
@@ -160,6 +160,12 @@ version = "0.5.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "23b62fc65de8e4e7f52534fb52b0f3ed04746ae267519eef2a83941e8085068b"

+[[package]]
+name = "arrayvec"
+version = "0.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5a2f58b0bb10c380af2b26e57212856b8c9a59e0925b4c20f4a174a49734eaf7"
+
 [[package]]
 name = "asn1_der"
 version = "0.6.3"
@@ -1320,6 +1326,17 @@ dependencies = [
 "syn",
 ]

+[[package]]
+name = "derivative"
+version = "2.2.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "fcc3dd5e9e9c0b295d6e1e4d811fb6f157d5ffd784b8d202fc62eac8035a770b"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "derive_more"
 version = "0.99.11"
@@ -5054,11 +5071,11 @@ dependencies = [

 [[package]]
 name = "parity-scale-codec"
-version = "2.0.0"
+version = "2.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "75c823fdae1bb5ff5708ee61a62697e6296175dc671710876871c853f48592b3"
+checksum = "731f4d179ed52b1c7eeb29baf29c604ea9301b889b23ce93660220a5465d5c6f"
 dependencies = [
- "arrayvec 0.5.2",
+ "arrayvec 0.7.0",
 "bitvec",
 "byte-slice-cast",
 "parity-scale-codec-derive",
@@ -5067,9 +5084,9 @@ dependencies = [

 [[package]]
 name = "parity-scale-codec-derive"
-version = "2.0.0"
+version = "2.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9029e65297c7fd6d7013f0579e193ec2b34ae78eabca854c9417504ad8a2d214"
+checksum = "f44c5f94427bd0b5076e8f7e15ca3f60a4d8ac0077e4793884e6fdfd8915344e"
 dependencies = [
 "proc-macro-crate 0.1.5",
 "proc-macro2",
@@ -11307,6 +11324,7 @@ dependencies = [
 name = "xcm"
 version = "0.8.30"
 dependencies = [
+ "derivative",
 "parity-scale-codec",
 ]

@@ -109,7 +109,7 @@ decl_module! {
 		/// The given parachain should exist and the payload should not exceed the preconfigured size
 		/// `config.max_downward_message_size`.
 		#[weight = (1_000, DispatchClass::Operational)]
-		pub fn sudo_queue_downward_xcm(origin, id: ParaId, xcm: xcm::VersionedXcm) -> DispatchResult {
+		pub fn sudo_queue_downward_xcm(origin, id: ParaId, xcm: xcm::opaque::VersionedXcm) -> DispatchResult {
 			ensure_root(origin)?;
 			ensure!(<paras::Module<T>>::is_valid_para(id), Error::<T>::ParaDoesntExist);
 			let config = <configuration::Module<T>>::config();
@@ -18,7 +18,7 @@

 use parity_scale_codec::Encode;
 use sp_std::marker::PhantomData;
-use xcm::{VersionedXcm, v0::{SendXcm, MultiLocation, Junction, Xcm, Result, Error}};
+use xcm::opaque::{VersionedXcm, v0::{SendXcm, MultiLocation, Junction, Xcm, Result, Error}};
 use runtime_parachains::{configuration, dmp};

 /// Xcm sender for relay chain. It only sends downward message.
@@ -36,7 +36,7 @@ impl<T: configuration::Config + dmp::Config> SendXcm for RelayChainXcmSender<T>
 			).map_err(Into::<Error>::into)?;
 			Ok(())
 		} else {
-			Err(Error::CannotReachDestination)
+			Err(Error::CannotReachDestination("UnsupportedDestination"))
 		}
 	}
 }
@@ -995,7 +995,7 @@ impl<T: Config> Module<T> {
 		<Self as Store>::HrmpOpenChannelRequestsList::append(channel_id);

 		let notification_bytes = {
-			use xcm::{v0::Xcm, VersionedXcm};
+			use xcm::opaque::{v0::Xcm, VersionedXcm};
 			use parity_scale_codec::Encode as _;

 			VersionedXcm::from(Xcm::HrmpNewChannelOpenRequest {
@@ -1061,7 +1061,7 @@ impl<T: Config> Module<T> {

 		let notification_bytes = {
 			use parity_scale_codec::Encode as _;
-			use xcm::{v0::Xcm, VersionedXcm};
+			use xcm::opaque::{v0::Xcm, VersionedXcm};

 			VersionedXcm::from(Xcm::HrmpChannelAccepted {
 				recipient: u32::from(origin),
@@ -1104,7 +1104,7 @@ impl<T: Config> Module<T> {
 		let config = <configuration::Module<T>>::config();
 		let notification_bytes = {
 			use parity_scale_codec::Encode as _;
-			use xcm::{v0::Xcm, VersionedXcm};
+			use xcm::opaque::{v0::Xcm, VersionedXcm};

 			VersionedXcm::from(Xcm::HrmpChannelClosing {
 				initiator: u32::from(origin),
@@ -20,6 +20,7 @@ use crate::{
 };
 use sp_std::{fmt, prelude::*};
 use sp_std::collections::{btree_map::BTreeMap, vec_deque::VecDeque};
+use sp_runtime::traits::Zero;
 use frame_support::{decl_module, decl_storage, StorageMap, StorageValue, weights::Weight, traits::Get};
 use primitives::v1::{Id as ParaId, UpwardMessage};

@@ -65,15 +66,15 @@ impl<Config: xcm_executor::Config> UmpSink for XcmSink<Config> {
 		use xcm::v0::{Junction, MultiLocation, ExecuteXcm};
 		use xcm_executor::XcmExecutor;

-		let weight: Weight = 0;
-
-		if let Ok(versioned_xcm_message) = VersionedXcm::decode(&mut &msg[..]) {
+		// TODO: #2841 #UMPQUEUE Get a proper weight limit here. Probably from Relay Chain Config
+		let weight_limit = Weight::max_value();
+		let weight = if let Ok(versioned_xcm_message) = VersionedXcm::decode(&mut &msg[..]) {
 			match versioned_xcm_message {
 				VersionedXcm::V0(xcm_message) => {
 					let xcm_junction: Junction = Junction::Parachain { id: origin.into() };
 					let xcm_location: MultiLocation = xcm_junction.into();
-					// TODO: Do something with result.
-					let _result = XcmExecutor::<Config>::execute_xcm(xcm_location, xcm_message);
+					let result = XcmExecutor::<Config>::execute_xcm(xcm_location, xcm_message, weight_limit);
+					result.weight_used()
 				}
 			}
 		} else {
@@ -81,11 +82,12 @@ impl<Config: xcm_executor::Config> UmpSink for XcmSink<Config> {
 				target: LOG_TARGET,
 				"Failed to decode versioned XCM from upward message.",
 			);
-		}
+			Weight::zero()
+		};

-		// TODO: to be sound, this implementation must ensure that returned (and thus consumed)
-		// weight is limited to some small portion of the total block weight (as a ballpark, 1/4, 1/8
-		// or lower).
+		// TODO: #2841 #UMPQUEUE to be sound, this implementation must ensure that returned (and thus consumed)
+		//  weight is limited to some small portion of the total block weight (as a ballpark, 1/4, 1/8
+		//  or lower).
 		weight
 	}
 }
@@ -81,11 +81,11 @@ pub use pallet_balances::Call as BalancesCall;

 use polkadot_parachain::primitives::Id as ParaId;
 use xcm::v0::{MultiLocation, NetworkId};
-use xcm_executor::traits::IsConcrete;
 use xcm_builder::{
 	AccountId32Aliases, ChildParachainConvertsVia, SovereignSignedViaLocation,
 	CurrencyAdapter as XcmCurrencyAdapter, ChildParachainAsNative,
 	SignedAccountId32AsNative, ChildSystemParachainAsSuperuser, LocationInverter,
+	IsConcrete, FixedWeightBounds, FixedRateOfConcreteFungible,
 };
 use constants::{time::*, currency::*, fee::*};
 use frame_support::traits::InstanceFilter;
@@ -616,6 +616,11 @@ type LocalOriginConverter = (
 	ChildSystemParachainAsSuperuser<ParaId, Origin>,
 );

+parameter_types! {
+	pub const BaseXcmWeight: Weight = 100_000;
+	pub const RocFee: (MultiLocation, u128) = (RocLocation::get(), 1 * CENTS);
+}
+
 pub struct XcmConfig;
 impl xcm_executor::Config for XcmConfig {
 	type Call = Call;
@@ -625,6 +630,10 @@ impl xcm_executor::Config for XcmConfig {
 	type IsReserve = ();
 	type IsTeleporter = ();
 	type LocationInverter = LocationInverter<Ancestry>;
+	type Barrier = ();
+	type Weigher = FixedWeightBounds<BaseXcmWeight, Call>;
+	type Trader = FixedRateOfConcreteFungible<RocFee>;
+	type ResponseHandler = ();
 }

 impl parachains_session_info::Config for Runtime {}
@@ -7,6 +7,7 @@ edition = "2018"

 [dependencies]
 parity-scale-codec = { version = "2.0.0", default-features = false, features = [ "derive" ] }
+derivative = {version = "2.2.0", default-features = false, features = [ "use_core" ] }

 [features]
 default = ["std"]
@@ -0,0 +1,74 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use alloc::vec::Vec;
+use parity_scale_codec::{Encode, Decode};
+
+#[derive(Encode, Decode)]
+#[codec(encode_bound())]
+#[codec(decode_bound())]
+pub struct DoubleEncoded<T> {
+	encoded: Vec<u8>,
+	#[codec(skip)]
+	decoded: Option<T>,
+}
+
+impl<T> Clone for DoubleEncoded<T> {
+	fn clone(&self) -> Self { Self { encoded: self.encoded.clone(), decoded: None } }
+}
+impl<T> Eq for DoubleEncoded<T> {
+}
+impl<T> PartialEq for DoubleEncoded<T> {
+	fn eq(&self, other: &Self) -> bool { self.encoded.eq(&other.encoded) }
+}
+impl<T> core::fmt::Debug for DoubleEncoded<T> {
+	fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result { self.encoded.fmt(f) }
+}
+
+impl<T> From<Vec<u8>> for DoubleEncoded<T> {
+	fn from(encoded: Vec<u8>) -> Self {
+		Self { encoded, decoded: None }
+	}
+}
+
+impl<T> DoubleEncoded<T> {
+	pub fn into<S>(self) -> DoubleEncoded<S> { DoubleEncoded::from(self) }
+	pub fn from<S>(e: DoubleEncoded<S>) -> Self {
+		Self {
+			encoded: e.encoded,
+			decoded: None,
+		}
+	}
+	pub fn as_ref(&self) -> Option<&T> {
+		self.decoded.as_ref()
+	}
+}
+
+impl<T: Decode> DoubleEncoded<T> {
+	pub fn ensure_decoded(&mut self) -> Result<&T, ()> {
+		if self.decoded.is_none() {
+			self.decoded = T::decode(&mut &self.encoded[..]).ok();
+		}
+		self.decoded.as_ref().ok_or(())
+	}
+	pub fn take_decoded(&mut self) -> Result<T, ()> {
+		self.decoded.take().or_else(|| T::decode(&mut &self.encoded[..]).ok()).ok_or(())
+	}
+	pub fn try_into(mut self) -> Result<T, ()> {
+		self.ensure_decoded()?;
+		self.decoded.ok_or(())
+	}
+}
@@ -24,13 +24,32 @@
 extern crate alloc;

 use parity_scale_codec::{Encode, Decode};
+use derivative::Derivative;

 pub mod v0;

+mod double_encoded;
+pub use double_encoded::DoubleEncoded;
+
 /// A single XCM message, together with its version code.
-#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
-pub enum VersionedXcm {
-	V0(v0::Xcm),
+#[derive(Derivative, Encode, Decode)]
+#[derivative(Clone(bound=""), Eq(bound=""), PartialEq(bound=""), Debug(bound=""))]
+#[codec(encode_bound())]
+#[codec(decode_bound())]
+pub enum VersionedXcm<Call> {
+	V0(v0::Xcm<Call>),
+}
+
+pub mod opaque {
+	pub mod v0 {
+		// Everything from v0
+		pub use crate::v0::*;
+		// Then override with the opaque types in v0
+		pub use crate::v0::opaque::{Xcm, Order};
+	}
+
+	/// The basic VersionedXcm type which just uses the `Vec<u8>` as an encoded call.
+	pub type VersionedXcm = super::VersionedXcm<()>;
 }

 /// A versioned multi-location, a relative location of a cross-consensus system identifier.
@@ -45,6 +45,10 @@ pub enum Junction {
 	/// An indexed parachain belonging to and operated by the context.
 	///
 	/// Generally used when the context is a Polkadot Relay-chain.
+	///
+	/// There is also `Parachain` which can be used in tests to avoid the faffy `{ id: ... }` syntax. Production
+	/// code should use this.
+	// TODO: parity-scale-codec#262: Change to be `Parachain(#[codec(compact)] u32)`
 	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.
@@ -64,7 +68,7 @@ pub enum Junction {
 	/// 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 },
+	PalletInstance(u8),
 	/// A non-descript index within the context location.
 	///
 	/// Usage will vary widely owing to its generality.
@@ -16,11 +16,11 @@

 //! Version 0 of the Cross-Consensus Message format data structures.

-use core::{result, convert::TryFrom};
-use alloc::{boxed::Box, vec::Vec};
-
+use core::{result, convert::TryFrom, fmt::Debug};
+use derivative::Derivative;
+use alloc::vec::Vec;
 use parity_scale_codec::{self, Encode, Decode};
-use super::{VersionedXcm, VersionedMultiAsset};
+use crate::{VersionedMultiAsset, DoubleEncoded, VersionedXcm};

 mod junction;
 mod multi_asset;
@@ -31,10 +31,10 @@ 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};
+pub use traits::{Error, Result, SendXcm, ExecuteXcm, Outcome};

-// TODO: Efficient encodings for Vec<MultiAsset>, Vec<Order>, using initial byte values 128+ to encode the number of
-//   items in the vector.
+// TODO: #2841 #XCMENCODE 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)]
@@ -52,6 +52,13 @@ pub enum OriginKind {
 	Superuser,
 }

+/// Response data to a query.
+#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
+pub enum Response {
+	/// Some assets.
+	Assets(Vec<MultiAsset>),
+}
+
 /// Cross-Consensus Message: A message from one consensus system to another.
 ///
 /// Consensus systems that may send and receive messages include blockchains and smart contracts.
@@ -60,8 +67,11 @@ pub enum OriginKind {
 ///
 /// 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 {
+#[derive(Derivative, Encode, Decode)]
+#[derivative(Clone(bound = ""), Eq(bound = ""), PartialEq(bound = ""), Debug(bound = ""))]
+#[codec(encode_bound())]
+#[codec(decode_bound())]
+pub enum Xcm<Call> {
 	/// Withdraw asset(s) (`assets`) from the ownership of `origin` and place them into `holding`. Execute the
 	/// orders (`effects`).
 	///
@@ -71,7 +81,8 @@ pub enum Xcm {
 	/// Kind: *Instruction*.
 	///
 	/// Errors:
-	WithdrawAsset { assets: Vec<MultiAsset>, effects: Vec<Order> },
+	#[codec(index = 0)]
+	WithdrawAsset { assets: Vec<MultiAsset>, effects: Vec<Order<Call>> },

 	/// Asset(s) (`assets`) have been received into the ownership of this system on the `origin` system.
 	///
@@ -87,7 +98,8 @@ pub enum Xcm {
 	/// Kind: *Trusted Indication*.
 	///
 	/// Errors:
-	ReserveAssetDeposit { assets: Vec<MultiAsset>, effects: Vec<Order> },
+	#[codec(index = 1)]
+	ReserveAssetDeposit { assets: Vec<MultiAsset>, effects: Vec<Order<Call>> },

 	/// Asset(s) (`assets`) have been destroyed on the `origin` system and equivalent assets should be
 	/// created on this system.
@@ -104,7 +116,8 @@ pub enum Xcm {
 	/// Kind: *Trusted Indication*.
 	///
 	/// Errors:
-	TeleportAsset { assets: Vec<MultiAsset>, effects: Vec<Order> },
+	#[codec(index = 2)]
+	TeleportAsset { assets: Vec<MultiAsset>, effects: Vec<Order<Call>> },

 	/// Indication of the contents of the holding account corresponding to the `QueryHolding` order of `query_id`.
 	///
@@ -116,12 +129,47 @@ pub enum Xcm {
 	/// Kind: *Information*.
 	///
 	/// Errors:
-	Balances { #[codec(compact)] query_id: u64, assets: Vec<MultiAsset> },
+	#[codec(index = 3)]
+	QueryResponse { #[codec(compact)] query_id: u64, response: Response },
+
+	/// Withdraw asset(s) (`assets`) from the ownership of `origin` and place equivalent assets under the
+	/// ownership of `dest` within this consensus system.
+	///
+	/// - `assets`: The asset(s) to be withdrawn.
+	/// - `dest`: The new owner for the assets.
+	///
+	/// Safety: No concerns.
+	///
+	/// Kind: *Instruction*.
+	///
+	/// Errors:
+	#[codec(index = 4)]
+	TransferAsset { assets: Vec<MultiAsset>, dest: MultiLocation },
+
+	/// Withdraw asset(s) (`assets`) from the ownership of `origin` and place equivalent assets under the
+	/// ownership of `dest` within this consensus system.
+	///
+	/// Send an onward XCM message to `dest` of `ReserveAssetDeposit` with the given `effects`.
+	///
+	/// - `assets`: The asset(s) to be withdrawn.
+	/// - `dest`: The new owner for the assets.
+	/// - `effects`: The orders that should be contained in the `ReserveAssetDeposit` which is sent onwards to
+	///   `dest.
+	///
+	/// Safety: No concerns.
+	///
+	/// Kind: *Instruction*.
+	///
+	/// Errors:
+	#[codec(index = 5)]
+	TransferReserveAsset { assets: Vec<MultiAsset>, dest: MultiLocation, effects: Vec<Order<()>> },

 	/// 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.
+	/// - `max_weight`: The weight of `call`; this should be at least the chain's calculated weight and will
+	///   be used in the weight determination arithmetic.
 	/// - `call`: The encoded transaction to be applied.
 	///
 	/// Safety: No concerns.
@@ -129,35 +177,8 @@ pub enum Xcm {
 	/// 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> },
+	#[codec(index = 6)]
+	Transact { origin_type: OriginKind, require_weight_at_most: u64, call: DoubleEncoded<Call> },

