DeFI / XCM Use of XCM ExchangeAsset

[hbulgarini]

There is a new instruction in XCM v3 called ExchangeAsset which permits to exchange assets. This use case will be based on the integration of the swap functionality defined in the substrate-dex and this new instruction.

Very high level, the idea is to execute this XCM instruction and configure the AssetExchange type in a way that the substrate-dex pallet is performs the swap, and if successful, the ExchangeAsset makes the swap on the holding register.

As this is a new XCM instruction we need to some time to understanding it and the use case goal might be modified according to the learnt experience.

Contextual information here: #42 (comment)

[Wiezzel]

I've read the specification of ExchangeAsset instruction and I'm not sure how could we implement the Holding register. Exchange functionality offered by pallet-dex operates on accounts, while the mentioned register is described as a kind of temporary, non-persistent asset storage. We need to figure this out first.

[hbulgarini]

Still there is some some more investigation required until we confirm the XCM ExchangeAsset works in the way as we expect.

[bernardoaraujor]

I have written an implementation for the AssetExchange trait. It's on the bar/xcm-exchange-asset branch of substrate-dex.

The implementation is quite simplistic, with many naive assumptions.

• Only one asset pair is supported per swap. While that is not necessarily a restriction on the ExchangeAsset instruction, I did impose it to simplify my code.
• The parsing of give/want as currency/asset is rather dirty. There's probably a more elegant way to do it.
• A correct implementation would require coupling with pallet-asset-registry. I didn't have time to do it, so I wrote a static conversion with AssetId = 100. That is actually pretty dirty and I'm not proud of it 🙈.

bar/xcm-v3-multiple-runtimes-asset-exchange branch has trappist enabled with this implementation, along with an integration test that does a reserve-transfer of xUSD from Base to Trappist and swaps 50% for HOP before depositing into Bob's account.