Remove Default bound on AccountId types under the xcm directory (#4712)

* Refactor ParentIsDefault to ParentIsAllZeroes

* Remove Default bound on all AccountId types under the xcm directory

* Change to ParentIs<A: Get<AccountId>, AccountId>

* Provide a better account for ParentIs

* Fixes

* Fixes

* Fixes

* Fixes

* Update xcm/xcm-builder/src/currency_adapter.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Use preset account ID value for parent MultiLocations

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

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

@@ -52,9 +52,11 @@ impl From<Error> for XcmError {
 ///
 /// # Example
 /// ```
-/// use frame_support::parameter_types;
+/// use parity_scale_codec::Decode;
+/// use frame_support::{parameter_types, PalletId};
+/// use sp_runtime::traits::{AccountIdConversion, TrailingZeroInput};
 /// use xcm::latest::prelude::*;
-/// use xcm_builder::{ParentIsDefault, CurrencyAdapter, IsConcrete};
+/// use xcm_builder::{ParentIsPreset, CurrencyAdapter, IsConcrete};
 ///
 /// /// Our chain's account id.
 /// type AccountId = sp_runtime::AccountId32;
@@ -62,12 +64,12 @@ impl From<Error> for XcmError {
 /// /// Our relay chain's location.
 /// parameter_types! {
 ///     pub RelayChain: MultiLocation = Parent.into();
-///     pub CheckingAccount: AccountId = Default::default();
+///     pub CheckingAccount: AccountId = PalletId(*b"checking").into_account();
 /// }
 ///
 /// /// Some items that implement `Convert<MultiLocation, AccountId>`. Can be more, but for now we just assume we accept
 /// /// messages from the parent (relay chain).
-/// pub type LocationConvertor = (ParentIsDefault<RelayChain>);
+/// pub type LocationConverter = (ParentIsPreset<AccountId>);
 ///
 /// /// Final currency adapter. This can be used in `xcm::Config` to specify how asset related transactions happen.
 /// pub type AssetTransactor = CurrencyAdapter<
@@ -75,8 +77,8 @@ impl From<Error> for XcmError {
 ///     u128,
 ///     // The matcher: use the currency when the asset is a concrete asset in our relay chain.
 ///     IsConcrete<RelayChain>,
-///     // The local convertor: default account of the parent relay chain.
-///     LocationConvertor,
+///     // The local converter: default account of the parent relay chain.
+///     LocationConverter,
 ///     // Our chain's account ID type.
 ///     AccountId,
 ///     // The checking account. Can be any deterministic inaccessible account.

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

@@ -31,7 +31,7 @@ pub mod test_utils;
 mod location_conversion;
 pub use location_conversion::{
 	Account32Hash, AccountId32Aliases, AccountKey20Aliases, ChildParachainConvertsVia,
-	LocationInverter, ParentIsDefault, SiblingParachainConvertsVia,
+	LocationInverter, ParentIsPreset, SiblingParachainConvertsVia,
 };
 
 mod origin_conversion;

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

@@ -15,9 +15,9 @@
 // along with Polkadot.  If not, see <http://www.gnu.org/licenses/>.
 
 use frame_support::traits::Get;
-use parity_scale_codec::Encode;
+use parity_scale_codec::{Decode, Encode};
 use sp_io::hashing::blake2_256;
-use sp_runtime::traits::AccountIdConversion;
+use sp_runtime::traits::{AccountIdConversion, TrailingZeroInput};
 use sp_std::{borrow::Borrow, marker::PhantomData};
 use xcm::latest::{Junction::*, Junctions::*, MultiLocation, NetworkId, Parent};
 use xcm_executor::traits::{Convert, InvertLocation};
@@ -36,21 +36,26 @@ impl<Network: Get<NetworkId>, AccountId: From<[u8; 32]> + Into<[u8; 32]> + Clone
 }
 
 /// A [`MultiLocation`] consisting of a single `Parent` [`Junction`] will be converted to the
-/// default value of `AccountId` (e.g. all zeros for `AccountId32`).
-pub struct ParentIsDefault<AccountId>(PhantomData<AccountId>);
-impl<AccountId: Default + Eq + Clone> Convert<MultiLocation, AccountId>
-	for ParentIsDefault<AccountId>
+/// parent `AccountId`.
+pub struct ParentIsPreset<AccountId>(PhantomData<AccountId>);
+impl<AccountId: Decode + Eq + Clone> Convert<MultiLocation, AccountId>
+	for ParentIsPreset<AccountId>
 {
 	fn convert_ref(location: impl Borrow<MultiLocation>) -> Result<AccountId, ()> {
 		if location.borrow().contains_parents_only(1) {
-			Ok(AccountId::default())
+			Ok(b"Parent"
+				.using_encoded(|b| AccountId::decode(&mut TrailingZeroInput::new(b)))
+				.expect("infinite length input; no invalid inputs for type; qed"))
 		} else {
 			Err(())
 		}
 	}
 
 	fn reverse_ref(who: impl Borrow<AccountId>) -> Result<MultiLocation, ()> {
-		if who.borrow() == &AccountId::default() {
+		let parent_account = b"Parent"
+			.using_encoded(|b| AccountId::decode(&mut TrailingZeroInput::new(b)))
+			.expect("infinite length input; no invalid inputs for type; qed");
+		if who.borrow() == &parent_account {
 			Ok(Parent.into())
 		} else {
 			Err(())