 	/// A message to notify about a new incoming HRMP channel. This message is meant to be sent by the
 	/// relay-chain to a para.
@@ -169,6 +190,7 @@ pub enum Xcm {
 	/// Safety: The message should originate directly from the relay-chain.
 	///
 	/// Kind: *System Notification*
+	#[codec(index = 7)]
 	HrmpNewChannelOpenRequest {
 		#[codec(compact)] sender: u32,
 		#[codec(compact)] max_message_size: u32,
@@ -184,6 +206,7 @@ pub enum Xcm {
 	/// Kind: *System Notification*
 	///
 	/// Errors:
+	#[codec(index = 8)]
 	HrmpChannelAccepted {
 		#[codec(compact)] recipient: u32,
 	},
@@ -198,6 +221,7 @@ pub enum Xcm {
 	/// Kind: *System Notification*
 	///
 	/// Errors:
+	#[codec(index = 9)]
 	HrmpChannelClosing {
 		#[codec(compact)] initiator: u32,
 		#[codec(compact)] sender: u32,
@@ -205,17 +229,54 @@ pub enum Xcm {
 	},
 }

-impl From<Xcm> for VersionedXcm {
-	fn from(x: Xcm) -> Self {
+impl<Call> From<Xcm<Call>> for VersionedXcm<Call> {
+	fn from(x: Xcm<Call>) -> Self {
 		VersionedXcm::V0(x)
 	}
 }

-impl TryFrom<VersionedXcm> for Xcm {
+impl<Call> TryFrom<VersionedXcm<Call>> for Xcm<Call> {
 	type Error = ();
-	fn try_from(x: VersionedXcm) -> result::Result<Self, ()> {
+	fn try_from(x: VersionedXcm<Call>) -> result::Result<Self, ()> {
 		match x {
 			VersionedXcm::V0(x) => Ok(x),
 		}
 	}
 }
+
+impl<Call> Xcm<Call> {
+	pub fn into<C>(self) -> Xcm<C> { Xcm::from(self) }
+	pub fn from<C>(xcm: Xcm<C>) -> Self {
+		use Xcm::*;
+		match xcm {
+			WithdrawAsset { assets, effects }
+			=> WithdrawAsset { assets, effects: effects.into_iter().map(Order::into).collect() },
+			ReserveAssetDeposit { assets, effects }
+			=> ReserveAssetDeposit { assets, effects: effects.into_iter().map(Order::into).collect() },
+			TeleportAsset { assets, effects }
+			=> TeleportAsset { assets, effects: effects.into_iter().map(Order::into).collect() },
+			QueryResponse { query_id: u64, response }
+			=> QueryResponse { query_id: u64, response },
+			TransferAsset { assets, dest }
+			=> TransferAsset { assets, dest },
+			TransferReserveAsset { assets, dest, effects }
+			=> TransferReserveAsset { assets, dest, effects },
+			HrmpNewChannelOpenRequest { sender, max_message_size, max_capacity}
+			=> HrmpNewChannelOpenRequest { sender, max_message_size, max_capacity},
+			HrmpChannelAccepted { recipient}
+			=> HrmpChannelAccepted { recipient},
+			HrmpChannelClosing { initiator, sender, recipient}
+			=> HrmpChannelClosing { initiator, sender, recipient},
+			Transact { origin_type, require_weight_at_most, call}
+			=> Transact { origin_type, require_weight_at_most, call: call.into() }
+		}
+	}
+}
+
+pub mod opaque {
+	/// The basic concrete type of `generic::Xcm`, which doesn't make any assumptions about the format of a
+	/// call other than it is pre-encoded.
+	pub type Xcm = super::Xcm<()>;
+
+	pub use super::order::opaque::*;
+}
@@ -146,6 +146,159 @@ pub enum MultiAsset {
 	ConcreteNonFungible { class: MultiLocation, instance: AssetInstance },
 }

+impl MultiAsset {
+	pub fn is_wildcard(&self) -> bool {
+		match self {
+			MultiAsset::None
+			| MultiAsset::AbstractFungible {..}
+			| MultiAsset::AbstractNonFungible {..}
+			| MultiAsset::ConcreteFungible {..}
+			| MultiAsset::ConcreteNonFungible {..}
+			=> false,
+
+			MultiAsset::All
+			| MultiAsset::AllFungible
+			| MultiAsset::AllNonFungible
+			| MultiAsset::AllAbstractFungible {..}
+			| MultiAsset::AllConcreteFungible {..}
+			| MultiAsset::AllAbstractNonFungible {..}
+			| MultiAsset::AllConcreteNonFungible {..}
+			=> true,
+		}
+	}
+
+	fn is_none(&self) -> bool {
+		match self {
+			MultiAsset::None
+			| MultiAsset::AbstractFungible { amount: 0, .. }
+			| MultiAsset::ConcreteFungible { amount: 0, .. }
+			=> true,
+
+			_ => false,
+		}
+	}
+
+	fn is_fungible(&self) -> bool {
+		match self {
+			MultiAsset::All
+			| MultiAsset::AllFungible
+			| MultiAsset::AllAbstractFungible {..}
+			| MultiAsset::AllConcreteFungible {..}
+			| MultiAsset::AbstractFungible {..}
+			| MultiAsset::ConcreteFungible {..}
+			=> true,
+
+			_ => false,
+		}
+	}
+
+	fn is_non_fungible(&self) -> bool {
+		match self {
+			MultiAsset::All
+			| MultiAsset::AllNonFungible
+			| MultiAsset::AllAbstractNonFungible {..}
+			| MultiAsset::AllConcreteNonFungible {..}
+			| MultiAsset::AbstractNonFungible {..}
+			| MultiAsset::ConcreteNonFungible {..}
+			=> true,
+
+			_ => false,
+		}
+	}
+
+	fn is_concrete_fungible(&self, id: &MultiLocation) -> bool {
+		match self {
+			MultiAsset::AllFungible => true,
+
+			MultiAsset::AllConcreteFungible { id: i }
+			| MultiAsset::ConcreteFungible { id: i, .. }
+			=> i == id,
+
+			_ => false,
+		}
+	}
+
+	fn is_abstract_fungible(&self, id: &[u8]) -> bool {
+		match self {
+			MultiAsset::AllFungible => true,
+			MultiAsset::AllAbstractFungible { id: i }
+			| MultiAsset::AbstractFungible { id: i, .. }
+			=> i == id,
+			_ => false,
+		}
+	}
+
+	fn is_concrete_non_fungible(&self, class: &MultiLocation) -> bool {
+		match self {
+			MultiAsset::AllNonFungible => true,
+			MultiAsset::AllConcreteNonFungible { class: i }
+			| MultiAsset::ConcreteNonFungible { class: i, .. }
+			=> i == class,
+			_ => false,
+		}
+	}
+
+	fn is_abstract_non_fungible(&self, class: &[u8]) -> bool {
+		match self {
+			MultiAsset::AllNonFungible => true,
+			MultiAsset::AllAbstractNonFungible { class: i }
+			| MultiAsset::AbstractNonFungible { class: i, .. }
+			=> i == class,
+			_ => false,
+		}
+	}
+
+	fn is_all(&self) -> bool { matches!(self, MultiAsset::All) }
+
+	pub fn contains(&self, inner: &MultiAsset) -> bool {
+		use MultiAsset::*;
+		// Inner cannot be wild
+		if inner.is_wildcard() { return false }
+		// Everything contains nothing.
+		if inner.is_none() { return true }
+
+		// Everything contains anything.
+		if self.is_all() { return true }
+		// Nothing contains nothing.
+		if self.is_none() { return false }
+
+		match self {
+			// Anything fungible contains "all fungibles"
+			AllFungible => inner.is_fungible(),
+			// Anything non-fungible contains "all non-fungibles"
+			AllNonFungible => inner.is_non_fungible(),
+
+			AllConcreteFungible { id } => inner.is_concrete_fungible(id),
+			AllAbstractFungible { id } => inner.is_abstract_fungible(id),
+			AllConcreteNonFungible { class } => inner.is_concrete_non_fungible(class),
+			AllAbstractNonFungible { class } => inner.is_abstract_non_fungible(class),
+
+			ConcreteFungible { id, amount }
+			=> matches!(inner, ConcreteFungible { id: i , amount: a } if i == id && a >= amount),
+			AbstractFungible { id, amount }
+			=> matches!(inner, AbstractFungible { id: i , amount: a } if i == id && a >= amount),
+			ConcreteNonFungible { class, instance }
+			=> matches!(inner, ConcreteNonFungible { class: i , instance: a } if i == class && a == instance),
+			AbstractNonFungible { class, instance }
+			=> matches!(inner, AbstractNonFungible { class: i , instance: a } if i == class && a == instance),
+
+			_ => false,
+		}
+	}
+
+	pub fn reanchor(&mut self, prepend: &MultiLocation) -> Result<(), ()> {
+		use MultiAsset::*;
+		match self {
+			AllConcreteFungible { ref mut id }
+			| AllConcreteNonFungible { class: ref mut id }
+			| ConcreteFungible { ref mut id, .. }
+			| ConcreteNonFungible { class: ref mut id, .. }
+			=> id.prepend_with(prepend.clone()).map_err(|_| ()),
+			_ => Ok(()),
+		}
+	}
+}
+
 impl From<MultiAsset> for VersionedMultiAsset {
 	fn from(x: MultiAsset) -> Self {
 		VersionedMultiAsset::V0(x)
@@ -17,13 +17,18 @@
 //! Version 0 of the Cross-Consensus Message format data structures.

 use alloc::vec::Vec;
+use derivative::Derivative;
 use parity_scale_codec::{self, Encode, Decode};
-use super::{MultiAsset, MultiLocation};
+use super::{MultiAsset, MultiLocation, Xcm};

 /// An instruction to be executed on some or all of the assets in holding, used by asset-related XCM messages.
-#[derive(Clone, Eq, PartialEq, Encode, Decode, Debug)]
-pub enum Order {
+#[derive(Derivative, Encode, Decode)]
+#[derivative(Clone(bound=""), Eq(bound=""), PartialEq(bound=""), Debug(bound=""))]
+#[codec(encode_bound())]
+#[codec(decode_bound())]
+pub enum Order<Call> {
 	/// Do nothing. Not generally used.
+	#[codec(index = 0)]
 	Null,

 	/// Remove the asset(s) (`assets`) from holding and place equivalent assets under the ownership of `dest` within
@@ -33,12 +38,13 @@ pub enum Order {
 	/// - `dest`: The new owner for the assets.
 	///
 	/// Errors:
+	#[codec(index = 1)]
 	DepositAsset { assets: Vec<MultiAsset>, dest: MultiLocation },

 	/// Remove the asset(s) (`assets`) from holding and place equivalent assets under the ownership of `dest` within
 	/// this consensus system.
 	///
-	/// Send an onward XCM message to `dest` of `ReserveAssetDeposit` with the
+	/// Send an onward XCM message to `dest` of `ReserveAssetDeposit` with the given `effects`.
 	///
 	/// - `assets`: The asset(s) to remove from holding.
 	/// - `dest`: The new owner for the assets.
@@ -46,7 +52,8 @@ pub enum Order {
 	///   `dest.
 	///
 	/// Errors:
-	DepositReserveAsset { assets: Vec<MultiAsset>, dest: MultiLocation, effects: Vec<Order> },
+	#[codec(index = 2)]
+	DepositReserveAsset { assets: Vec<MultiAsset>, dest: MultiLocation, effects: Vec<Order<()>> },

 	/// Remove the asset(s) (`give`) from holding and replace them with alternative assets.
 	///
@@ -57,6 +64,7 @@ pub enum Order {
 	///   is undefined and they should be not be used.
 	///
 	/// Errors:
+	#[codec(index = 3)]
 	ExchangeAsset { give: Vec<MultiAsset>, receive: Vec<MultiAsset> },

 	/// Remove the asset(s) (`assets`) from holding and send a `WithdrawAsset` XCM message to a reserve location.
@@ -68,7 +76,8 @@ pub enum Order {
 	/// - `effects`: The orders to execute on the assets once withdrawn *on the reserve location*.
 	///
 	/// Errors:
-	InitiateReserveWithdraw { assets: Vec<MultiAsset>, reserve: MultiLocation, effects: Vec<Order> },
+	#[codec(index = 4)]
+	InitiateReserveWithdraw { assets: Vec<MultiAsset>, reserve: MultiLocation, effects: Vec<Order<()>> },

 	/// Remove the asset(s) (`assets`) from holding and send a `TeleportAsset` XCM message to a destination location.
 	///
@@ -77,7 +86,8 @@ pub enum Order {
 	/// - `effects`: The orders to execute on the assets once arrived *on the destination location*.
 	///
 	/// Errors:
-	InitiateTeleport { assets: Vec<MultiAsset>, dest: MultiLocation, effects: Vec<Order> },
+	#[codec(index = 5)]
+	InitiateTeleport { assets: Vec<MultiAsset>, dest: MultiLocation, effects: Vec<Order<()>> },

 	/// Send a `Balances` XCM message with the `assets` value equal to the holding contents, or a portion thereof.
 	///
@@ -88,5 +98,43 @@ pub enum Order {
 	///   back.
 	///
 	/// Errors:
+	#[codec(index = 6)]
 	QueryHolding { #[codec(compact)] query_id: u64, dest: MultiLocation, assets: Vec<MultiAsset> },
+
+	/// Pay for the execution of some Xcm with up to `weight` picoseconds of execution time, paying for this with
+	/// up to `fees` from the holding account.
+	///
+	/// Errors:
+	#[codec(index = 7)]
+	BuyExecution { fees: MultiAsset, weight: u64, debt: u64, halt_on_error: bool, xcm: Vec<Xcm<Call>> },
+}
+
+pub mod opaque {
+	pub type Order = super::Order<()>;
+}
+
+impl<Call> Order<Call> {
+	pub fn into<C>(self) -> Order<C> { Order::from(self) }
+	pub fn from<C>(order: Order<C>) -> Self {
+		use Order::*;
+		match order {
+			Null => Null,
+			DepositAsset { assets, dest }
+				=> DepositAsset { assets, dest },
+			DepositReserveAsset { assets, dest, effects }
+				=> DepositReserveAsset { assets, dest, effects },
+			ExchangeAsset { give, receive }
+				=> ExchangeAsset { give, receive },
+			InitiateReserveWithdraw { assets, reserve, effects }
+				=> InitiateReserveWithdraw { assets, reserve, effects },
+			InitiateTeleport { assets, dest, effects }
+				=> InitiateTeleport { assets, dest, effects },
+			QueryHolding { query_id, dest, assets }
+				=> QueryHolding { query_id, dest, assets },
+			BuyExecution { fees, weight, debt, halt_on_error, xcm } => {
+				let xcm = xcm.into_iter().map(Xcm::from).collect();
+				BuyExecution { fees, weight, debt, halt_on_error, xcm }
+			},
+		}
+	}
 }
@@ -24,6 +24,7 @@ use super::{MultiLocation, Xcm};
 #[derive(Copy, Clone, Encode, Decode, Eq, PartialEq, Ord, PartialOrd, Debug)]
 pub enum Error {
 	Undefined,
+	Overflow,
 	Unimplemented,
 	UnhandledXcmVersion,
 	UnhandledXcmMessage,
@@ -32,12 +33,44 @@ pub enum Error {
 	UntrustedReserveLocation,
 	UntrustedTeleportLocation,
 	DestinationBufferOverflow,
-	CannotReachDestination,
+	CannotReachDestination(#[codec(skip)] &'static str),
 	MultiLocationFull,
 	FailedToDecode,
 	BadOrigin,
 	ExceedsMaxMessageSize,
 	FailedToTransactAsset(#[codec(skip)] &'static str),
+	WeightLimitReached,
+	Wildcard,
+	/// The case where an XCM message has specified a optional weight limit and the weight required for
+	/// processing is too great.
+	///
+	/// Used by:
+	/// - `Transact`
+	TooMuchWeightRequired,
+	/// The fees specified by the XCM message were not found in the holding account.
+	///
+	/// Used by:
+	/// - `BuyExecution`
+	NotHoldingFees,
+	/// The weight of an XCM message is not computable ahead of execution. This generally means at least part
+	/// of the message is invalid, which could be due to it containing overly nested structures or an invalid
+	/// nested data segment (e.g. for the call in `Transact`).
+	WeightNotComputable,
+	/// The XCM did noto pass the barrier condition for execution. The barrier condition differs on different
+	/// chains and in different circumstances, but generally it means that the conditions surrounding the message
+	/// were not such that the chain considers the message worth spending time executing. Since most chains
+	/// lift the barrier to execution on apropriate payment, presentation of an NFT voucher, or based on the
+	/// message origin, it means that none of those were the case.
+	Barrier,
+	/// Indicates that it is not possible for a location to have an asset be withdrawn or transferred from its
+	/// ownership. This probably means it doesn't own (enough of) it, but may also indicate that it is under a
+	/// lock, hold, freeze or is otherwise unavailable.
+	NotWithdrawable,
+	/// Indicates that the consensus system cannot deposit an asset under the ownership of a particular location.
+	LocationCannotHold,
+	/// We attempted to send an XCM to the local consensus system. Execution was not possible probably due to
+	/// no execution weight being assigned.
+	DestinationIsLocal,
 }

 impl From<()> for Error {
@@ -48,22 +81,61 @@ impl From<()> for Error {

 pub type Result = result::Result<(), Error>;

-pub trait ExecuteXcm {
-	fn execute_xcm(origin: MultiLocation, msg: Xcm) -> Result;
+/// Local weight type; execution time in picoseconds.
+pub type Weight = u64;
+
+/// Outcome of an XCM excution.
+#[derive(Copy, Clone, Encode, Decode, Eq, PartialEq, Debug)]
+pub enum Outcome {
+	/// Execution completed successfully; given weight was used.
+	Complete(Weight),
+	/// Execution started, but did not complete successfully due to the given error; given weight was used.
+	Incomplete(Weight, Error),
+	/// Execution did not start due to the given error.
+	Error(Error),
 }

