Remove xcm::v3 from assets-common nits (#4037)

This PR mainly removes `xcm::v3` stuff from `assets-common` to make it
more generic and facilitate the transition to newer XCM versions. Some
of the implementations here used hard-coded `xcm::v3::Location`, but now
it's up to the runtime to configure according to its needs.

Additional/consequent changes:
- `penpal` runtime uses now `xcm::latest::Location` for `pallet_assets`
as `AssetId`, because we don't care about migrations here
- it pretty much simplify xcm-emulator integration tests, where we don't
need now a lots of boilerplate conversions:
      ```
      v3::Location::try_from(...).expect("conversion works")`
      ```
- xcm-emulator tests
- split macro `impl_assets_helpers_for_parachain` to the
`impl_assets_helpers_for_parachain` and
`impl_foreign_assets_helpers_for_parachain` (avoids using hard-coded
`xcm::v3::Location`)

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

@@ -107,17 +107,6 @@ impl<
 #[deprecated = "Use `ConvertedConcreteId` instead"]
 pub type ConvertedConcreteAssetId<A, B, C, O> = ConvertedConcreteId<A, B, C, O>;
 
-pub struct V4V3LocationConverter;
-impl MaybeEquivalence<xcm::v4::Location, xcm::v3::Location> for V4V3LocationConverter {
-	fn convert(old: &xcm::v4::Location) -> Option<xcm::v3::Location> {
-		(*old).clone().try_into().ok()
-	}
-
-	fn convert_back(new: &xcm::v3::Location) -> Option<xcm::v4::Location> {
-		(*new).try_into().ok()
-	}
-}
-
 pub struct MatchedConvertedConcreteId<AssetId, Balance, MatchAssetId, ConvertAssetId, ConvertOther>(
 	PhantomData<(AssetId, Balance, MatchAssetId, ConvertAssetId, ConvertOther)>,
 );

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

@@ -30,7 +30,7 @@ mod asset_conversion;
 #[allow(deprecated)]
 pub use asset_conversion::ConvertedConcreteAssetId;
 pub use asset_conversion::{
-	AsPrefixedGeneralIndex, ConvertedConcreteId, MatchedConvertedConcreteId, V4V3LocationConverter,
+	AsPrefixedGeneralIndex, ConvertedConcreteId, MatchedConvertedConcreteId,
 };
 
 mod barriers;
@@ -81,7 +81,9 @@ pub use location_conversion::{
 };
 
 mod matches_location;
-pub use matches_location::{StartsWith, StartsWithExplicitGlobalConsensus};
+pub use matches_location::{
+	StartsWith, StartsWithExplicitGlobalConsensus, WithLatestLocationConverter,
+};
 
 mod matches_token;
 pub use matches_token::IsConcrete;

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

@@ -18,6 +18,8 @@
 //! `InteriorLocation` types.
 
 use frame_support::traits::{Contains, Get};
+use sp_runtime::traits::MaybeEquivalence;
+use sp_std::marker::PhantomData;
 use xcm::latest::{InteriorLocation, Location, NetworkId};
 
 /// An implementation of `Contains` that checks for `Location` or
@@ -51,3 +53,18 @@ impl<T: Get<NetworkId>> Contains<InteriorLocation> for StartsWithExplicitGlobalC
 		matches!(location.global_consensus(), Ok(requested_network) if requested_network.eq(&T::get()))
 	}
 }
+
+/// An adapter implementation of `MaybeEquivalence` which can convert between the latest `Location`
+/// and other versions that implement `TryInto<Location>` and `TryFrom<Location>`.
+pub struct WithLatestLocationConverter<Target>(PhantomData<Target>);
+impl<Target: TryInto<Location> + TryFrom<Location> + Clone> MaybeEquivalence<Location, Target>
+	for WithLatestLocationConverter<Target>
+{
+	fn convert(old: &Location) -> Option<Target> {
+		(*old).clone().try_into().ok()
+	}
+
+	fn convert_back(new: &Target) -> Option<Location> {
+		new.clone().try_into().ok()
+	}
+}