Xtokens seems incompatible with what statemint expects

[girazoki]

Test Scenario: We have an asset created in Statemint, and transferred (reserve transfer asset) to Moonbeam. This transfer works as expected. But when willing to transfer back this asset with Xtokens, Statemint is expecting a GeneralIndex(asset_id) but Xtokens, after inverting its location and reanchoring the asset, sends MultiLocation { parents:1, interior: X2(ParachainId(statemint_para_id), GeneralIndex(asset_id))}. Therefore we obtain a FailedAssetTransact

I have just made a couple of tests on this, would like to investigate further.

[xlc]

Related: #806

[yrong]

@xlc I think in orml-xtokens we may need some api to support transfering multiple assets(now only one) and with the ability to specific which asset as fee, details here

[xlc]

Please write an issue to https://github.com/open-web3-stack/open-runtime-module-library/issues but yeah I agree we need that.

[gavofyork]

Yes, Statemint will expect an XCM with its local asset ID (GeneralIndex(asset_id)). The reanchoring process from Xtokens to Statemint should result in an asset from Xtokens known as (Parent, Parachain(Statemint), GeneralIndex(asset_id)) being translated to GeneralIndex(asset_id) which is what Statemint expects. So all third-party chains must ensure that reserve assets based on Statemint are known in XCM as (Parent, Parachain(Statemint), GeneralIndex(asset_id)). This is pretty easy to do with the AsPrefixedGeneralIndex. If chains want to have assets in them other than the assets sent by Statemint, then they can have two separate instances of the Assets pallet, one whose assets are known in XCM as (Parent, Parachain(Statemint), GeneralIndex(asset_id)), and the other with a different XCM location prefix. This namespaces the asset IDs and keeps a locally-created asset from interfering with a Statemint reserve asset of the same index.