-impl ExecuteXcm for () {
-	fn execute_xcm(_origin: MultiLocation, _msg: Xcm) -> Result {
-		Err(Error::Unimplemented)
+impl Outcome {
+	pub fn ensure_complete(self) -> Result {
+		match self {
+			Outcome::Complete(_) => Ok(()),
+			Outcome::Incomplete(_, e) => Err(e),
+			Outcome::Error(e) => Err(e),
+		}
+	}
+	pub fn ensure_execution(self) -> result::Result<Weight, Error> {
+		match self {
+			Outcome::Complete(w) => Ok(w),
+			Outcome::Incomplete(w, _) => Ok(w),
+			Outcome::Error(e) => Err(e),
+		}
+	}
+	/// How much weight was used by the XCM execution attempt.
+	pub fn weight_used(&self) -> Weight {
+		match self {
+			Outcome::Complete(w) => *w,
+			Outcome::Incomplete(w, _) => *w,
+			Outcome::Error(_) => 0,
+		}
+	}
+}
+
+pub trait ExecuteXcm<Call> {
+	fn execute_xcm(origin: MultiLocation, message: Xcm<Call>, weight_limit: Weight) -> Outcome;
+}
+
+impl<C> ExecuteXcm<C> for () {
+	fn execute_xcm(_origin: MultiLocation, _message: Xcm<C>, _weight_limit: Weight) -> Outcome {
+		Outcome::Error(Error::Unimplemented)
 	}
 }

 pub trait SendXcm {
-	fn send_xcm(dest: MultiLocation, msg: Xcm) -> Result;
+	fn send_xcm(dest: MultiLocation, msg: Xcm<()>) -> Result;
 }

 impl SendXcm for () {
-	fn send_xcm(_dest: MultiLocation, _msg: Xcm) -> Result {
+	fn send_xcm(_dest: MultiLocation, _msg: Xcm<()>) -> Result {
 		Err(Error::Unimplemented)
 	}
 }
@@ -0,0 +1,90 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::{result::Result, marker::PhantomData};
+use xcm::v0::{Xcm, Order, MultiLocation};
+use frame_support::{ensure, traits::Contains, weights::Weight};
+use xcm_executor::traits::{OnResponse, ShouldExecute};
+
+pub struct TakeWeightCredit;
+impl ShouldExecute for TakeWeightCredit {
+	fn should_execute<Call>(
+		_origin: &MultiLocation,
+		_top_level: bool,
+		_message: &Xcm<Call>,
+		shallow_weight: Weight,
+		weight_credit: &mut Weight,
+	) -> Result<(), ()> {
+		*weight_credit = weight_credit.checked_sub(shallow_weight).ok_or(())?;
+		Ok(())
+	}
+}
+
+pub struct AllowTopLevelPaidExecutionFrom<T>(PhantomData<T>);
+impl<T: Contains<MultiLocation>> ShouldExecute for AllowTopLevelPaidExecutionFrom<T> {
+	fn should_execute<Call>(
+		origin: &MultiLocation,
+		top_level: bool,
+		message: &Xcm<Call>,
+		shallow_weight: Weight,
+		_weight_credit: &mut Weight,
+	) -> Result<(), ()> {
+		ensure!(T::contains(origin), ());
+		ensure!(top_level, ());
+		match message {
+			Xcm::TeleportAsset { effects, .. }
+			| Xcm::WithdrawAsset { effects, ..}
+			| Xcm::ReserveAssetDeposit { effects, ..}
+			if matches!(
+					effects.first(),
+					Some(Order::BuyExecution { debt, ..}) if *debt >= shallow_weight
+				)
+			=> Ok(()),
+			_ => Err(()),
+		}
+	}
+}
+
+pub struct AllowUnpaidExecutionFrom<T>(PhantomData<T>);
+impl<T: Contains<MultiLocation>> ShouldExecute for AllowUnpaidExecutionFrom<T> {
+	fn should_execute<Call>(
+		origin: &MultiLocation,
+		_top_level: bool,
+		_message: &Xcm<Call>,
+		_shallow_weight: Weight,
+		_weight_credit: &mut Weight,
+	) -> Result<(), ()> {
+		ensure!(T::contains(origin), ());
+		Ok(())
+	}
+}
+
+pub struct AllowKnownQueryResponses<ResponseHandler>(PhantomData<ResponseHandler>);
+impl<ResponseHandler: OnResponse> ShouldExecute for AllowKnownQueryResponses<ResponseHandler> {
+	fn should_execute<Call>(
+		origin: &MultiLocation,
+		_top_level: bool,
+		message: &Xcm<Call>,
+		_shallow_weight: Weight,
+		_weight_credit: &mut Weight,
+	) -> Result<(), ()> {
+		match message {
+			Xcm::QueryResponse { query_id, .. } if ResponseHandler::expecting_response(origin, *query_id)
+			=> Ok(()),
+			_ => Err(()),
+		}
+	}
+}
@@ -18,7 +18,8 @@ use sp_std::{result, convert::TryInto, marker::PhantomData};
 use xcm::v0::{Error as XcmError, Result, MultiAsset, MultiLocation};
 use sp_runtime::traits::SaturatedConversion;
 use frame_support::traits::{ExistenceRequirement::AllowDeath, WithdrawReasons};
-use xcm_executor::traits::{MatchesFungible, LocationConversion, TransactAsset};
+use xcm_executor::traits::{MatchesFungible, Convert, TransactAsset};
+use xcm_executor::Assets;

 /// Asset transaction errors.
 enum Error {
@@ -48,9 +49,9 @@ pub struct CurrencyAdapter<Currency, Matcher, AccountIdConverter, AccountId>(

 impl<
 	Matcher: MatchesFungible<Currency::Balance>,
-	AccountIdConverter: LocationConversion<AccountId>,
+	AccountIdConverter: Convert<MultiLocation, AccountId>,
 	Currency: frame_support::traits::Currency<AccountId>,
-	AccountId,	// can't get away without it since Currency is generic over it.
+	AccountId: Clone,	// can't get away without it since Currency is generic over it.
 > TransactAsset for CurrencyAdapter<Currency, Matcher, AccountIdConverter, AccountId> {

 	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> Result {
@@ -58,8 +59,8 @@ impl<
 		let amount: u128 = Matcher::matches_fungible(&what)
 			.ok_or(Error::AssetNotFound)?
 			.saturated_into();
-		let who = AccountIdConverter::from_location(who)
-			.ok_or(Error::AccountIdConversionFailed)?;
+		let who = AccountIdConverter::convert_ref(who)
+			.map_err(|()| Error::AccountIdConversionFailed)?;
 		let balance_amount = amount
 			.try_into()
 			.map_err(|_| Error::AmountToBalanceConversionFailed)?;
@@ -70,18 +71,18 @@ impl<
 	fn withdraw_asset(
 		what: &MultiAsset,
 		who: &MultiLocation
-	) -> result::Result<MultiAsset, XcmError> {
+	) -> result::Result<Assets, XcmError> {
 		// Check we handle this asset.
 		let amount: u128 = Matcher::matches_fungible(what)
 			.ok_or(Error::AssetNotFound)?
 			.saturated_into();
-		let who = AccountIdConverter::from_location(who)
-			.ok_or(Error::AccountIdConversionFailed)?;
+		let who = AccountIdConverter::convert_ref(who)
+			.map_err(|()| Error::AccountIdConversionFailed)?;
 		let balance_amount = amount
 			.try_into()
 			.map_err(|_| Error::AmountToBalanceConversionFailed)?;
 		Currency::withdraw(&who, balance_amount, WithdrawReasons::TRANSFER, AllowDeath)
 			.map_err(|e| XcmError::FailedToTransactAsset(e.into()))?;
-		Ok(what.clone())
+		Ok(what.clone().into())
 	}
 }
@@ -0,0 +1,36 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::marker::PhantomData;
+use xcm::v0::{MultiAsset, MultiLocation};
+use frame_support::traits::Get;
+use xcm_executor::traits::FilterAssetLocation;
+
+pub struct NativeAsset;
+impl FilterAssetLocation for NativeAsset {
+	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool {
+		matches!(asset, MultiAsset::ConcreteFungible { ref id, .. } if id == origin)
+	}
+}
+
+
+pub struct Case<T>(PhantomData<T>);
+impl<T: Get<(MultiAsset, MultiLocation)>> FilterAssetLocation for Case<T> {
+	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool {
+		let (a, o) = T::get();
+		&a == asset && &o == origin
+	}
+}
@@ -14,10 +14,10 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

-use sp_std::{prelude::*, result, convert::TryFrom, marker::PhantomData, borrow::Borrow};
+use sp_std::{prelude::*, result, marker::PhantomData, borrow::Borrow};
 use xcm::v0::{Error as XcmError, Result, MultiAsset, MultiLocation, Junction};
-use frame_support::traits::{Get, tokens::fungibles::Mutate as Fungibles};
-use xcm_executor::traits::{LocationConversion, TransactAsset};
+use frame_support::traits::{Get, tokens::fungibles};
+use xcm_executor::traits::{TransactAsset, Convert};

 /// Asset transaction errors.
 pub enum Error {
@@ -45,104 +45,6 @@ impl From<Error> for XcmError {
 	}
 }

-/// Generic third-party conversion trait. Use this when you don't want to force the user to use default
-/// impls of `From` and `Into` for the types you wish to convert between.
-///
-/// One of `convert`/`convert_ref` and `reverse`/`reverse_ref` MUST be implemented. If possible, implement
-/// `convert_ref`, since this will never result in a clone. Use `convert` when you definitely need to consume
-/// the source value.
-pub trait Convert<A: Clone, B: Clone> {
-	/// Convert from `value` (of type `A`) into an equivalent value of type `B`, `Err` if not possible.
-	fn convert(value: A) -> result::Result<B, A> { Self::convert_ref(&value).map_err(|_| value) }
-	fn convert_ref(value: impl Borrow<A>) -> result::Result<B, ()> {
-		Self::convert(value.borrow().clone()).map_err(|_| ())
-	}
-	/// Convert from `value` (of type `B`) into an equivalent value of type `A`, `Err` if not possible.
-	fn reverse(value: B) -> result::Result<A, B> { Self::reverse_ref(&value).map_err(|_| value) }
-	fn reverse_ref(value: impl Borrow<B>) -> result::Result<A, ()> {
-		Self::reverse(value.borrow().clone()).map_err(|_| ())
-	}
-}
-
-#[impl_trait_for_tuples::impl_for_tuples(30)]
-impl<A: Clone, B: Clone> Convert<A, B> for Tuple {
-	fn convert(value: A) -> result::Result<B, A> {
-		for_tuples!( #(
-			let value = match Tuple::convert(value) {
-				Ok(result) => return Ok(result),
-				Err(v) => v,
-			};
-		)* );
-		Err(value)
-	}
-	fn reverse(value: B) -> result::Result<A, B> {
-		for_tuples!( #(
-			let value = match Tuple::reverse(value) {
-				Ok(result) => return Ok(result),
-				Err(v) => v,
-			};
-		)* );
-		Err(value)
-	}
-	fn convert_ref(value: impl Borrow<A>) -> result::Result<B, ()> {
-		let value = value.borrow();
-		for_tuples!( #(
-			match Tuple::convert_ref(value) {
-				Ok(result) => return Ok(result),
-				Err(_) => (),
-			}
-		)* );
-		Err(())
-	}
-	fn reverse_ref(value: impl Borrow<B>) -> result::Result<A, ()> {
-		let value = value.borrow();
-		for_tuples!( #(
-			match Tuple::reverse_ref(value.clone()) {
-				Ok(result) => return Ok(result),
-				Err(_) => (),
-			}
-		)* );
-		Err(())
-	}
-}
-
-/// Simple pass-through which implements `BytesConversion` while not doing any conversion.
-pub struct Identity;
-impl<T: Clone> Convert<T, T> for Identity {
-	fn convert(value: T) -> result::Result<T, T> { Ok(value) }
-	fn reverse(value: T) -> result::Result<T, T> { Ok(value) }
-}
-
-/// Implementation of `Convert` trait using `TryFrom`.
-pub struct JustTry;
-impl<Source: TryFrom<Dest> + Clone, Dest: TryFrom<Source> + Clone> Convert<Source, Dest> for JustTry {
-	fn convert(value: Source) -> result::Result<Dest, Source> {
-		Dest::try_from(value.clone()).map_err(|_| value)
-	}
-	fn reverse(value: Dest) -> result::Result<Source, Dest> {
-		Source::try_from(value.clone()).map_err(|_| value)
-	}
-}
-
-use parity_scale_codec::{Encode, Decode};
-/// Implementation of `Convert<_, Vec<u8>>` using the parity scale codec.
-pub struct Encoded;
-impl<T: Clone + Encode + Decode> Convert<T, Vec<u8>> for Encoded {
-	fn convert_ref(value: impl Borrow<T>) -> result::Result<Vec<u8>, ()> { Ok(value.borrow().encode()) }
-	fn reverse_ref(bytes: impl Borrow<Vec<u8>>) -> result::Result<T, ()> {
-		T::decode(&mut &bytes.borrow()[..]).map_err(|_| ())
-	}
-}
-
-/// Implementation of `Convert<Vec<u8>, _>` using the parity scale codec.
-pub struct Decoded;
-impl<T: Clone + Encode + Decode> Convert<Vec<u8>, T> for Decoded {
-	fn convert_ref(bytes: impl Borrow<Vec<u8>>) -> result::Result<T, ()> {
-		T::decode(&mut &bytes.borrow()[..]).map_err(|_| ())
-	}
-	fn reverse_ref(value: impl Borrow<T>) -> result::Result<Vec<u8>, ()> { Ok(value.borrow().encode()) }
-}
-
 /// Converter struct implementing `AssetIdConversion` converting a numeric asset ID (must be TryFrom/TryInto<u128>)
 /// into a `GeneralIndex` junction, prefixed by some `MultiLocation` value. The `MultiLocation` value will
 /// typically be a `PalletInstance` junction.
@@ -232,22 +134,47 @@ impl<
 	}
 }

-pub struct FungiblesAdapter<Assets, Matcher, AccountIdConverter, AccountId>(
+pub struct FungiblesTransferAdapter<Assets, Matcher, AccountIdConverter, AccountId>(
 	PhantomData<(Assets, Matcher, AccountIdConverter, AccountId)>
 );
-
 impl<
-	Assets: Fungibles<AccountId>,
+	Assets: fungibles::Transfer<AccountId>,
 	Matcher: MatchesFungibles<Assets::AssetId, Assets::Balance>,
-	AccountIdConverter: LocationConversion<AccountId>,
-	AccountId,	// can't get away without it since Currency is generic over it.
-> TransactAsset for FungiblesAdapter<Assets, Matcher, AccountIdConverter, AccountId> {
+	AccountIdConverter: Convert<MultiLocation, AccountId>,
+	AccountId: Clone,	// can't get away without it since Currency is generic over it.
+> TransactAsset for FungiblesTransferAdapter<Assets, Matcher, AccountIdConverter, AccountId> {
+	fn transfer_asset(
+		what: &MultiAsset,
+		from: &MultiLocation,
+		to: &MultiLocation,
+	) -> result::Result<xcm_executor::Assets, XcmError> {
+		// Check we handle this asset.
+		let (asset_id, amount) = Matcher::matches_fungibles(what)?;
+		let source = AccountIdConverter::convert_ref(from)
+			.map_err(|()| Error::AccountIdConversionFailed)?;
+		let dest = AccountIdConverter::convert_ref(to)
+			.map_err(|()| Error::AccountIdConversionFailed)?;
+		Assets::transfer(asset_id, &source, &dest, amount, true)
+			.map_err(|e| XcmError::FailedToTransactAsset(e.into()))?;
+		Ok(what.clone().into())
+	}
+}
+
+pub struct FungiblesMutateAdapter<Assets, Matcher, AccountIdConverter, AccountId>(
+	PhantomData<(Assets, Matcher, AccountIdConverter, AccountId)>
+);
+impl<
+	Assets: fungibles::Mutate<AccountId>,
+	Matcher: MatchesFungibles<Assets::AssetId, Assets::Balance>,
+	AccountIdConverter: Convert<MultiLocation, AccountId>,
+	AccountId: Clone,	// can't get away without it since Currency is generic over it.
+> TransactAsset for FungiblesMutateAdapter<Assets, Matcher, AccountIdConverter, AccountId> {

 	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> Result {
 		// Check we handle this asset.
 		let (asset_id, amount) = Matcher::matches_fungibles(what)?;
-		let who = AccountIdConverter::from_location(who)
-			.ok_or(Error::AccountIdConversionFailed)?;
+		let who = AccountIdConverter::convert_ref(who)
+			.map_err(|()| Error::AccountIdConversionFailed)?;
 		Assets::mint_into(asset_id, &who, amount)
 			.map_err(|e| XcmError::FailedToTransactAsset(e.into()))
 	}
@@ -255,13 +182,43 @@ impl<
 	fn withdraw_asset(
 		what: &MultiAsset,
 		who: &MultiLocation
-	) -> result::Result<MultiAsset, XcmError> {
+	) -> result::Result<xcm_executor::Assets, XcmError> {
 		// Check we handle this asset.
 		let (asset_id, amount) = Matcher::matches_fungibles(what)?;
-		let who = AccountIdConverter::from_location(who)
-			.ok_or(Error::AccountIdConversionFailed)?;
+		let who = AccountIdConverter::convert_ref(who)
+			.map_err(|()| Error::AccountIdConversionFailed)?;
 		Assets::burn_from(asset_id, &who, amount)
 			.map_err(|e| XcmError::FailedToTransactAsset(e.into()))?;
-		Ok(what.clone())
+		Ok(what.clone().into())
+	}
+}
+
+pub struct FungiblesAdapter<Assets, Matcher, AccountIdConverter, AccountId>(
+	PhantomData<(Assets, Matcher, AccountIdConverter, AccountId)>
+);
+impl<
+	Assets: fungibles::Mutate<AccountId> + fungibles::Transfer<AccountId>,
+	Matcher: MatchesFungibles<Assets::AssetId, Assets::Balance>,
+	AccountIdConverter: Convert<MultiLocation, AccountId>,
+	AccountId: Clone,	// can't get away without it since Currency is generic over it.
+> TransactAsset for FungiblesAdapter<Assets, Matcher, AccountIdConverter, AccountId> {
+
+	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> Result {
+		FungiblesMutateAdapter::<Assets, Matcher, AccountIdConverter, AccountId>::deposit_asset(what, who)
+	}
+
+	fn withdraw_asset(
+		what: &MultiAsset,
+		who: &MultiLocation
+	) -> result::Result<xcm_executor::Assets, XcmError> {
+		FungiblesMutateAdapter::<Assets, Matcher, AccountIdConverter, AccountId>::withdraw_asset(what, who)
+	}
+
+	fn transfer_asset(
+		what: &MultiAsset,
+		from: &MultiLocation,
+		to: &MultiLocation,
+	) -> result::Result<xcm_executor::Assets, XcmError> {
+		FungiblesTransferAdapter::<Assets, Matcher, AccountIdConverter, AccountId>::transfer_asset(what, from, to)
 	}
 }
@@ -16,43 +16,40 @@

