xcm-builder: PayOverXcm supports fallible convertors for asset kind and beneficiary conversion (#1572)

`PayOverXcm` type accepts two converters to transform the `AssetKind`
and `Beneficiary` parameter types into recognized `xcm` types. In this
PR, we've modified the bounds for these converters, transitioning from
`Convert` to `TryConvert`.

One such use case for this adjustment is when dealing with versioned xcm
types for `AssetKind` and `Beneficiary`. These types might be not
convertible to the latest xcm version, hence the need for fallible
conversion.

This changes required for
https://github.com/paritytech/polkadot-sdk/pull/1333

polkadot/xcm/xcm-builder/src/location_conversion.rs

@@ -18,7 +18,7 @@ use crate::universal_exports::ensure_is_remote;
 use frame_support::traits::Get;
 use parity_scale_codec::{Compact, Decode, Encode};
 use sp_io::hashing::blake2_256;
-use sp_runtime::traits::{AccountIdConversion, Convert, TrailingZeroInput};
+use sp_runtime::traits::{AccountIdConversion, TrailingZeroInput, TryConvert};
 use sp_std::{marker::PhantomData, prelude::*};
 use xcm::latest::prelude::*;
 use xcm_executor::traits::ConvertLocation;
@@ -322,10 +322,10 @@ impl<Network: Get<Option<NetworkId>>, AccountId: From<[u8; 32]> + Into<[u8; 32]>
 /// network (provided by `Network`) and the `AccountId`'s `[u8; 32]` datum for the `id`.
 pub struct AliasesIntoAccountId32<Network, AccountId>(PhantomData<(Network, AccountId)>);
 impl<'a, Network: Get<Option<NetworkId>>, AccountId: Clone + Into<[u8; 32]> + Clone>
-	Convert<&'a AccountId, MultiLocation> for AliasesIntoAccountId32<Network, AccountId>
+	TryConvert<&'a AccountId, MultiLocation> for AliasesIntoAccountId32<Network, AccountId>
 {
-	fn convert(who: &AccountId) -> MultiLocation {
+	fn try_convert(who: &AccountId) -> Result<MultiLocation, &AccountId> {
-		AccountId32 { network: Network::get(), id: who.clone().into() }.into()
+		Ok(AccountId32 { network: Network::get(), id: who.clone().into() }.into())
 	}
 }
 

polkadot/xcm/xcm-builder/src/pay.rs

@@ -20,7 +20,7 @@ use frame_support::traits::{
 	tokens::{Pay, PaymentStatus},
 	Get,
 };
-use sp_runtime::traits::Convert;
+use sp_runtime::traits::TryConvert;
 use sp_std::{marker::PhantomData, vec};
 use xcm::{opaque::lts::Weight, prelude::*};
 use xcm_executor::traits::{QueryHandler, QueryResponseStatus};
@@ -71,8 +71,8 @@ impl<
 		Timeout: Get<Querier::BlockNumber>,
 		Beneficiary: Clone,
 		AssetKind,
-		AssetKindToLocatableAsset: Convert<AssetKind, LocatableAssetId>,
+		AssetKindToLocatableAsset: TryConvert<AssetKind, LocatableAssetId>,
-		BeneficiaryRefToLocation: for<'a> Convert<&'a Beneficiary, MultiLocation>,
+		BeneficiaryRefToLocation: for<'a> TryConvert<&'a Beneficiary, MultiLocation>,
 	> Pay
 	for PayOverXcm<
 		Interior,
@@ -96,12 +96,14 @@ impl<
 		asset_kind: Self::AssetKind,
 		amount: Self::Balance,
 	) -> Result<Self::Id, Self::Error> {
-		let locatable = AssetKindToLocatableAsset::convert(asset_kind);
+		let locatable = AssetKindToLocatableAsset::try_convert(asset_kind)
+			.map_err(|_| xcm::latest::Error::InvalidLocation)?;
 		let LocatableAssetId { asset_id, location: asset_location } = locatable;
 		let destination = Querier::UniversalLocation::get()
 			.invert_target(&asset_location)
 			.map_err(|()| Self::Error::LocationNotInvertible)?;
-		let beneficiary = BeneficiaryRefToLocation::convert(&who);
+		let beneficiary = BeneficiaryRefToLocation::try_convert(&who)
+			.map_err(|_| xcm::latest::Error::InvalidLocation)?;
 
 		let query_id = Querier::new_query(asset_location, Timeout::get(), Interior::get());
 
@@ -196,10 +198,10 @@ pub struct LocatableAssetId {
 /// Adapter `struct` which implements a conversion from any `AssetKind` into a [`LocatableAssetId`]
 /// value using a fixed `Location` for the `location` field.
 pub struct FixedLocation<Location>(sp_std::marker::PhantomData<Location>);
-impl<Location: Get<MultiLocation>, AssetKind: Into<AssetId>> Convert<AssetKind, LocatableAssetId>
+impl<Location: Get<MultiLocation>, AssetKind: Into<AssetId>> TryConvert<AssetKind, LocatableAssetId>
 	for FixedLocation<Location>
 {
-	fn convert(value: AssetKind) -> LocatableAssetId {
+	fn try_convert(value: AssetKind) -> Result<LocatableAssetId, AssetKind> {
-		LocatableAssetId { asset_id: value.into(), location: Location::get() }
+		Ok(LocatableAssetId { asset_id: value.into(), location: Location::get() })
 	}
 }

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

@@ -29,9 +29,9 @@ pub struct AssetKind {
 }
 
 pub struct LocatableAssetKindConverter;
-impl sp_runtime::traits::Convert<AssetKind, LocatableAssetId> for LocatableAssetKindConverter {
+impl sp_runtime::traits::TryConvert<AssetKind, LocatableAssetId> for LocatableAssetKindConverter {
-	fn convert(value: AssetKind) -> LocatableAssetId {
+	fn try_convert(value: AssetKind) -> Result<LocatableAssetId, AssetKind> {
-		LocatableAssetId { asset_id: value.asset_id, location: value.destination }
+		Ok(LocatableAssetId { asset_id: value.asset_id, location: value.destination })
 	}
 }