FRAME: Unity Balance Conversion for Different IDs of Native Asset (#3659)

Introduce types to define 1:1 balance conversion for different relative
asset ids/locations of native asset.

Examples:
native asset on Asset Hub presented as `VersionedLocatableAsset` type in
the context of Relay Chain is
```
{
  `location`: (0, Parachain(1000)),
  `asset_id`: (1, Here),
}
```
and it's balance should be converted 1:1 by implementations of
`ConversionToAssetBalance` trait.

---------

Co-authored-by: Branislav Kontur <bkontur@gmail.com>

substrate/frame/support/src/traits.rs

@@ -36,7 +36,7 @@ mod members;
 pub use members::{AllowAll, DenyAll, Filter};
 pub use members::{
 	AsContains, ChangeMembers, Contains, ContainsLengthBound, ContainsPair, Equals, Everything,
-	EverythingBut, FromContainsPair, InitializeMembers, InsideBoth, IsInVec, Nothing,
+	EverythingBut, FromContains, FromContainsPair, InitializeMembers, InsideBoth, IsInVec, Nothing,
 	RankedMembers, RankedMembersSwapHandler, SortedMembers, TheseExcept,
 };
 

substrate/frame/support/src/traits/members.rs

@@ -66,6 +66,15 @@ impl<A, B, CP: ContainsPair<A, B>> Contains<(A, B)> for FromContainsPair<CP> {
 	}
 }
 
+/// A [`ContainsPair`] implementation that has a `Contains` implementation for each member of the
+/// pair.
+pub struct FromContains<CA, CB>(PhantomData<(CA, CB)>);
+impl<A, B, CA: Contains<A>, CB: Contains<B>> ContainsPair<A, B> for FromContains<CA, CB> {
+	fn contains(a: &A, b: &B) -> bool {
+		CA::contains(a) && CB::contains(b)
+	}
+}
+
 /// A [`Contains`] implementation that contains every value.
 pub enum Everything {}
 impl<T> Contains<T> for Everything {

substrate/frame/support/src/traits/tokens.rs

@@ -31,7 +31,7 @@ pub mod pay;
 pub use misc::{
 	AssetId, Balance, BalanceStatus, ConversionFromAssetBalance, ConversionToAssetBalance,
 	ConvertRank, DepositConsequence, ExistenceRequirement, Fortitude, GetSalary, Locker, Precision,
-	Preservation, Provenance, Restriction, UnityAssetBalanceConversion, WithdrawConsequence,
-	WithdrawReasons,
+	Preservation, Provenance, Restriction, UnityAssetBalanceConversion, UnityOrOuterConversion,
+	WithdrawConsequence, WithdrawReasons,
 };
 pub use pay::{Pay, PayFromAccount, PaymentStatus};

substrate/frame/support/src/traits/tokens/misc.rs

@@ -17,6 +17,7 @@
 
 //! Miscellaneous types.
 
+use crate::traits::Contains;
 use codec::{Decode, Encode, FullCodec, MaxEncodedLen};
 use sp_arithmetic::traits::{AtLeast32BitUnsigned, Zero};
 use sp_core::RuntimeDebug;
@@ -299,6 +300,33 @@ where
 	fn ensure_successful(_: AssetId) {}
 }
 
+/// Implements [`ConversionFromAssetBalance`], allowing for a 1:1 balance conversion of the asset
+/// when it meets the conditions specified by `C`. If the conditions are not met, the conversion is
+/// delegated to `O`.
+pub struct UnityOrOuterConversion<C, O>(core::marker::PhantomData<(C, O)>);
+impl<AssetBalance, AssetId, OutBalance, C, O>
+	ConversionFromAssetBalance<AssetBalance, AssetId, OutBalance> for UnityOrOuterConversion<C, O>
+where
+	C: Contains<AssetId>,
+	O: ConversionFromAssetBalance<AssetBalance, AssetId, OutBalance>,
+	AssetBalance: Into<OutBalance>,
+{
+	type Error = O::Error;
+	fn from_asset_balance(
+		balance: AssetBalance,
+		asset_id: AssetId,
+	) -> Result<OutBalance, Self::Error> {
+		if C::contains(&asset_id) {
+			return Ok(balance.into());
+		}
+		O::from_asset_balance(balance, asset_id)
+	}
+	#[cfg(feature = "runtime-benchmarks")]
+	fn ensure_successful(asset_id: AssetId) {
+		O::ensure_successful(asset_id)
+	}
+}
+
 /// Trait to handle NFT locking mechanism to ensure interactions with the asset can be implemented
 /// downstream to extend logic of Uniques/Nfts current functionality.
 pub trait Locker<CollectionId, ItemId> {