 #![cfg_attr(not(feature = "std"), no_std)]

+#[cfg(test)]
+mod mock;
+#[cfg(test)]
+mod tests;
+
 mod location_conversion;
 pub use location_conversion::{
-	Account32Hash, ParentIsDefault, ChildParachainConvertsVia, SiblingParachainConvertsVia, AccountId32Aliases, AccountKey20Aliases,
+	Account32Hash, ParentIsDefault, ChildParachainConvertsVia, SiblingParachainConvertsVia, AccountId32Aliases,
+	AccountKey20Aliases, LocationInverter,
 };

 mod origin_conversion;
 pub use origin_conversion::{
 	SovereignSignedViaLocation, ParentAsSuperuser, ChildSystemParachainAsSuperuser, SiblingSystemParachainAsSuperuser,
-	ChildParachainAsNative, SiblingParachainAsNative, RelayChainAsNative, SignedAccountId32AsNative, SignedAccountKey20AsNative,
+	ChildParachainAsNative, SiblingParachainAsNative, RelayChainAsNative, SignedAccountId32AsNative,
+	SignedAccountKey20AsNative,
+};
+
+mod barriers;
+pub use barriers::{
+	TakeWeightCredit, AllowUnpaidExecutionFrom, AllowTopLevelPaidExecutionFrom, AllowKnownQueryResponses,
 };

 mod currency_adapter;
-mod fungibles_adapter;
 pub use currency_adapter::CurrencyAdapter;
+
+mod fungibles_adapter;
 pub use fungibles_adapter::FungiblesAdapter;

-use sp_std::marker::PhantomData;
-use xcm_executor::traits::InvertLocation;
-use xcm::v0::{MultiLocation, Junction};
-use frame_support::traits::Get;
+mod weight;
+pub use weight::{FixedRateOfConcreteFungible, FixedWeightBounds};

-/// Simple location inverter; give it this location's ancestry and it'll
-pub struct LocationInverter<Ancestry>(PhantomData<Ancestry>);
+mod matches_fungible;
+pub use matches_fungible::{IsAbstract, IsConcrete};

-impl<Ancestry: Get<MultiLocation>> InvertLocation for LocationInverter<Ancestry> {
-	fn invert_location(location: &MultiLocation) -> MultiLocation {
-		let mut ancestry = Ancestry::get();
-		let mut result = location.clone();
-		for (i, j) in location.iter_rev()
-			.map(|j| match j {
-				Junction::Parent => ancestry.take_first().unwrap_or(Junction::OnlyChild),
-				_ => Junction::Parent,
-			})
-			.enumerate()
-		{
-			*result.at_mut(i).expect("location and result begin equal; same size; qed") = j;
-		}
-		result
-	}
-}
+mod filter_asset_location;
+pub use filter_asset_location::{Case, NativeAsset};
@@ -14,70 +14,68 @@
 // You should have received a copy of the GNU General Public License
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.

-use sp_std::marker::PhantomData;
+use sp_std::{marker::PhantomData, borrow::Borrow};
 use sp_io::hashing::blake2_256;
 use sp_runtime::traits::AccountIdConversion;
 use frame_support::traits::Get;
 use parity_scale_codec::Encode;
 use xcm::v0::{MultiLocation, NetworkId, Junction};
-use xcm_executor::traits::LocationConversion;
+use xcm_executor::traits::{InvertLocation, Convert};

 pub struct Account32Hash<Network, AccountId>(PhantomData<(Network, AccountId)>);

 impl<
 	Network: Get<NetworkId>,
-	AccountId: From<[u8; 32]> + Into<[u8; 32]>,
-> LocationConversion<AccountId> for Account32Hash<Network, AccountId> {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		Some(("multiloc", location).using_encoded(blake2_256).into())
+	AccountId: From<[u8; 32]> + Into<[u8; 32]> + Clone,
+> Convert<MultiLocation, AccountId> for Account32Hash<Network, AccountId> {
+	fn convert_ref(location: impl Borrow<MultiLocation>) -> Result<AccountId, ()> {
+		Ok(("multiloc", location.borrow()).using_encoded(blake2_256).into())
 	}

-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
-		Err(who)
+	fn reverse_ref(_: impl Borrow<AccountId>) -> Result<MultiLocation, ()> {
+		Err(())
 	}
 }

 pub struct ParentIsDefault<AccountId>(PhantomData<AccountId>);
-
 impl<
-	AccountId: Default + Eq,
-> LocationConversion<AccountId> for ParentIsDefault<AccountId> {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		if let MultiLocation::X1(Junction::Parent) = location {
-			Some(AccountId::default())
+	AccountId: Default + Eq + Clone,
+> Convert<MultiLocation, AccountId> for ParentIsDefault<AccountId> {
+	fn convert_ref(location: impl Borrow<MultiLocation>) -> Result<AccountId, ()> {
+		if let &MultiLocation::X1(Junction::Parent) = location.borrow() {
+			Ok(AccountId::default())
 		} else {
-			None
+			Err(())
 		}
 	}

-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
-		if who == AccountId::default() {
+	fn reverse_ref(who: impl Borrow<AccountId>) -> Result<MultiLocation, ()> {
+		if who.borrow() == &AccountId::default() {
 			Ok(Junction::Parent.into())
 		} else {
-			Err(who)
+			Err(())
 		}
 	}
 }

 pub struct ChildParachainConvertsVia<ParaId, AccountId>(PhantomData<(ParaId, AccountId)>);
-
 impl<
 	ParaId: From<u32> + Into<u32> + AccountIdConversion<AccountId>,
-	AccountId,
-> LocationConversion<AccountId> for ChildParachainConvertsVia<ParaId, AccountId> {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		if let MultiLocation::X1(Junction::Parachain { id }) = location {
-			Some(ParaId::from(*id).into_account())
+	AccountId: Clone,
+> Convert<MultiLocation, AccountId> for ChildParachainConvertsVia<ParaId, AccountId> {
+	fn convert_ref(location: impl Borrow<MultiLocation>) -> Result<AccountId, ()> {
+		if let &MultiLocation::X1(Junction::Parachain { id }) = location.borrow() {
+			Ok(ParaId::from(id).into_account())
 		} else {
-			None
+			Err(())
 		}
 	}

-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
-		if let Some(id) = ParaId::try_from_account(&who) {
+	fn reverse_ref(who: impl Borrow<AccountId>) -> Result<MultiLocation, ()> {
+		if let Some(id) = ParaId::try_from_account(who.borrow()) {
 			Ok(Junction::Parachain { id: id.into() }.into())
 		} else {
-			Err(who)
+			Err(())
 		}
 	}
 }
@@ -86,65 +84,79 @@ pub struct SiblingParachainConvertsVia<ParaId, AccountId>(PhantomData<(ParaId, A

 impl<
 	ParaId: From<u32> + Into<u32> + AccountIdConversion<AccountId>,
-	AccountId,
-> LocationConversion<AccountId> for SiblingParachainConvertsVia<ParaId, AccountId> {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		if let MultiLocation::X2(Junction::Parent, Junction::Parachain { id }) = location {
-			Some(ParaId::from(*id).into_account())
+	AccountId: Clone,
+> Convert<MultiLocation, AccountId> for SiblingParachainConvertsVia<ParaId, AccountId> {
+	fn convert_ref(location: impl Borrow<MultiLocation>) -> Result<AccountId, ()> {
+		if let &MultiLocation::X2(Junction::Parent, Junction::Parachain { id }) = location.borrow() {
+			Ok(ParaId::from(id).into_account())
 		} else {
-			None
+			Err(())
 		}
 	}

-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
-		if let Some(id) = ParaId::try_from_account(&who) {
+	fn reverse_ref(who: impl Borrow<AccountId>) -> Result<MultiLocation, ()> {
+		if let Some(id) = ParaId::try_from_account(who.borrow()) {
 			Ok([Junction::Parent, Junction::Parachain { id: id.into() }].into())
 		} else {
-			Err(who)
+			Err(())
 		}
 	}
 }

 pub struct AccountId32Aliases<Network, AccountId>(PhantomData<(Network, AccountId)>);
-
 impl<
 	Network: Get<NetworkId>,
-	AccountId: From<[u8; 32]> + Into<[u8; 32]>,
-> LocationConversion<AccountId> for AccountId32Aliases<Network, AccountId> {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		if let MultiLocation::X1(Junction::AccountId32 { id, network }) = location {
-			if matches!(network, NetworkId::Any) || network == &Network::get() {
-				return Some((*id).into())
-			}
-		}
-		None
+	AccountId: From<[u8; 32]> + Into<[u8; 32]> + Clone,
+> Convert<MultiLocation, AccountId> for AccountId32Aliases<Network, AccountId> {
+	fn convert(location: MultiLocation) -> Result<AccountId, MultiLocation> {
+		let id = match location {
+			MultiLocation::X1(Junction::AccountId32 { id, network: NetworkId::Any }) => id,
+			MultiLocation::X1(Junction::AccountId32 { id, network }) if &network == &Network::get() => id,
+			l => return Err(l),
+		};
+		Ok(id.into())
 	}

-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
+	fn reverse(who: AccountId) -> Result<MultiLocation, AccountId> {
 		Ok(Junction::AccountId32 { id: who.into(), network: Network::get() }.into())
 	}
 }

 pub struct AccountKey20Aliases<Network, AccountId>(PhantomData<(Network, AccountId)>);
-
 impl<
 	Network: Get<NetworkId>,
-	AccountId: From<[u8; 20]> + Into<[u8; 20]>
-> LocationConversion<AccountId> for AccountKey20Aliases<Network, AccountId> {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		if let MultiLocation::X1(Junction::AccountKey20 { key, network }) = location {
-			if matches!(network, NetworkId::Any) || network == &Network::get() {
-				return Some((*key).into());
-			}
-		}
-		None
+	AccountId: From<[u8; 20]> + Into<[u8; 20]> + Clone,
+> Convert<MultiLocation, AccountId> for AccountKey20Aliases<Network, AccountId> {
+	fn convert(location: MultiLocation) -> Result<AccountId, MultiLocation> {
+		let key = match location {
+			MultiLocation::X1(Junction::AccountKey20 { key, network: NetworkId::Any }) => key,
+			MultiLocation::X1(Junction::AccountKey20 { key, network }) if &network == &Network::get() => key,
+			l => return Err(l),
+		};
+		Ok(key.into())
 	}

-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
-		Ok(Junction::AccountKey20 {
-			key: who.into(),
-			network: Network::get(),
-		}
-		.into())
+	fn reverse(who: AccountId) -> Result<MultiLocation, AccountId> {
+		let j = Junction::AccountKey20 { key: who.into(), network: Network::get() };
+		Ok(j.into())
+	}
+}
+
+/// Simple location inverter; give it this location's ancestry and it'll figure out the inverted location.
+pub struct LocationInverter<Ancestry>(PhantomData<Ancestry>);
+impl<Ancestry: Get<MultiLocation>> InvertLocation for LocationInverter<Ancestry> {
+	fn invert_location(location: &MultiLocation) -> MultiLocation {
+		let mut ancestry = Ancestry::get();
+		let mut result = location.clone();
+		for (i, j) in location.iter_rev()
+			.map(|j| match j {
+				Junction::Parent => ancestry.take_first().unwrap_or(Junction::OnlyChild),
+				_ => Junction::Parent,
+			})
+			.enumerate()
+		{
+			*result.at_mut(i).expect("location and result begin equal; same size; qed") = j;
+		}
+		result
 	}
 }
@@ -0,0 +1,42 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::{marker::PhantomData, convert::TryFrom};
+use sp_runtime::traits::CheckedConversion;
+use xcm::v0::{MultiAsset, MultiLocation};
+use frame_support::traits::Get;
+use xcm_executor::traits::MatchesFungible;
+
+pub struct IsConcrete<T>(PhantomData<T>);
+impl<T: Get<MultiLocation>, B: TryFrom<u128>> MatchesFungible<B> for IsConcrete<T> {
+	fn matches_fungible(a: &MultiAsset) -> Option<B> {
+		match a {
+			MultiAsset::ConcreteFungible { id, amount } if id == &T::get() =>
+				CheckedConversion::checked_from(*amount),
+			_ => None,
+		}
+	}
+}
+pub struct IsAbstract<T>(PhantomData<T>);
+impl<T: Get<&'static [u8]>, B: TryFrom<u128>> MatchesFungible<B> for IsAbstract<T> {
+	fn matches_fungible(a: &MultiAsset) -> Option<B> {
+		match a {
+			MultiAsset::AbstractFungible { id, amount } if &id[..] == T::get() =>
+				CheckedConversion::checked_from(*amount),
+			_ => None,
+		}
+	}
+}
@@ -0,0 +1,282 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+pub use sp_std::{fmt::Debug, marker::PhantomData, cell::RefCell};
+pub use sp_std::collections::{btree_map::BTreeMap, btree_set::BTreeSet};
+pub use parity_scale_codec::{Encode, Decode};
+pub use xcm::v0::{
+	SendXcm, MultiLocation::*, Junction::*, MultiAsset, Xcm, Order, Result as XcmResult, Error as XcmError,
+	OriginKind, MultiLocation, Junction, opaque,
+};
+pub use frame_support::{
+	ensure, parameter_types,
+	dispatch::{Dispatchable, Parameter, Weight, DispatchError, DispatchResultWithPostInfo, DispatchInfo},
+	weights::{PostDispatchInfo, GetDispatchInfo},
+	sp_runtime::DispatchErrorWithPostInfo,
+	traits::{Get, Contains},
+};
+pub use xcm_executor::{
+	Assets, Config, traits::{TransactAsset, ConvertOrigin, FilterAssetLocation, InvertLocation, OnResponse}
+};
+pub use crate::{
+	TakeWeightCredit, AllowTopLevelPaidExecutionFrom, AllowUnpaidExecutionFrom, FixedWeightBounds,
+	FixedRateOfConcreteFungible, AllowKnownQueryResponses, LocationInverter,
+};
+
+pub enum TestOrigin { Root, Relay, Signed(u64), Parachain(u32) }
+
+#[derive(Debug, Encode, Decode, Eq, PartialEq, Clone, Copy)]
+pub enum TestCall {
+	OnlyRoot(Weight, Option<Weight>),
+	OnlyParachain(Weight, Option<Weight>, Option<u32>),
+	OnlySigned(Weight, Option<Weight>, Option<u64>),
+	Any(Weight, Option<Weight>),
+}
+impl Dispatchable for TestCall {
+	type Origin = TestOrigin;
+	type Config = ();
+	type Info = ();
+	type PostInfo = PostDispatchInfo;
+	fn dispatch(self, origin: Self::Origin) -> DispatchResultWithPostInfo {
+		let mut post_info = PostDispatchInfo::default();
+		post_info.actual_weight = match self {
+			TestCall::OnlyRoot(_, maybe_actual)
+			| TestCall::OnlySigned(_, maybe_actual, _)
+			| TestCall::OnlyParachain(_, maybe_actual, _)
+			| TestCall::Any(_, maybe_actual)
+			=> maybe_actual,
+		};
+		if match (&origin, &self) {
+			(TestOrigin::Parachain(i), TestCall::OnlyParachain(_, _, Some(j)))
+			=> i == j,
+			(TestOrigin::Signed(i), TestCall::OnlySigned(_, _, Some(j)))
+			=> i == j,
+
+			(TestOrigin::Root, TestCall::OnlyRoot(..))
+			| (TestOrigin::Parachain(_), TestCall::OnlyParachain(_, _, None))
+			| (TestOrigin::Signed(_), TestCall::OnlySigned(_, _, None))
+			| (_, TestCall::Any(..))
+			=> true,
+
+			_ => false,
+		} {
+			Ok(post_info)
+		} else {
+			Err(DispatchErrorWithPostInfo { error: DispatchError::BadOrigin, post_info })
+		}
+	}
+}
+
+impl GetDispatchInfo for TestCall {
+	fn get_dispatch_info(&self) -> DispatchInfo {
+		let weight = *match self {
+			TestCall::OnlyRoot(estimate, ..)
+			| TestCall::OnlyParachain(estimate, ..)
+			| TestCall::OnlySigned(estimate, ..)
+			| TestCall::Any(estimate, ..)
+			=> estimate,
+		};
+		DispatchInfo { weight, .. Default::default() }
+	}
+}
+
+thread_local! {
+	pub static SENT_XCM: RefCell<Vec<(MultiLocation, opaque::Xcm)>> = RefCell::new(Vec::new());
+}
+pub fn sent_xcm() -> Vec<(MultiLocation, opaque::Xcm)> {
+	SENT_XCM.with(|q| (*q.borrow()).clone())
+}
+pub struct TestSendXcm;
+impl SendXcm for TestSendXcm {
+	fn send_xcm(dest: MultiLocation, msg: opaque::Xcm) -> XcmResult {
+		SENT_XCM.with(|q| q.borrow_mut().push((dest, msg)));
+		Ok(())
+	}
+}
+
+thread_local! {
+	pub static ASSETS: RefCell<BTreeMap<u64, Assets>> = RefCell::new(BTreeMap::new());
+}
+pub fn assets(who: u64) -> Vec<MultiAsset> {
+	ASSETS.with(|a| a.borrow().get(&who).map_or(vec![], |a| a.clone().into()))
+}
+pub fn add_asset(who: u64, what: MultiAsset) {
+	ASSETS.with(|a| a.borrow_mut()
+		.entry(who)
+		.or_insert(Assets::new())
+		.saturating_subsume(what)
+	);
+}
+
+pub struct TestAssetTransactor;
+impl TransactAsset for TestAssetTransactor {
+	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> Result<(), XcmError> {
+		let who = to_account(who.clone()).map_err(|_| XcmError::LocationCannotHold)?;
+		add_asset(who, what.clone());
+		Ok(())
+	}
+
+	fn withdraw_asset(what: &MultiAsset, who: &MultiLocation) -> Result<Assets, XcmError> {
+		let who = to_account(who.clone()).map_err(|_| XcmError::LocationCannotHold)?;
+		ASSETS.with(|a| a.borrow_mut()
+			.get_mut(&who)
+			.ok_or(XcmError::NotWithdrawable)?
+			.try_take(what.clone())
+			.map_err(|()| XcmError::NotWithdrawable)
+		)
+	}
+}
+
+
+pub fn to_account(l: MultiLocation) -> Result<u64, MultiLocation> {
+	Ok(match l {
+		// Siblings at 2000+id
+		X2(Parent, Parachain { id }) => 2000 + id as u64,
+		// Accounts are their number
+		X1(AccountIndex64 { index, .. }) => index,
+		// Children at 1000+id
+		X1(Parachain { id }) => 1000 + id as u64,
+		// Self at 3000
+		Null => 3000,
+		// Parent at 3000
+		X1(Parent) => 3001,
+		l => return Err(l),
+	})
+}
+
+pub struct TestOriginConverter;
+impl ConvertOrigin<TestOrigin> for TestOriginConverter {
+	fn convert_origin(origin: MultiLocation, kind: OriginKind) -> Result<TestOrigin, MultiLocation> {
+		use {OriginKind::*};
+		match (kind, origin) {
+			(Superuser, _) => Ok(TestOrigin::Root),
+			(SovereignAccount, l) => Ok(TestOrigin::Signed(to_account(l)?)),
+			(Native, X1(Parachain { id })) => Ok(TestOrigin::Parachain(id)),
+			(Native, X1(Parent)) => Ok(TestOrigin::Relay),
+			(Native, X1(AccountIndex64 {index, ..})) => Ok(TestOrigin::Signed(index)),
+			(_, origin) => Err(origin),
+		}
+	}
+}
+
+thread_local! {
+	pub static IS_RESERVE: RefCell<BTreeMap<MultiLocation, Vec<MultiAsset>>> = RefCell::new(BTreeMap::new());
+	pub static IS_TELEPORTER: RefCell<BTreeMap<MultiLocation, Vec<MultiAsset>>> = RefCell::new(BTreeMap::new());
+}
+pub fn add_reserve(from: MultiLocation, asset: MultiAsset) {
+	IS_RESERVE.with(|r| r.borrow_mut().entry(from).or_default().push(asset));
+}
+#[allow(dead_code)]
+pub fn add_teleporter(from: MultiLocation, asset: MultiAsset) {
+	IS_TELEPORTER.with(|r| r.borrow_mut().entry(from).or_default().push(asset));
+}
+pub struct TestIsReserve;
+impl FilterAssetLocation for TestIsReserve {
+	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool {
+		IS_RESERVE.with(|r| r.borrow().get(origin)
+			.map_or(false, |v| v.iter().any(|a| a.contains(asset)))
+		)
+	}
+}
+pub struct TestIsTeleporter;
+impl FilterAssetLocation for TestIsTeleporter {
+	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool {
+		IS_TELEPORTER.with(|r| r.borrow().get(origin)
+			.map_or(false, |v| v.iter().any(|a| a.contains(asset)))
+		)
+	}
+}
+
+use xcm::v0::Response;
+pub enum ResponseSlot {
+	Expecting(MultiLocation),
+	Received(Response),
+}
+thread_local! {
+	pub static QUERIES: RefCell<BTreeMap<u64, ResponseSlot>> = RefCell::new(BTreeMap::new());
+}
+pub struct TestResponseHandler;
+impl OnResponse for TestResponseHandler {
+	fn expecting_response(origin: &MultiLocation, query_id: u64) -> bool {
+		QUERIES.with(|q| match q.borrow().get(&query_id) {
+			Some(ResponseSlot::Expecting(ref l)) => l == origin,
+			_ => false,
+		})
+	}
+	fn on_response(_origin: MultiLocation, query_id: u64, response: xcm::v0::Response) -> Weight {
+		QUERIES.with(|q| {
+			q.borrow_mut()
+				.entry(query_id)
+				.and_modify(|v| if matches!(*v, ResponseSlot::Expecting(..)) {
+					*v = ResponseSlot::Received(response);
+				});
+		});
+		10
+	}
+}
+pub fn expect_response(query_id: u64, from: MultiLocation) {
+	QUERIES.with(|q| q.borrow_mut()
+		.insert(query_id, ResponseSlot::Expecting(from))
+	);
+}
+pub fn response(query_id: u64) -> Option<Response> {
+	QUERIES.with(|q| q.borrow()
+		.get(&query_id)
+		.and_then(|v| match v {
+			ResponseSlot::Received(r) => Some(r.clone()),
+			_ => None,
+		})
+	)
+}
+
+parameter_types! {
+	pub TestAncestry: MultiLocation = X1(Parachain{id: 42});
+	pub UnitWeightCost: Weight = 10;
+}
+parameter_types! {
+	// Nothing is allowed to be paid/unpaid by default.
+	pub static AllowUnpaidFrom: Vec<MultiLocation> = vec![];
+	pub static AllowPaidFrom: Vec<MultiLocation> = vec![];
+	// 1_000_000_000_000 => 1 unit of asset for 1 unit of Weight.
+	pub static WeightPrice: (MultiLocation, u128) = (Null, 1_000_000_000_000);
+}
+
+pub struct IsInVec<T>(PhantomData<T>);
+impl<X: Ord + PartialOrd, T: Get<Vec<X>>> Contains<X> for IsInVec<T> {
+	fn sorted_members() -> Vec<X> { let mut r = T::get(); r.sort(); r }
+}
+
+pub type TestBarrier = (
+	TakeWeightCredit,
+	AllowKnownQueryResponses<TestResponseHandler>,
+	AllowTopLevelPaidExecutionFrom<IsInVec<AllowPaidFrom>>,
+	AllowUnpaidExecutionFrom<IsInVec<AllowUnpaidFrom>>,
+);
+
+pub struct TestConfig;
+impl Config for TestConfig {
+	type Call = TestCall;
+	type XcmSender = TestSendXcm;
+	type AssetTransactor = TestAssetTransactor;
+	type OriginConverter = TestOriginConverter;
+	type IsReserve = TestIsReserve;
+	type IsTeleporter = TestIsTeleporter;
+	type LocationInverter = LocationInverter<TestAncestry>;
+	type Barrier = TestBarrier;
+	type Weigher = FixedWeightBounds<UnitWeightCost, TestCall>;
+	type Trader = FixedRateOfConcreteFungible<WeightPrice>;
+	type ResponseHandler = TestResponseHandler;
+}
@@ -17,7 +17,7 @@
 use sp_std::marker::PhantomData;
 use frame_support::traits::{Get, OriginTrait};
 use xcm::v0::{MultiLocation, OriginKind, NetworkId, Junction};
-use xcm_executor::traits::{LocationConversion, ConvertOrigin};
+use xcm_executor::traits::{Convert, ConvertOrigin};
 use polkadot_parachain::primitives::IsSystem;

