pallet-asset-conversion: Decoupling Native Currency Dependancy (#2031)

closes https://github.com/paritytech/polkadot-sdk/issues/1842

Decoupling Pallet from the Concept of Native Currency

Currently, the pallet is intrinsically linked with the concept of native
currency, requiring users to provide implementations of the
`fungible::*` and `fungibles::*` traits to interact with native and non
native assets. This incapsulates some non-related to the pallet
complexity and makes it less adaptable in contexts where the native
currency concept is absent.

With this PR, the dependence on `fungible::*` for liquidity-supplying
assets has been removed. Instead, the native and non-native currencies'
handling is now overseen by a single type that implements the
`fungibles::*` traits. To simplify this integration, types have been
introduced to facilitate the creation of a union between `fungible::*`
and `fungibles::*` implementations, producing a unified `fungibles::*`
type.

One of the reasons driving these changes is the ambition to create a
more user-friendly API for the `SwapCredit` implementation. Given that
it interacts with two distinct credit types from `fungible` and
`fungibles`, a unified type was introduced. Clients now manage potential
conversion failures for those credit types. In certain contexts, it's
vital to guarantee that operations are fail-safe, like in this impl -
[PR](https://github.com/paritytech/polkadot-sdk/pull/1845), place in
[code](https://github.com/paritytech/polkadot-sdk/blob/20b85a5fada8f55c98ba831964f5866ffeadf4da/cumulus/primitives/utility/src/lib.rs#L429).

Additional Updates:
- abstracted the pool ID and its account derivation logic via trait
bounds, along with common implementation offerings;
- removed `inc_providers` on a pool creation for the pool account;
- benchmarks:
-- swap complexity is N, not const;
-- removed `From<u128> + Into<u128>` bound from `T::Balance`;
-- removed swap/liquidity/.. amount constants, resolve them dynamically
based on pallet configuration;
-- migrated to v2 API;
- `OnUnbalanced` handler for the pool creation fee, replacing direct
transfers to a specified account ID;
- renamed `MultiAssetId` to `AssetKind` aligning with naming across
frame crates;

related PRs:
- (depends) https://github.com/paritytech/polkadot-sdk/pull/1677
- (caused) https://github.com/paritytech/polkadot-sdk/pull/2033
- (caused) https://github.com/paritytech/polkadot-sdk/pull/1876

---------

Co-authored-by: joe petrowski <25483142+joepetrowski@users.noreply.github.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

substrate/frame/asset-conversion/src/mock.rs

@@ -19,12 +19,17 @@
 
 use super::*;
 use crate as pallet_asset_conversion;
-
 use frame_support::{
 	construct_runtime, derive_impl,
 	instances::{Instance1, Instance2},
 	ord_parameter_types, parameter_types,
-	traits::{AsEnsureOriginWithArg, ConstU128, ConstU32, ConstU64},
+	traits::{
+		tokens::{
+			fungible::{NativeFromLeft, NativeOrWithId, UnionOf},
+			imbalance::ResolveAssetTo,
+		},
+		AsEnsureOriginWithArg, ConstU128, ConstU32, ConstU64,
+	},
 	PalletId,
 };
 use frame_system::{EnsureSigned, EnsureSignedBy};
@@ -34,6 +39,7 @@ use sp_runtime::{
 	traits::{AccountIdConversion, BlakeTwo256, IdentityLookup},
 	BuildStorage,
 };
+use sp_std::default::Default;
 
 type Block = frame_system::mocking::MockBlock<Test>;
 
@@ -143,37 +149,37 @@ impl pallet_assets::Config<Instance2> for Test {
 
 parameter_types! {
 	pub const AssetConversionPalletId: PalletId = PalletId(*b"py/ascon");
-	pub storage AllowMultiAssetPools: bool = true;
-	pub storage LiquidityWithdrawalFee: Permill = Permill::from_percent(0); // should be non-zero if AllowMultiAssetPools is true, otherwise can be zero
+	pub const Native: NativeOrWithId<u32> = NativeOrWithId::Native;
+	pub storage LiquidityWithdrawalFee: Permill = Permill::from_percent(0);
 }
 
 ord_parameter_types! {
 	pub const AssetConversionOrigin: u128 = AccountIdConversion::<u128>::into_account_truncating(&AssetConversionPalletId::get());
 }
 
+pub type NativeAndAssets = UnionOf<Balances, Assets, NativeFromLeft, NativeOrWithId<u32>, u128>;
+pub type AscendingLocator = Ascending<u128, NativeOrWithId<u32>>;
+pub type WithFirstAssetLocator = WithFirstAsset<Native, u128, NativeOrWithId<u32>>;
+
 impl Config for Test {
 	type RuntimeEvent = RuntimeEvent;
-	type Currency = Balances;
-	type AssetId = u32;
+	type Balance = <Self as pallet_balances::Config>::Balance;
+	type HigherPrecisionBalance = sp_core::U256;
+	type AssetKind = NativeOrWithId<u32>;
+	type Assets = NativeAndAssets;
+	type PoolId = (Self::AssetKind, Self::AssetKind);
+	type PoolLocator = Chain<WithFirstAssetLocator, AscendingLocator>;
 	type PoolAssetId = u32;
-	type Assets = Assets;
 	type PoolAssets = PoolAssets;
+	type PoolSetupFee = ConstU128<100>; // should be more or equal to the existential deposit
+	type PoolSetupFeeAsset = Native;
+	type PoolSetupFeeTarget = ResolveAssetTo<AssetConversionOrigin, Self::Assets>;
 	type PalletId = AssetConversionPalletId;
 	type WeightInfo = ();
 	type LPFee = ConstU32<3>; // means 0.3%
-	type PoolSetupFee = ConstU128<100>; // should be more or equal to the existential deposit
-	type PoolSetupFeeReceiver = AssetConversionOrigin;
 	type LiquidityWithdrawalFee = LiquidityWithdrawalFee;
-	type AllowMultiAssetPools = AllowMultiAssetPools;
 	type MaxSwapPathLength = ConstU32<4>;
 	type MintMinLiquidity = ConstU128<100>; // 100 is good enough when the main currency has 12 decimals.
-
-	type Balance = <Self as pallet_balances::Config>::Balance;
-	type HigherPrecisionBalance = sp_core::U256;
-
-	type MultiAssetId = NativeOrAssetId<u32>;
-	type MultiAssetIdConverter = NativeOrAssetIdConverter<u32>;
-
 	#[cfg(feature = "runtime-benchmarks")]
 	type BenchmarkHelper = ();
 }