 /// Sovereign accounts use the system's `Signed` origin with an account ID derived from the
@@ -26,12 +26,12 @@ pub struct SovereignSignedViaLocation<LocationConverter, Origin>(
 	PhantomData<(LocationConverter, Origin)>
 );
 impl<
-	LocationConverter: LocationConversion<Origin::AccountId>,
+	LocationConverter: Convert<MultiLocation, Origin::AccountId>,
 	Origin: OriginTrait,
-> ConvertOrigin<Origin> for SovereignSignedViaLocation<LocationConverter, Origin> {
+> ConvertOrigin<Origin> for SovereignSignedViaLocation<LocationConverter, Origin> where Origin::AccountId: Clone {
 	fn convert_origin(origin: MultiLocation, kind: OriginKind) -> Result<Origin, MultiLocation> {
 		if let OriginKind::SovereignAccount = kind {
-			let location = LocationConverter::from_location(&origin).ok_or(origin)?;
+			let location = LocationConverter::convert(origin)?;
 			Ok(Origin::signed(location).into())
 		} else {
 			Err(origin)
@@ -0,0 +1,338 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use super::*;
+use super::mock::*;
+use {MultiAsset::*, Option::None};
+use xcm::v0::{Order, NetworkId::Any, Outcome, Response, ExecuteXcm};
+use xcm_executor::{XcmExecutor, Config, traits::*};
+
+#[test]
+fn basic_setup_works() {
+	add_reserve(X1(Parent), AllConcreteFungible { id: X1(Parent) });
+	assert!(<TestConfig as Config>::IsReserve::filter_asset_location(
+		&ConcreteFungible { id: X1(Parent), amount: 100 },
+		&X1(Parent),
+	));
+
+	assert_eq!(to_account(X1(Parachain{id:1})), Ok(1001));
+	assert_eq!(to_account(X1(Parachain{id:50})), Ok(1050));
+	assert_eq!(to_account(X2(Parent, Parachain{id:1})), Ok(2001));
+	assert_eq!(to_account(X2(Parent, Parachain{id:50})), Ok(2050));
+	assert_eq!(to_account(X1(AccountIndex64{index:1, network:Any})), Ok(1));
+	assert_eq!(to_account(X1(AccountIndex64{index:42, network:Any})), Ok(42));
+	assert_eq!(to_account(Null), Ok(3000));
+}
+
+#[test]
+fn weigher_should_work() {
+	let mut message = opaque::Xcm::ReserveAssetDeposit {
+		assets: vec![ConcreteFungible { id: X1(Parent), amount: 100 }],
+		effects: vec![
+			Order::BuyExecution { fees: All, weight: 0, debt: 30, halt_on_error: true, xcm: vec![] },
+			Order::DepositAsset { assets: vec![All], dest: Null },
+		],
+	}.into();
+	assert_eq!(<TestConfig as Config>::Weigher::shallow(&mut message), Ok(30));
+}
+
+#[test]
+fn take_weight_credit_barrier_should_work() {
+	let mut message = opaque::Xcm::TransferAsset {
+		assets: vec![ConcreteFungible { id: X1(Parent), amount: 100 }],
+		dest: Null,
+	};
+
+	let mut weight_credit = 10;
+	let r = TakeWeightCredit::should_execute(
+		&X1(Parent),
+		true,
+		&mut message,
+		10,
+		&mut weight_credit,
+	);
+	assert_eq!(r, Ok(()));
+	assert_eq!(weight_credit, 0);
+
+	let r = TakeWeightCredit::should_execute(
+		&X1(Parent),
+		true,
+		&mut message,
+		10,
+		&mut weight_credit,
+	);
+	assert_eq!(r, Err(()));
+	assert_eq!(weight_credit, 0);
+}
+
+#[test]
+fn allow_unpaid_should_work() {
+	let mut message = opaque::Xcm::TransferAsset {
+		assets: vec![ConcreteFungible { id: X1(Parent), amount: 100 }],
+		dest: Null,
+	};
+
+	AllowUnpaidFrom::set(vec![ X1(Parent) ]);
+
+	let r = AllowUnpaidExecutionFrom::<IsInVec<AllowUnpaidFrom>>::should_execute(
+		&X1(Parachain { id: 1 }),
+		true,
+		&mut message,
+		10,
+		&mut 0,
+	);
+	assert_eq!(r, Err(()));
+
+	let r = AllowUnpaidExecutionFrom::<IsInVec<AllowUnpaidFrom>>::should_execute(
+		&X1(Parent),
+		true,
+		&mut message,
+		10,
+		&mut 0,
+	);
+	assert_eq!(r, Ok(()));
+}
+
+#[test]
+fn allow_paid_should_work() {
+	AllowPaidFrom::set(vec![ X1(Parent) ]);
+
+	let mut message = opaque::Xcm::TransferAsset {
+		assets: vec![ConcreteFungible { id: X1(Parent), amount: 100 }],
+		dest: Null,
+	};
+
+	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
+		&X1(Parachain { id: 1 }),
+		true,
+		&mut message,
+		10,
+		&mut 0,
+	);
+	assert_eq!(r, Err(()));
+
+	let mut underpaying_message = opaque::Xcm::ReserveAssetDeposit {
+		assets: vec![ConcreteFungible { id: X1(Parent), amount: 100 }],
+		effects: vec![
+			Order::BuyExecution { fees: All, weight: 0, debt: 20, halt_on_error: true, xcm: vec![] },
+			Order::DepositAsset { assets: vec![All], dest: Null },
+		],
+	};
+
+	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
+		&X1(Parent),
+		true,
+		&mut underpaying_message,
+		30,
+		&mut 0,
+	);
+	assert_eq!(r, Err(()));
+
+	let mut paying_message = opaque::Xcm::ReserveAssetDeposit {
+		assets: vec![ConcreteFungible { id: X1(Parent), amount: 100 }],
+		effects: vec![
+			Order::BuyExecution { fees: All, weight: 0, debt: 30, halt_on_error: true, xcm: vec![] },
+			Order::DepositAsset { assets: vec![All], dest: Null },
+		],
+	};
+
+	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
+		&X1(Parachain { id: 1 }),
+		true,
+		&mut paying_message,
+		30,
+		&mut 0,
+	);
+	assert_eq!(r, Err(()));
+
+	let r = AllowTopLevelPaidExecutionFrom::<IsInVec<AllowPaidFrom>>::should_execute(
+		&X1(Parent),
+		true,
+		&mut paying_message,
+		30,
+		&mut 0,
+	);
+	assert_eq!(r, Ok(()));
+}
+
+#[test]
+fn paying_reserve_deposit_should_work() {
+	AllowPaidFrom::set(vec![ X1(Parent) ]);
+	add_reserve(X1(Parent), AllConcreteFungible { id: X1(Parent) });
+	WeightPrice::set((X1(Parent), 1_000_000_000_000));
+
+	let origin = X1(Parent);
+	let message = Xcm::<TestCall>::ReserveAssetDeposit {
+		assets: vec![ ConcreteFungible { id: X1(Parent), amount: 100 } ],
+		effects: vec![
+			Order::<TestCall>::BuyExecution { fees: All, weight: 0, debt: 30, halt_on_error: true, xcm: vec![] },
+			Order::<TestCall>::DepositAsset { assets: vec![ All ], dest: Null },
+		],
+	};
+	let weight_limit = 50;
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin, message, weight_limit);
+	assert_eq!(r, Outcome::Complete(30));
+	assert_eq!(assets(3000), vec![ ConcreteFungible { id: X1(Parent), amount: 70 } ]);
+}
+
+#[test]
+fn transfer_should_work() {
+	// we'll let them have message execution for free.
+	AllowUnpaidFrom::set(vec![ X1(Parachain{id:1}) ]);
+	// Child parachain #1 owns 1000 tokens held by us in reserve.
+	add_asset(1001, ConcreteFungible { id: Null, amount: 1000 });
+	// They want to transfer 100 of them to their sibling parachain #2
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		X1(Parachain{id:1}),
+		Xcm::TransferAsset {
+			assets: vec![ ConcreteFungible { id: Null, amount: 100 } ],
+			dest: X1(AccountIndex64{index:3, network:Any}),
+		},
+		50,
+	);
+	assert_eq!(r, Outcome::Complete(10));
+	assert_eq!(assets(3), vec![ ConcreteFungible { id: Null, amount: 100 } ]);
+	assert_eq!(assets(1001), vec![ ConcreteFungible { id: Null, amount: 900 } ]);
+	assert_eq!(sent_xcm(), vec![]);
+}
+
+#[test]
+fn reserve_transfer_should_work() {
+	AllowUnpaidFrom::set(vec![ X1(Parachain{id:1}) ]);
+	// Child parachain #1 owns 1000 tokens held by us in reserve.
+	add_asset(1001, ConcreteFungible { id: Null, amount: 1000 });
+	// The remote account owned by gav.
+	let three = X1(AccountIndex64{index:3, network:Any});
+
+	// They want to transfer 100 of our native asset from sovereign account of parachain #1 into #2
+	// and let them know to hand it to account #3.
+	let r = XcmExecutor::<TestConfig>::execute_xcm(
+		X1(Parachain{id:1}),
+		Xcm::TransferReserveAsset {
+			assets: vec![ ConcreteFungible { id: Null, amount: 100 } ],
+			dest: X1(Parachain{id:2}),
+			effects: vec![ Order::DepositAsset { assets: vec![ All ], dest: three.clone() } ],
+		},
+		50,
+	);
+	assert_eq!(r, Outcome::Complete(10));
+
+	assert_eq!(assets(1002), vec![ ConcreteFungible { id: Null, amount: 100 } ]);
+	assert_eq!(sent_xcm(), vec![(
+		X1(Parachain { id: 2 }),
+		Xcm::ReserveAssetDeposit {
+			assets: vec![ ConcreteFungible { id: X1(Parent), amount: 100 } ],
+			effects: vec![ Order::DepositAsset { assets: vec![ All ], dest: three } ],
+		})
+	]);
+}
+
+#[test]
+fn transacting_should_work() {
+	AllowUnpaidFrom::set(vec![ X1(Parent) ]);
+
+	let origin = X1(Parent);
+	let message = Xcm::<TestCall>::Transact {
+		origin_type: OriginKind::Native,
+		require_weight_at_most: 50,
+		call: TestCall::Any(50, None).encode().into(),
+	};
+	let weight_limit = 60;
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin, message, weight_limit);
+	assert_eq!(r, Outcome::Complete(60));
+}
+
+#[test]
+fn transacting_should_respect_max_weight_requirement() {
+	AllowUnpaidFrom::set(vec![ X1(Parent) ]);
+
+	let origin = X1(Parent);
+	let message = Xcm::<TestCall>::Transact {
+		origin_type: OriginKind::Native,
+		require_weight_at_most: 40,
+		call: TestCall::Any(50, None).encode().into(),
+	};
+	let weight_limit = 60;
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin, message, weight_limit);
+	assert_eq!(r, Outcome::Incomplete(60, XcmError::TooMuchWeightRequired));
+}
+
+#[test]
+fn transacting_should_refund_weight() {
+	AllowUnpaidFrom::set(vec![ X1(Parent) ]);
+
+	let origin = X1(Parent);
+	let message = Xcm::<TestCall>::Transact {
+		origin_type: OriginKind::Native,
+		require_weight_at_most: 50,
+		call: TestCall::Any(50, Some(30)).encode().into(),
+	};
+	let weight_limit = 60;
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin, message, weight_limit);
+	assert_eq!(r, Outcome::Complete(40));
+}
+
+#[test]
+fn paid_transacting_should_refund_payment_for_unused_weight() {
+	let one = X1(AccountIndex64{index:1, network:Any});
+	AllowPaidFrom::set(vec![ one.clone() ]);
+	add_asset(1, ConcreteFungible { id: X1(Parent), amount: 100 });
+	WeightPrice::set((X1(Parent), 1_000_000_000_000));
+
+	let origin = one.clone();
+	let message = Xcm::<TestCall>::WithdrawAsset {
+		assets: vec![ ConcreteFungible { id: X1(Parent), amount: 100 } ],	// enough for 100 units of weight.
+		effects: vec![
+			Order::<TestCall>::BuyExecution { fees: All, weight: 70, debt: 30, halt_on_error: true, xcm: vec![
+				Xcm::<TestCall>::Transact {
+					origin_type: OriginKind::Native,
+					require_weight_at_most: 60,
+					// call estimated at 70 but only takes 10.
+					call: TestCall::Any(60, Some(10)).encode().into(),
+				}
+			] },
+			Order::<TestCall>::DepositAsset { assets: vec![ All ], dest: one.clone() },
+		],
+	};
+	let weight_limit = 100;
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin, message, weight_limit);
+	assert_eq!(r, Outcome::Complete(50));
+	assert_eq!(assets(1), vec![ ConcreteFungible { id: X1(Parent), amount: 50 } ]);
+}
+
+#[test]
+fn prepaid_result_of_query_should_get_free_execution() {
+	let query_id = 33;
+	let origin = X1(Parent);
+	// We put this in manually here, but normally this would be done at the point of crafting the message.
+	expect_response(query_id, origin.clone());
+
+	let the_response = Response::Assets(vec![ ConcreteFungible { id: X1(Parent), amount: 100 } ]);
+	let message = Xcm::<TestCall>::QueryResponse {
+		query_id,
+		response: the_response.clone(),
+	};
+	let weight_limit = 10;
+
+	// First time the response gets through since we're expecting it...
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin.clone(), message.clone(), weight_limit);
+	assert_eq!(r, Outcome::Complete(10));
+	assert_eq!(response(query_id).unwrap(), the_response);
+
+	// Second time it doesn't, since we're not.
+	let r = XcmExecutor::<TestConfig>::execute_xcm(origin.clone(), message.clone(), weight_limit);
+	assert_eq!(r, Outcome::Incomplete(10, XcmError::Barrier));
+}
@@ -0,0 +1,98 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::{result::Result, marker::PhantomData};
+use parity_scale_codec::Decode;
+use xcm::v0::{Xcm, Order, MultiAsset, MultiLocation};
+use frame_support::{traits::Get, weights::{Weight, GetDispatchInfo}};
+use xcm_executor::{Assets, traits::{WeightBounds, WeightTrader}};
+
+pub struct FixedWeightBounds<T, C>(PhantomData<(T, C)>);
+impl<T: Get<Weight>, C: Decode + GetDispatchInfo> WeightBounds<C> for FixedWeightBounds<T, C> {
+	fn shallow(message: &mut Xcm<C>) -> Result<Weight, ()> {
+		let min = match message {
+			Xcm::Transact { call, .. } => {
+				call.ensure_decoded()?.get_dispatch_info().weight + T::get()
+			}
+			Xcm::WithdrawAsset { effects, .. }
+			| Xcm::ReserveAssetDeposit { effects, .. }
+			| Xcm::TeleportAsset { effects, .. } => {
+				let inner: Weight = effects.iter_mut()
+					.map(|effect| match effect {
+						Order::BuyExecution { .. } => {
+							// On success, execution of this will result in more weight being consumed but
+							// we don't count it here since this is only the *shallow*, non-negotiable weight
+							// spend and doesn't count weight placed behind a `BuyExecution` since it will not
+							// be definitely consumed from any existing weight credit if execution of the message
+							// is attempted.
+							T::get()
+						},
+						_ => T::get(),
+					}).sum();
+				T::get() + inner
+			}
+			_ => T::get(),
+		};
+		Ok(min)
+	}
+	fn deep(message: &mut Xcm<C>) -> Result<Weight, ()> {
+		let mut extra = 0;
+		match message {
+			Xcm::Transact { .. } => {}
+			Xcm::WithdrawAsset { effects, .. }
+			| Xcm::ReserveAssetDeposit { effects, .. }
+			| Xcm::TeleportAsset { effects, .. } => {
+				for effect in effects.iter_mut() {
+					match effect {
+						Order::BuyExecution { xcm, .. } => {
+							for message in xcm.iter_mut() {
+								extra += Self::shallow(message)? + Self::deep(message)?;
+							}
+						},
+						_ => {}
+					}
+				}
+			}
+			_ => {}
+		};
+		Ok(extra)
+	}
+}
+
+/// Simple fee calculator that requires payment in a single concrete fungible at a fixed rate.
+///
+/// The constant `Get` type parameter should be the concrete fungible ID and the amount of it required for
+/// one second of weight.
+pub struct FixedRateOfConcreteFungible<T>(Weight, PhantomData<T>);
+impl<T: Get<(MultiLocation, u128)>> WeightTrader for FixedRateOfConcreteFungible<T> {
+	fn new() -> Self { Self(0, PhantomData) }
+	fn buy_weight(&mut self, weight: Weight, payment: Assets) -> Result<Assets, ()> {
+		let (id, units_per_second) = T::get();
+		let amount = units_per_second * (weight as u128) / 1_000_000_000_000u128;
+		let required = MultiAsset::ConcreteFungible { amount, id };
+		let (used, _) = payment.less(required).map_err(|_| ())?;
+		self.0 = self.0.saturating_add(weight);
+		Ok(used)
+	}
+	fn refund_weight(&mut self, weight: Weight) -> MultiAsset {
+		let weight = weight.min(self.0);
+		self.0 -= weight;
+		let (id, units_per_second) = T::get();
+		let amount = units_per_second * (weight as u128) / 1_000_000_000_000u128;
+		let result = MultiAsset::ConcreteFungible { amount, id };
+		result
+	}
+}
@@ -27,18 +27,41 @@ pub enum AssetId {

 impl AssetId {
 	/// Prepend a MultiLocation to a concrete asset, giving it a new root location.
-	pub fn reanchor(&mut self, prepend: &MultiLocation) -> Result<(), ()> {
+	pub fn prepend_location(&mut self, prepend: &MultiLocation) -> Result<(), ()> {
 		if let AssetId::Concrete(ref mut l) = self {
 			l.prepend_with(prepend.clone()).map_err(|_| ())?;
 		}
 		Ok(())
 	}
+
+	/// Use the value of `self` along with an `amount to create the corresponding `MultiAsset` value for a
+	/// fungible asset.
+	pub fn into_fungible_multiasset(self, amount: u128) -> MultiAsset {
+		match self {
+			AssetId::Concrete(id) => MultiAsset::ConcreteFungible { id, amount },
+			AssetId::Abstract(id) => MultiAsset::AbstractFungible { id, amount },
+		}
+	}
+
+	/// Use the value of `self` along with an `instance to create the corresponding `MultiAsset` value for a
+	/// non-fungible asset.
+	pub fn into_non_fungible_multiasset(self, instance: AssetInstance) -> MultiAsset {
+		match self {
+			AssetId::Concrete(class) => MultiAsset::ConcreteNonFungible { class, instance },
+			AssetId::Abstract(class) => MultiAsset::AbstractNonFungible { class, instance },
+		}
+	}
 }

-/// List of concretely identified fungible and non-fungible assets.
-#[derive(Default, Clone, RuntimeDebug)]
+/// List of non-wildcard fungible and non-fungible assets.
+#[derive(Default, Clone, RuntimeDebug, Eq, PartialEq)]
 pub struct Assets {
+	/// The fungible assets.
 	pub fungible: BTreeMap<AssetId, u128>,
+
+	/// The non-fungible assets.
+	// OPTIMIZE: Consider BTreeMap<AssetId, BTreeSet<AssetInstance>>
+	//   or even BTreeMap<AssetId, SortedVec<AssetInstance>>
 	pub non_fungible: BTreeSet<(AssetId, AssetInstance)>,
 }

@@ -58,7 +81,18 @@ impl From<Assets> for Vec<MultiAsset> {
 	}
 }

+impl From<MultiAsset> for Assets {
+	fn from(asset: MultiAsset) -> Assets {
+		let mut result = Self::default();
+		result.saturating_subsume(asset);
+		result
+	}
+}
+
 impl Assets {
+	/// New value, containing no assets.
+	pub fn new() -> Self { Self::default() }
+
 	/// An iterator over the fungible assets.
 	pub fn fungible_assets_iter<'a>(&'a self) -> impl Iterator<Item=MultiAsset> + 'a {
 		self.fungible.iter()
@@ -99,8 +133,20 @@ impl Assets {
 		fungible.chain(non_fungible)
 	}

-	/// Modify `self` to include a `MultiAsset`, saturating if necessary.
-	/// Only works on concretely identified assets; wildcards will be swallowed without error.
+	/// Mutate `self` to contain all given `assets`, saturating if necessary.
+	///
+	/// Wildcards in `assets` are ignored.
+	pub fn saturating_subsume_all(&mut self, assets: Assets) {
+		// OPTIMIZE: Could be done with a much faster btree entry merge and only sum the entries with the
+		// same key.
+		for asset in assets.into_assets_iter() {
+			self.saturating_subsume(asset)
+		}
+	}
+
+	/// Mutate `self` to contain the given `asset`, saturating if necessary.
+	///
+	/// Wildcard values of `asset` do nothing.
 	pub fn saturating_subsume(&mut self, asset: MultiAsset) {
 		match asset {
 			MultiAsset::ConcreteFungible { id, amount } => {
@@ -119,6 +165,100 @@ impl Assets {
 		}
 	}

+	/// Consumes `self` and returns its original value excluding `asset` iff it contains at least `asset`.
+	///
+	/// Wildcard assets in `self` will result in an error.
+	///
+	/// `asset` may be a wildcard and are evaluated in the context of `self`.
+	///
+	/// Returns `Ok` with the `self` minus `asset` and the non-wildcard equivalence of `asset` taken if `self`
+	/// contains `asset`, and `Err` with `self` otherwise.
+	pub fn less(mut self, asset: MultiAsset) -> Result<(Self, Assets), Self> {
+		match self.try_take(asset) {
+			Ok(taken) => Ok((self, taken)),
+			Err(()) => Err(self),
+		}
+	}
+
+	/// Mutates `self` to its original value less `asset` and returns `true` iff it contains at least `asset`.
+	///
+	/// Wildcard assets in `self` will result in an error.
+	///
+	/// `asset` may be a wildcard and are evaluated in the context of `self`.
+	///
+	/// Returns `Ok` with the non-wildcard equivalence of `asset` taken and mutates `self` to its value minus
+	/// `asset` if `self` contains `asset`, and return `Err` otherwise.
+	pub fn try_take(&mut self, asset: MultiAsset) -> Result<Assets, ()> {
+		match asset {
+			MultiAsset::None => Ok(Assets::new()),
+			MultiAsset::ConcreteFungible { id, amount } => self.try_take_fungible(AssetId::Concrete(id), amount),
+			MultiAsset::AbstractFungible { id, amount } => self.try_take_fungible(AssetId::Abstract(id), amount),
+			MultiAsset::ConcreteNonFungible { class, instance} => self.try_take_non_fungible(AssetId::Concrete(class), instance),
+			MultiAsset::AbstractNonFungible { class, instance} => self.try_take_non_fungible(AssetId::Abstract(class), instance),
+			MultiAsset::AllAbstractFungible { id } => Ok(self.take_fungible(&AssetId::Abstract(id))),
+			MultiAsset::AllConcreteFungible { id } => Ok(self.take_fungible(&AssetId::Concrete(id))),
+			MultiAsset::AllAbstractNonFungible { class } => Ok(self.take_non_fungible(&AssetId::Abstract(class))),
+			MultiAsset::AllConcreteNonFungible { class } => Ok(self.take_non_fungible(&AssetId::Concrete(class))),
+			MultiAsset::AllFungible => {
+				let mut taken = Assets::new();
+				mem::swap(&mut self.fungible, &mut taken.fungible);
+				Ok(taken)
+			},
+			MultiAsset::AllNonFungible => {
+				let mut taken = Assets::new();
+				mem::swap(&mut self.non_fungible, &mut taken.non_fungible);
+				Ok(taken)
+			},
+			MultiAsset::All => Ok(self.swapped(Assets::new())),
+		}
+	}
+
+	pub fn try_take_fungible(&mut self, id: AssetId, amount: u128) -> Result<Assets, ()> {
+		self.try_remove_fungible(&id, amount)?;
+		Ok(id.into_fungible_multiasset(amount).into())
+	}
+
+	pub fn try_take_non_fungible(&mut self, id: AssetId, instance: AssetInstance) -> Result<Assets, ()> {
+		let asset_id_instance = (id, instance);
+		self.try_remove_non_fungible(&asset_id_instance)?;
+		let (asset_id, instance) = asset_id_instance;
+		Ok(asset_id.into_non_fungible_multiasset(instance).into())
+	}
+
+	pub fn take_fungible(&mut self, id: &AssetId) -> Assets {
+		let mut taken = Assets::new();
+		if let Some((id, amount)) = self.fungible.remove_entry(&id) {
+			taken.fungible.insert(id, amount);
+		}
+		taken
+	}
+
+	pub fn take_non_fungible(&mut self, id: &AssetId) -> Assets {
+		let mut taken = Assets::new();
+		let non_fungible = mem::replace(&mut self.non_fungible, Default::default());
+		non_fungible.into_iter().for_each(|(c, instance)| {
+			if &c == id {
+				taken.non_fungible.insert((c, instance));
+			} else {
+				self.non_fungible.insert((c, instance));
+			}
+		});
+		taken
+	}
+
+	pub fn try_remove_fungible(&mut self, id: &AssetId, amount: u128) -> Result<(), ()> {
+		let self_amount = self.fungible.get_mut(&id).ok_or(())?;
+		*self_amount = self_amount.checked_sub(amount).ok_or(())?;
+		Ok(())
+	}
+
+	pub fn try_remove_non_fungible(&mut self, class_instance: &(AssetId, AssetInstance)) -> Result<(), ()> {
+		match self.non_fungible.remove(class_instance) {
+			true => Ok(()),
+			false => Err(()),
+		}
+	}
+
 	/// Modify `self` to include a new fungible asset by `id` and `amount`,
 	/// saturating if necessary.
 	pub fn saturating_subsume_fungible(&mut self, id: AssetId, amount: u128) {
@@ -133,20 +273,20 @@ impl Assets {
 		self.non_fungible.insert((class, instance));
 	}

-	/// Alter any concretely identified assets according to the given `MultiLocation`.
+	/// Alter any concretely identified assets by prepending the given `MultiLocation`.
 	///
 	/// WARNING: For now we consider this infallible and swallow any errors. It is thus the caller's responsibility to
 	/// ensure that any internal asset IDs are able to be prepended without overflow.
-	pub fn reanchor(&mut self, prepend: &MultiLocation) {
+	pub fn prepend_location(&mut self, prepend: &MultiLocation) {
 		let mut fungible = Default::default();
 		mem::swap(&mut self.fungible, &mut fungible);
 		self.fungible = fungible.into_iter()
-			.map(|(mut id, amount)| { let _ = id.reanchor(prepend); (id, amount) })
+			.map(|(mut id, amount)| { let _ = id.prepend_location(prepend); (id, amount) })
 			.collect();
 		let mut non_fungible = Default::default();
 		mem::swap(&mut self.non_fungible, &mut non_fungible);
 		self.non_fungible = non_fungible.into_iter()
-			.map(|(mut class, inst)| { let _ = class.reanchor(prepend); (class, inst) })
+			.map(|(mut class, inst)| { let _ = class.prepend_location(prepend); (class, inst) })
 			.collect();
 	}

@@ -16,12 +16,16 @@

 use xcm::v0::SendXcm;
 use frame_support::dispatch::{Dispatchable, Parameter};
-use crate::traits::{TransactAsset, ConvertOrigin, FilterAssetLocation, InvertLocation};
+use frame_support::weights::{PostDispatchInfo, GetDispatchInfo};
+use crate::traits::{
+	TransactAsset, ConvertOrigin, FilterAssetLocation, InvertLocation, ShouldExecute, WeightTrader, WeightBounds,
+	OnResponse,
+};

 /// The trait to parametrize the `XcmExecutor`.
 pub trait Config {
 	/// The outer call dispatch type.
-	type Call: Parameter + Dispatchable;
+	type Call: Parameter + Dispatchable<PostInfo=PostDispatchInfo> + GetDispatchInfo;

 	/// How to send an onward XCM message.
 	type XcmSender: SendXcm;
@@ -40,4 +44,16 @@ pub trait Config {

 	/// Means of inverting a location.
 	type LocationInverter: InvertLocation;
+
+	/// Whether we should execute the given XCM at all.
+	type Barrier: ShouldExecute;
+
+	/// The means of determining an XCM message's weight.
+	type Weigher: WeightBounds<Self::Call>;
+
+	/// The means of purchasing weight credit for XCM execution.
+	type Trader: WeightTrader;
+
+	/// What to do when a response of a query is found.
+	type ResponseHandler: OnResponse;
 }
@@ -16,120 +16,197 @@

 #![cfg_attr(not(feature = "std"), no_std)]

-use sp_std::{prelude::*, marker::PhantomData, convert::TryInto};
-use frame_support::{ensure, dispatch::Dispatchable};
-use parity_scale_codec::Decode;
+use sp_std::{prelude::*, marker::PhantomData};
+use frame_support::{
+	ensure, weights::GetDispatchInfo,
+	dispatch::{Weight, Dispatchable}
+};
 use xcm::v0::{
-	Xcm, Order, ExecuteXcm, SendXcm, Error as XcmError, Result as XcmResult,
-	MultiLocation, MultiAsset, Junction,
+	ExecuteXcm, SendXcm, Error as XcmError, Outcome,
+	MultiLocation, MultiAsset, Xcm, Order, Response,
 };

 pub mod traits;
-mod assets;
-mod config;
+use traits::{
+	TransactAsset, ConvertOrigin, FilterAssetLocation, InvertLocation, WeightBounds, WeightTrader, ShouldExecute,
+	OnResponse
+};

-use traits::{TransactAsset, ConvertOrigin, FilterAssetLocation, InvertLocation};
+mod assets;
 pub use assets::{Assets, AssetId};
+mod config;
 pub use config::Config;

 pub struct XcmExecutor<Config>(PhantomData<Config>);

-impl<Config: config::Config> ExecuteXcm for XcmExecutor<Config> {
-	fn execute_xcm(origin: MultiLocation, msg: Xcm) -> XcmResult {
-		let (mut holding, effects) = match (origin.clone(), msg) {
-			(origin, Xcm::RelayedFrom { superorigin, inner }) => {
-				// We ensure that it doesn't contain any `Parent` Junctions which would imply a privilege escalation.
-				let mut new_origin = origin;
-				for j in superorigin.into_iter() {
-					ensure!(j.is_sub_consensus(), XcmError::EscalationOfPrivilege);
-					new_origin.push(j).map_err(|_| XcmError::MultiLocationFull)?;
-				}
-				return Self::execute_xcm(
-					new_origin,
-					(*inner).try_into().map_err(|_| XcmError::UnhandledXcmVersion)?
-				)
-			}
-			(origin, Xcm::WithdrawAsset { assets, effects }) => {
-				// Take `assets` from the origin account (on-chain) and place in holding.
-				let mut holding = Assets::default();
-				for asset in assets {
-					let withdrawn = Config::AssetTransactor::withdraw_asset(&asset, &origin)?;
-					holding.saturating_subsume(withdrawn);
-				}
-				(holding, effects)
-			}
-			(origin, Xcm::ReserveAssetDeposit { assets, effects }) => {
-				// check whether we trust origin to be our reserve location for this asset.
-				if assets.iter().all(|asset| Config::IsReserve::filter_asset_location(asset, &origin)) {
-					// We only trust the origin to send us assets that they identify as their
-					// sovereign assets.
-					(Assets::from(assets), effects)
-				} else {
-					Err(XcmError::UntrustedReserveLocation)?
-				}
-			}
-			(origin, Xcm::TeleportAsset { assets, effects }) => {
-				// check whether we trust origin to teleport this asset to us via config trait.
-				// TODO: should de-wildcard `assets` before passing in.
-				log::debug!(target: "runtime::xcm-executor", "Teleport from {:?}", origin);
-				if assets.iter().all(|asset| Config::IsTeleporter::filter_asset_location(asset, &origin)) {
-					// We only trust the origin to send us assets that they identify as their
-					// sovereign assets.
-					(Assets::from(assets), effects)
-				} else {
-					Err(XcmError::UntrustedTeleportLocation)?
-				}
-			}
-			(origin, Xcm::Transact { origin_type, call }) => {
-				// We assume that the Relay-chain is allowed to use transact on this parachain.
-
-				// TODO: Weight fees should be paid.
-
-				// TODO: allow this to be configurable in the trait.
-				// TODO: allow the trait to issue filters for the relay-chain
-				let message_call = Config::Call::decode(&mut &call[..]).map_err(|_| XcmError::FailedToDecode)?;
-				let dispatch_origin = Config::OriginConverter::convert_origin(origin, origin_type)
-					.map_err(|_| XcmError::BadOrigin)?;
-				let _ok = message_call.dispatch(dispatch_origin).is_ok();
-				// Not much to do with the result as it is. It's up to the parachain to ensure that the
-				// message makes sense.
-				return Ok(());
-			}
-			(origin, Xcm::RelayTo { dest: MultiLocation::X1(Junction::Parachain { id }), inner }) => {
-				let msg = Xcm::RelayedFrom { superorigin: origin, inner }.into();
-				return Config::XcmSender::send_xcm(Junction::Parachain { id }.into(), msg)
-			},
-			_ => Err(XcmError::UnhandledXcmMessage)?,	// Unhandled XCM message.
+impl<Config: config::Config> ExecuteXcm<Config::Call> for XcmExecutor<Config> {
+	fn execute_xcm(origin: MultiLocation, message: Xcm<Config::Call>, weight_limit: Weight) -> Outcome {
+		// TODO: #2841 #HARDENXCM We should identify recursive bombs here and bail.
+		let mut message = Xcm::<Config::Call>::from(message);
+		let shallow_weight = match Config::Weigher::shallow(&mut message) {
+			Ok(x) => x,
+			Err(()) => return Outcome::Error(XcmError::WeightNotComputable),
 		};
-
-		// TODO: stuff that should happen after holding is populated but before effects,
-		//   including depositing fees for effects from holding account.
-
-		for effect in effects.into_iter() {
-			let _ = Self::execute_effects(&origin, &mut holding, effect)?;
+		let deep_weight = match Config::Weigher::deep(&mut message) {
+			Ok(x) => x,
+			Err(()) => return Outcome::Error(XcmError::WeightNotComputable),
+		};
+		let maximum_weight = match shallow_weight.checked_add(deep_weight) {
+			Some(x) => x,
+			None => return Outcome::Error(XcmError::WeightLimitReached),
+		};
+		if maximum_weight > weight_limit {
+			return Outcome::Error(XcmError::WeightLimitReached);
+		}
+		let mut trader = Config::Trader::new();
+		match Self::do_execute_xcm(origin, true, message, &mut 0, Some(shallow_weight), &mut trader) {
+			Ok(surplus) => Outcome::Complete(maximum_weight.saturating_sub(surplus)),
+			// TODO: #2841 #REALWEIGHT We can do better than returning `maximum_weight` here, and we should otherwise
+			//  we'll needlessly be disregarding block execution time.
+			Err(e) => Outcome::Incomplete(maximum_weight, e),
 		}
-
-		// TODO: stuff that should happen after effects including refunding unused fees.
-
-		Ok(())
 	}
 }

 impl<Config: config::Config> XcmExecutor<Config> {
 	fn reanchored(mut assets: Assets, dest: &MultiLocation) -> Vec<MultiAsset> {
 		let inv_dest = Config::LocationInverter::invert_location(&dest);
-		assets.reanchor(&inv_dest);
+		assets.prepend_location(&inv_dest);
 		assets.into_assets_iter().collect::<Vec<_>>()
 	}

-	fn execute_effects(_origin: &MultiLocation, holding: &mut Assets, effect: Order) -> XcmResult {
+	/// Execute the XCM and return any unexpected and unknowable surplus weight.
+	fn do_execute_xcm(
+		origin: MultiLocation,
+		top_level: bool,
+		mut message: Xcm<Config::Call>,
+		weight_credit: &mut Weight,
+		maybe_shallow_weight: Option<Weight>,
+		trader: &mut Config::Trader,
+	) -> Result<Weight, XcmError> {
+		// This is the weight of everything that cannot be paid for. This basically means all computation
+		// except any XCM which is behind an Order::BuyExecution.
+		let shallow_weight = maybe_shallow_weight
+			.or_else(|| Config::Weigher::shallow(&mut message).ok())
+			.ok_or(XcmError::WeightNotComputable)?;
+
+		Config::Barrier::should_execute(&origin, top_level, &message, shallow_weight, weight_credit)
+			.map_err(|()| XcmError::Barrier)?;
+
+		// The surplus weight, defined as the amount by which `shallow_weight` plus all nested
+		// `shallow_weight` values (ensuring no double-counting) is an overestimate of the actual weight
+		// consumed.
+		let mut total_surplus: Weight = 0;
+
+		let maybe_holding_effects = match (origin.clone(), message) {
+			(origin, Xcm::WithdrawAsset { assets, effects }) => {
+				// Take `assets` from the origin account (on-chain) and place in holding.
+				let mut holding = Assets::default();
+				for asset in assets {
+					ensure!(!asset.is_wildcard(), XcmError::Wildcard);
+					let withdrawn = Config::AssetTransactor::withdraw_asset(&asset, &origin)?;
+					holding.saturating_subsume_all(withdrawn);
+				}
+				Some((holding, effects))
+			}
+			(origin, Xcm::ReserveAssetDeposit { assets, effects }) => {
+				// check whether we trust origin to be our reserve location for this asset.
+				for asset in assets.iter() {
+					ensure!(!asset.is_wildcard(), XcmError::Wildcard);
+					// We only trust the origin to send us assets that they identify as their
+					// sovereign assets.
+					ensure!(Config::IsReserve::filter_asset_location(asset, &origin), XcmError::UntrustedReserveLocation);
+				}
+				Some((Assets::from(assets), effects))
+			}
+			(origin, Xcm::TransferAsset { assets, dest }) => {
+				// Take `assets` from the origin account (on-chain) and place into dest account.
+				for asset in assets {
+					ensure!(!asset.is_wildcard(), XcmError::Wildcard);
+					Config::AssetTransactor::teleport_asset(&asset, &origin, &dest)?;
+				}
+				None
+			}
+			(origin, Xcm::TransferReserveAsset { mut assets, dest, effects }) => {
+				// Take `assets` from the origin account (on-chain) and place into dest account.
+				let inv_dest = Config::LocationInverter::invert_location(&dest);
+				for asset in assets.iter_mut() {
+					ensure!(!asset.is_wildcard(), XcmError::Wildcard);
+					Config::AssetTransactor::teleport_asset(&asset, &origin, &dest)?;
+					asset.reanchor(&inv_dest)?;
+				}
+				Config::XcmSender::send_xcm(dest, Xcm::ReserveAssetDeposit { assets, effects })?;
+				None
+			}
+			(origin, Xcm::TeleportAsset { assets, effects }) => {
+				// check whether we trust origin to teleport this asset to us via config trait.
+				for asset in assets.iter() {
+					ensure!(!asset.is_wildcard(), XcmError::Wildcard);
+					// We only trust the origin to send us assets that they identify as their
+					// sovereign assets.
+					ensure!(Config::IsTeleporter::filter_asset_location(asset, &origin), XcmError::UntrustedTeleportLocation);
+				}
+				Some((Assets::from(assets), effects))
+			}
+			(origin, Xcm::Transact { origin_type, require_weight_at_most,  mut call }) => {
+				// We assume that the Relay-chain is allowed to use transact on this parachain.
+
+				// TODO: #2841 #TRANSACTFILTER allow the trait to issue filters for the relay-chain
+				let message_call = call.take_decoded().map_err(|_| XcmError::FailedToDecode)?;
+				let dispatch_origin = Config::OriginConverter::convert_origin(origin, origin_type)
+					.map_err(|_| XcmError::BadOrigin)?;
+				let weight = message_call.get_dispatch_info().weight;
+				ensure!(weight <= require_weight_at_most, XcmError::TooMuchWeightRequired);
+				let actual_weight = match message_call.dispatch(dispatch_origin) {
+					Ok(post_info) => post_info.actual_weight,
+					Err(error_and_info) => {
+						// Not much to do with the result as it is. It's up to the parachain to ensure that the
+						// message makes sense.
+						error_and_info.post_info.actual_weight
+					}
+				}.unwrap_or(weight);
+				let surplus = weight.saturating_sub(actual_weight);
+				// Credit any surplus weight that we bought. This should be safe since it's work we
+				// didn't realise that we didn't have to do.
+				// It works because we assume that the `Config::Weigher` will always count the `call`'s
+				// `get_dispatch_info` weight into its `shallow` estimate.
+				*weight_credit = weight_credit.saturating_add(surplus);
+				// Do the same for the total surplus, which is reported to the caller and eventually makes its way
+				// back up the stack to be subtracted from the deep-weight.
+				total_surplus = total_surplus.saturating_add(surplus);
+				// Return the overestimated amount so we can adjust our expectations on how much this entire
+				// execution has taken.
+				None
+			}
+			(origin, Xcm::QueryResponse { query_id, response }) => {
+				Config::ResponseHandler::on_response(origin, query_id, response);
+				None
+			}
+			_ => Err(XcmError::UnhandledXcmMessage)?,	// Unhandled XCM message.
+		};
+
+		if let Some((mut holding, effects)) = maybe_holding_effects {
+			for effect in effects.into_iter() {
+				total_surplus += Self::execute_effects(&origin, &mut holding, effect, trader)?;
+			}
+		}
+
+		Ok(total_surplus)
+	}
+
+	fn execute_effects(
+		origin: &MultiLocation,
+		holding: &mut Assets,
+		effect: Order<Config::Call>,
+		trader: &mut Config::Trader,
+	) -> Result<Weight, XcmError> {
+		let mut total_surplus = 0;
 		match effect {
 			Order::DepositAsset { assets, dest } => {
 				let deposited = holding.saturating_take(assets);
 				for asset in deposited.into_assets_iter() {
 					Config::AssetTransactor::deposit_asset(&asset, &dest)?;
 				}
-				Ok(())
 			},
 			Order::DepositReserveAsset { assets, dest, effects } => {
 				let deposited = holding.saturating_take(assets);
@@ -137,21 +214,39 @@ impl<Config: config::Config> XcmExecutor<Config> {
 					Config::AssetTransactor::deposit_asset(&asset, &dest)?;
 				}
 				let assets = Self::reanchored(deposited, &dest);
-				Config::XcmSender::send_xcm(dest, Xcm::ReserveAssetDeposit { assets, effects })
+				Config::XcmSender::send_xcm(dest, Xcm::ReserveAssetDeposit { assets, effects })?;
 			},
 			Order::InitiateReserveWithdraw { assets, reserve, effects} => {
 				let assets = Self::reanchored(holding.saturating_take(assets), &reserve);
-				Config::XcmSender::send_xcm(reserve, Xcm::WithdrawAsset { assets, effects })
+				Config::XcmSender::send_xcm(reserve, Xcm::WithdrawAsset { assets, effects })?;
 			}
 			Order::InitiateTeleport { assets, dest, effects} => {
 				let assets = Self::reanchored(holding.saturating_take(assets), &dest);
-				Config::XcmSender::send_xcm(dest, Xcm::TeleportAsset { assets, effects })
+				Config::XcmSender::send_xcm(dest, Xcm::TeleportAsset { assets, effects })?;
 			}
 			Order::QueryHolding { query_id, dest, assets } => {
 				let assets = Self::reanchored(holding.min(assets.iter()), &dest);
-				Config::XcmSender::send_xcm(dest, Xcm::Balances { query_id, assets })
+				Config::XcmSender::send_xcm(dest, Xcm::QueryResponse { query_id, response: Response::Assets(assets) })?;
 			}
-			_ => Err(XcmError::UnhandledEffect)?,
+			Order::BuyExecution { fees, weight, debt, halt_on_error, xcm } => {
+				// pay for `weight` using up to `fees` of the holding account.
+				let purchasing_weight = Weight::from(weight.checked_add(debt).ok_or(XcmError::Overflow)?);
+				let max_fee = holding.try_take(fees).map_err(|()| XcmError::NotHoldingFees)?;
+				let unspent = trader.buy_weight(purchasing_weight, max_fee)?;
+				holding.saturating_subsume_all(unspent);
+
+				let mut remaining_weight = weight;
+				for message in xcm.into_iter() {
+					match Self::do_execute_xcm(origin.clone(), false, message, &mut remaining_weight, None, trader) {
+						Err(e) if halt_on_error => return Err(e),
+						Err(_) => {}
+						Ok(surplus) => { total_surplus += surplus }
+					}
+				}
+				holding.saturating_subsume(trader.refund_weight(remaining_weight));
+			}
+			_ => return Err(XcmError::UnhandledEffect)?,
 		}
+		Ok(total_surplus)
 	}
 }
@@ -1,170 +0,0 @@
-// Copyright 2020 Parity Technologies (UK) Ltd.
-// This file is part of Polkadot.
-
-// Polkadot 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.
-
-// Polkadot 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/>.
-
-use sp_std::{result::Result, marker::PhantomData, convert::TryFrom};
-use sp_runtime::traits::CheckedConversion;
-use xcm::v0::{Error as XcmError, Result as XcmResult, MultiAsset, MultiLocation, OriginKind};
-use frame_support::traits::Get;
-
-pub trait FilterAssetLocation {
-	/// A filter to distinguish between asset/location pairs.
-	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool;
-}
-
-#[impl_trait_for_tuples::impl_for_tuples(30)]
-impl FilterAssetLocation for Tuple {
-	fn filter_asset_location(what: &MultiAsset, origin: &MultiLocation) -> bool {
-		for_tuples!( #(
-			if Tuple::filter_asset_location(what, origin) { return true }
-		)* );
-		false
-	}
-}
-
-pub struct NativeAsset;
-impl FilterAssetLocation for NativeAsset {
-	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool {
-		matches!(asset, MultiAsset::ConcreteFungible { ref id, .. } if id == origin)
-	}
-}
-
-
-pub struct Case<T>(PhantomData<T>);
-impl<T: Get<(MultiAsset, MultiLocation)>> FilterAssetLocation for Case<T> {
-	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool {
-		let (a, o) = T::get();
-		&a == asset && &o == origin
-	}
-}
-
-/// Facility for asset transacting.
-///
-/// This should work with as many asset/location combinations as possible. Locations to support may include non-
-/// account locations such as a `MultiLocation::X1(Junction::Parachain)`. Different chains may handle them in
-/// different ways.
-pub trait TransactAsset {
-	/// Deposit the `what` asset into the account of `who`.
-	///
-	/// Implementations should return `XcmError::FailedToTransactAsset` if deposit failed.
-	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> XcmResult;
-
-	/// Withdraw the given asset from the consensus system. Return the actual asset withdrawn. In
-	/// the case of `what` being a wildcard, this may be something more specific.
-	///
-	/// Implementations should return `XcmError::FailedToTransactAsset` if withdraw failed.
-	fn withdraw_asset(what: &MultiAsset, who: &MultiLocation) -> Result<MultiAsset, XcmError>;
-
-	/// Move an `asset` `from` one location in `to` another location.
-	///
-	/// Returns `XcmError::FailedToTransactAsset` if transfer failed.
-	fn transfer_asset(asset: &MultiAsset, from: &MultiLocation, to: &MultiLocation) -> Result<MultiAsset, XcmError> {
-		let withdrawn = Self::withdraw_asset(asset, from)?;
-		Self::deposit_asset(&withdrawn, to)?;
-		Ok(withdrawn)
-	}
-}
-
-#[impl_trait_for_tuples::impl_for_tuples(30)]
-impl TransactAsset for Tuple {
-	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> XcmResult {
-		for_tuples!( #(
-			match Tuple::deposit_asset(what, who) { o @ Ok(_) => return o, _ => () }
-		)* );
-		Err(XcmError::Unimplemented)
-	}
-	fn withdraw_asset(what: &MultiAsset, who: &MultiLocation) -> Result<MultiAsset, XcmError> {
-		for_tuples!( #(
-			match Tuple::withdraw_asset(what, who) { o @ Ok(_) => return o, _ => () }
-		)* );
-		Err(XcmError::Unimplemented)
-	}
-}
-
-
-pub trait MatchesFungible<Balance> {
-	fn matches_fungible(a: &MultiAsset) -> Option<Balance>;
-}
-pub struct IsConcrete<T>(PhantomData<T>);
-impl<T: Get<MultiLocation>, B: TryFrom<u128>> MatchesFungible<B> for IsConcrete<T> {
-	fn matches_fungible(a: &MultiAsset) -> Option<B> {
-		match a {
-			MultiAsset::ConcreteFungible { id, amount } if id == &T::get() =>
-				CheckedConversion::checked_from(*amount),
-			_ => None,
-		}
-	}
-}
-pub struct IsAbstract<T>(PhantomData<T>);
-impl<T: Get<&'static [u8]>, B: TryFrom<u128>> MatchesFungible<B> for IsAbstract<T> {
-	fn matches_fungible(a: &MultiAsset) -> Option<B> {
-		match a {
-			MultiAsset::AbstractFungible { id, amount } if &id[..] == T::get() =>
-				CheckedConversion::checked_from(*amount),
-			_ => None,
-		}
-	}
-}
-// TODO: impl for tuples
-impl<B: From<u128>, X: MatchesFungible<B>, Y: MatchesFungible<B>> MatchesFungible<B> for (X, Y) {
-	fn matches_fungible(a: &MultiAsset) -> Option<B> {
-		X::matches_fungible(a).or_else(|| Y::matches_fungible(a))
-	}
-}
-
-// TODO: change to use Convert trait.
-/// Attempt to convert a location into some value of type `T`, or vice-versa.
-pub trait LocationConversion<T> {
-	/// Convert `location` into `Some` value of `T`, or `None` if not possible.
-	// TODO: consider returning Result<T, ()> instead.
-	fn from_location(location: &MultiLocation) -> Option<T>;
-	/// Convert some value `value` into a `location`, `Err`oring with the original `value` if not possible.
-	// TODO: consider renaming `into_location`
-	fn try_into_location(value: T) -> Result<MultiLocation, T>;
-}
-
-#[impl_trait_for_tuples::impl_for_tuples(30)]
-impl<AccountId> LocationConversion<AccountId> for Tuple {
-	fn from_location(location: &MultiLocation) -> Option<AccountId> {
-		for_tuples!( #(
-			if let Some(result) = Tuple::from_location(location) { return Some(result) }
-		)* );
-		None
-	}
-	fn try_into_location(who: AccountId) -> Result<MultiLocation, AccountId> {
-		for_tuples!( #(
-			let who = match Tuple::try_into_location(who) { Err(w) => w, r => return r };
-		)* );
-		Err(who)
-	}
-}
-
-pub trait ConvertOrigin<Origin> {
-	fn convert_origin(origin: MultiLocation, kind: OriginKind) -> Result<Origin, MultiLocation>;
-}
-
-#[impl_trait_for_tuples::impl_for_tuples(30)]
-impl<O> ConvertOrigin<O> for Tuple {
-	fn convert_origin(origin: MultiLocation, kind: OriginKind) -> Result<O, MultiLocation> {
-		for_tuples!( #(
-			let origin = match Tuple::convert_origin(origin, kind) { Err(o) => o, r => return r };
-		)* );
-		Err(origin)
-	}
-}
-
-pub trait InvertLocation {
-	fn invert_location(l: &MultiLocation) -> MultiLocation;
-}
@@ -0,0 +1,136 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::{prelude::*, result::Result, borrow::Borrow, convert::TryFrom};
+use parity_scale_codec::{Encode, Decode};
+use xcm::v0::{MultiLocation, OriginKind};
+
+/// Generic third-party conversion trait. Use this when you don't want to force the user to use default
+/// impls of `From` and `Into` for the types you wish to convert between.
+///
+/// One of `convert`/`convert_ref` and `reverse`/`reverse_ref` MUST be implemented. If possible, implement
+/// `convert_ref`, since this will never result in a clone. Use `convert` when you definitely need to consume
+/// the source value.
+pub trait Convert<A: Clone, B: Clone> {
+	/// Convert from `value` (of type `A`) into an equivalent value of type `B`, `Err` if not possible.
+	fn convert(value: A) -> Result<B, A> { Self::convert_ref(&value).map_err(|_| value) }
+	fn convert_ref(value: impl Borrow<A>) -> Result<B, ()> {
+		Self::convert(value.borrow().clone()).map_err(|_| ())
+	}
+	/// Convert from `value` (of type `B`) into an equivalent value of type `A`, `Err` if not possible.
+	fn reverse(value: B) -> Result<A, B> { Self::reverse_ref(&value).map_err(|_| value) }
+	fn reverse_ref(value: impl Borrow<B>) -> Result<A, ()> {
+		Self::reverse(value.borrow().clone()).map_err(|_| ())
+	}
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl<A: Clone, B: Clone> Convert<A, B> for Tuple {
+	fn convert(value: A) -> Result<B, A> {
+		for_tuples!( #(
+			let value = match Tuple::convert(value) {
+				Ok(result) => return Ok(result),
+				Err(v) => v,
+			};
+		)* );
+		Err(value)
+	}
+	fn reverse(value: B) -> Result<A, B> {
+		for_tuples!( #(
+			let value = match Tuple::reverse(value) {
+				Ok(result) => return Ok(result),
+				Err(v) => v,
+			};
+		)* );
+		Err(value)
+	}
+	fn convert_ref(value: impl Borrow<A>) -> Result<B, ()> {
+		let value = value.borrow();
+		for_tuples!( #(
+			match Tuple::convert_ref(value) {
+				Ok(result) => return Ok(result),
+				Err(_) => (),
+			}
+		)* );
+		Err(())
+	}
+	fn reverse_ref(value: impl Borrow<B>) -> Result<A, ()> {
+		let value = value.borrow();
+		for_tuples!( #(
+			match Tuple::reverse_ref(value.clone()) {
+				Ok(result) => return Ok(result),
+				Err(_) => (),
+			}
+		)* );
+		Err(())
+	}
+}
+
+/// Simple pass-through which implements `BytesConversion` while not doing any conversion.
+pub struct Identity;
+impl<T: Clone> Convert<T, T> for Identity {
+	fn convert(value: T) -> Result<T, T> { Ok(value) }
+	fn reverse(value: T) -> Result<T, T> { Ok(value) }
+}
+
+/// Implementation of `Convert` trait using `TryFrom`.
+pub struct JustTry;
+impl<Source: TryFrom<Dest> + Clone, Dest: TryFrom<Source> + Clone> Convert<Source, Dest> for JustTry {
+	fn convert(value: Source) -> Result<Dest, Source> {
+		Dest::try_from(value.clone()).map_err(|_| value)
+	}
+	fn reverse(value: Dest) -> Result<Source, Dest> {
+		Source::try_from(value.clone()).map_err(|_| value)
+	}
+}
+
+/// Implementation of `Convert<_, Vec<u8>>` using the parity scale codec.
+pub struct Encoded;
+impl<T: Clone + Encode + Decode> Convert<T, Vec<u8>> for Encoded {
+	fn convert_ref(value: impl Borrow<T>) -> Result<Vec<u8>, ()> { Ok(value.borrow().encode()) }
+	fn reverse_ref(bytes: impl Borrow<Vec<u8>>) -> Result<T, ()> {
+		T::decode(&mut &bytes.borrow()[..]).map_err(|_| ())
+	}
+}
+
+/// Implementation of `Convert<Vec<u8>, _>` using the parity scale codec.
+pub struct Decoded;
+impl<T: Clone + Encode + Decode> Convert<Vec<u8>, T> for Decoded {
+	fn convert_ref(bytes: impl Borrow<Vec<u8>>) -> Result<T, ()> {
+		T::decode(&mut &bytes.borrow()[..]).map_err(|_| ())
+	}
+	fn reverse_ref(value: impl Borrow<T>) -> Result<Vec<u8>, ()> { Ok(value.borrow().encode()) }
+}
+
+pub trait ConvertOrigin<Origin> {
+	fn convert_origin(origin: MultiLocation, kind: OriginKind) -> Result<Origin, MultiLocation>;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl<O> ConvertOrigin<O> for Tuple {
+	fn convert_origin(origin: MultiLocation, kind: OriginKind) -> Result<O, MultiLocation> {
+		for_tuples!( #(
+			let origin = match Tuple::convert_origin(origin, kind) { Err(o) => o, r => return r };
+		)* );
+		Err(origin)
+	}
+}
+
+/// Means of inverting a location: given a location which describes a `target` interpreted from the `source`, this
+/// will provide the corresponding location which describes the `source`
+pub trait InvertLocation {
+	fn invert_location(l: &MultiLocation) -> MultiLocation;
+}
@@ -0,0 +1,32 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use xcm::v0::{MultiAsset, MultiLocation};
+
+pub trait FilterAssetLocation {
+	/// A filter to distinguish between asset/location pairs.
+	fn filter_asset_location(asset: &MultiAsset, origin: &MultiLocation) -> bool;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl FilterAssetLocation for Tuple {
+	fn filter_asset_location(what: &MultiAsset, origin: &MultiLocation) -> bool {
+		for_tuples!( #(
+			if Tuple::filter_asset_location(what, origin) { return true }
+		)* );
+		false
+	}
+}
@@ -0,0 +1,31 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use xcm::v0::MultiAsset;
+
+pub trait MatchesFungible<Balance> {
+	fn matches_fungible(a: &MultiAsset) -> Option<Balance>;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl<Balance> MatchesFungible<Balance> for Tuple {
+	fn matches_fungible(a: &MultiAsset) -> Option<Balance> {
+		for_tuples!( #(
+			match Tuple::matches_fungible(a) { o @ Some(_) => return o, _ => () }
+		)* );
+		None
+	}
+}
@@ -0,0 +1,32 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+//! Various traits used in configuring the executor.
+
+mod conversion;
+pub use conversion::{InvertLocation, ConvertOrigin, Convert, JustTry, Identity, Encoded, Decoded};
+mod filter_asset_location;
+pub use filter_asset_location::{FilterAssetLocation};
+mod matches_fungible;
+pub use matches_fungible::{MatchesFungible};
+mod on_response;
+pub use on_response::OnResponse;
+mod should_execute;
+pub use should_execute::ShouldExecute;
+mod transact_asset;
+pub use transact_asset::TransactAsset;
+mod weight;
+pub use weight::{WeightBounds, WeightTrader};
@@ -0,0 +1,27 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use xcm::v0::{Response, MultiLocation};
+use frame_support::weights::Weight;
+
+pub trait OnResponse {
+	fn expecting_response(origin: &MultiLocation, query_id: u64) -> bool;
+	fn on_response(origin: MultiLocation, query_id: u64, response: Response) -> Weight;
+}
+impl OnResponse for () {
+	fn expecting_response(_origin: &MultiLocation, _query_id: u64) -> bool { false }
+	fn on_response(_origin: MultiLocation, _query_id: u64, _response: Response) -> Weight { 0 }
+}
@@ -0,0 +1,60 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::result::Result;
+use xcm::v0::{Xcm, MultiLocation};
+use frame_support::weights::Weight;
+
+/// Trait to determine whether the execution engine should actually execute a given XCM.
+pub trait ShouldExecute {
+	/// Returns `true` if the given `message` may be executed.
+	///
+	/// - `origin`: The origin (sender) of the message.
+	/// - `top_level`: `true`` indicates the initial XCM coming from the `origin`, `false` indicates an embedded
+	///   XCM executed internally as part of another message or an `Order`.
+	/// - `message`: The message itself.
+	/// - `shallow_weight`: The weight of the non-negotiable execution of the message. This does not include any
+	///   embedded XCMs sat behind mechanisms like `BuyExecution` which would need to answer for their own weight.
+	/// - `weight_credit`: The pre-established amount of weight that the system has determined this message
+	///   may utilise in its execution. Typically non-zero only because of prior fee payment, but could
+	///   in principle be due to other factors.
+	fn should_execute<Call>(
+		origin: &MultiLocation,
+		top_level: bool,
+		message: &Xcm<Call>,
+		shallow_weight: Weight,
+		weight_credit: &mut Weight,
+	) -> Result<(), ()>;
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl ShouldExecute for Tuple {
+	fn should_execute<Call>(
+		origin: &MultiLocation,
+		top_level: bool,
+		message: &Xcm<Call>,
+		shallow_weight: Weight,
+		weight_credit: &mut Weight,
+	) -> Result<(), ()> {
+		for_tuples!( #(
+			match Tuple::should_execute(origin, top_level, message, shallow_weight, weight_credit) {
+				o @ Ok(()) => return o,
+				_ => (),
+			}
+		)* );
+		Err(())
+	}
+}
@@ -0,0 +1,86 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::result::Result;
+use xcm::v0::{Error as XcmError, Result as XcmResult, MultiAsset, MultiLocation};
+use crate::Assets;
+
+/// Facility for asset transacting.
+///
+/// This should work with as many asset/location combinations as possible. Locations to support may include non-
+/// account locations such as a `MultiLocation::X1(Junction::Parachain)`. Different chains may handle them in
+/// different ways.
+pub trait TransactAsset {
+	/// Deposit the `what` asset into the account of `who`.
+	///
+	/// Implementations should return `XcmError::FailedToTransactAsset` if deposit failed.
+	fn deposit_asset(_what: &MultiAsset, _who: &MultiLocation) -> XcmResult {
+		Err(XcmError::Unimplemented)
+	}
+
+	/// Withdraw the given asset from the consensus system. Return the actual asset(s) withdrawn. In
+	/// the case of `what` being a wildcard, this may be something more specific.
+	///
+	/// Implementations should return `XcmError::FailedToTransactAsset` if withdraw failed.
+	fn withdraw_asset(_what: &MultiAsset, _who: &MultiLocation) -> Result<Assets, XcmError> {
+		Err(XcmError::Unimplemented)
+	}
+
+	/// Move an `asset` `from` one location in `to` another location.
+	///
+	/// Returns `XcmError::FailedToTransactAsset` if transfer failed.
+	fn transfer_asset(_asset: &MultiAsset, _from: &MultiLocation, _to: &MultiLocation) -> Result<Assets, XcmError> {
+		Err(XcmError::Unimplemented)
+	}
+
+	/// Move an `asset` `from` one location in `to` another location.
+	///
+	/// Attempts to use `transfer_asset` and if not available then falls back to using a two-part withdraw/deposit.
+	fn teleport_asset(asset: &MultiAsset, from: &MultiLocation, to: &MultiLocation) -> Result<Assets, XcmError> {
+		match Self::transfer_asset(asset, from, to) {
+			Err(XcmError::Unimplemented) => {
+				let assets = Self::withdraw_asset(asset, from)?;
+				// Not a very forgiving attitude; once we implement roll-backs then it'll be nicer.
+				Self::deposit_asset(asset, to)?;
+				Ok(assets)
+			}
+			result => result
+		}
+	}
+}
+
+#[impl_trait_for_tuples::impl_for_tuples(30)]
+impl TransactAsset for Tuple {
+	fn deposit_asset(what: &MultiAsset, who: &MultiLocation) -> XcmResult {
+		for_tuples!( #(
+			match Tuple::deposit_asset(what, who) { o @ Ok(_) => return o, _ => () }
+		)* );
+		Err(XcmError::Unimplemented)
+	}
+	fn withdraw_asset(what: &MultiAsset, who: &MultiLocation) -> Result<Assets, XcmError> {
+		for_tuples!( #(
+			match Tuple::withdraw_asset(what, who) { o @ Ok(_) => return o, _ => () }
+		)* );
+		Err(XcmError::Unimplemented)
+	}
+	fn transfer_asset(what: &MultiAsset, from: &MultiLocation, to: &MultiLocation) -> Result<Assets, XcmError> {
+		for_tuples!( #(
+			match Tuple::transfer_asset(what, from, to) { o @ Ok(_) => return o, _ => () }
+		)* );
+		Err(XcmError::Unimplemented)
+	}
+}
+
@@ -0,0 +1,63 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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/>.
+
+use sp_std::result::Result;
+use xcm::v0::{Xcm, MultiAsset};
+use frame_support::weights::Weight;
+use crate::Assets;
+
+/// Determine the weight of an XCM message.
+pub trait WeightBounds<Call> {
+	/// Return the minimum amount of weight that an attempted execution of this message would definitely
+	/// consume.
+	///
+	/// This is useful to gauge how many fees should be paid up front to begin execution of the message.
+	/// It is not useful for determining whether execution should begin lest it result in surpassing weight
+	/// limits - in that case `deep` is the function to use.
+	fn shallow(message: &mut Xcm<Call>) -> Result<Weight, ()>;
+
+	/// Return the deep amount of weight, over `shallow` that complete, successful and worst-case execution of
+	/// `message` would incur.
+	///
+	/// This is perhaps overly pessimistic for determining how many fees should be paid for up-front since
+	/// fee payment (or any other way of offsetting the execution costs such as an voucher-style NFT) may
+	/// happen in stages throughout execution of the XCM.
+	///
+	/// A reminder: if it is possible that `message` may have alternative means of successful completion
+	/// (perhaps a conditional path), then the *worst case* weight must be reported.
+	///
+	/// This is guaranteed equal to the eventual sum of all `shallow` XCM messages that get executed through
+	/// any internal effects. Inner XCM messages may be executed by:
+	/// - Order::BuyExecution
+	fn deep(message: &mut Xcm<Call>) -> Result<Weight, ()>;
+}
+
+/// Charge for weight in order to execute XCM.
+pub trait WeightTrader {
+	/// Create a new trader instance.
+	fn new() -> Self;
+
+	/// Purchase execution weight credit in return for up to a given `fee`. If less of the fee is required
+	/// then the surplus is returned. If the `fee` cannot be used to pay for the `weight`, then an error is
+	/// returned.
+	fn buy_weight(&mut self, weight: Weight, payment: Assets) -> Result<Assets, ()>;
+
+	/// Attempt a refund of `weight` into some asset. The caller does not guarantee that the weight was
+	/// purchased using `buy_weight`.
+	///
+	/// Default implementation refunds nothing.
+	fn refund_weight(&mut self, _weight: Weight) -> MultiAsset { MultiAsset::None }
+}