Introduce new XCM instruction: InitiateAssetsTransfer for complex asset transfers

[acatangiu]

This is a continuation of polkadot-fellows/xcm-format#63 following the migration of XCM RFCs to this repo.

Summary

This RFC proposes a new instruction that provides a way to initiate asset transfers which transfer multiple types (teleports, local-reserve, destination-reserve) of assets, on remote chains using XCM alone.

The currently existing instructions are too opinionated and force each XCM asset transfer to a single transfer type (teleport, local-reserve, destination-reserve). This results in inability to combine different types of transfers in single transfer which results in overall poor UX when trying to move assets across chains.

Example usage

    // pay remote fees with ROCs
    let remote_fees = Some(
        AssetTransferFilter::ReserveWithdraw(
            AssetFilter::Wild(AllOf { id: rocs_id.into(), fun: WildFungibility::Fungible })
        )
    );
    // XCM to be executed locally
    let xcm = Xcm::<penpal_runtime::RuntimeCall>(vec![
        // Withdraw both ROCs and PENs from origin account
        WithdrawAsset(assets.clone().into()),
        // Execute the transfers while paying remote fees with ROCs
        InitiateAssetsTransfer {
            dest: local_asset_hub,
            assets: vec![
                // ROCs are reserve-withdrawn on AHR
                ReserveWithdraw(rocs.into()),
                // PENs are teleported to AHR
                Teleport(pens.into()),
            ],
            remote_fees,
            remote_xcm: xcm_on_ahr,
        },
    ]);

I also have a proof-of-concept implementation and a proof-of-concept asset transfer test, transferring 2 different types of assets across 3 chains.

[xlc]

not sure if I missed something but what about the order of the asset deposit?
e.g. transfer from Acala to AH, of 50 DOT and 10 USDT. it will result one UMP and one HRMP at Acala. On AH, it will receive one DMP and one HRMP. how exactly is this case handled?

[acatangiu]

not sure if I missed something but what about the order of the asset deposit? e.g. transfer from Acala to AH, of 50 DOT and 10 USDT. it will result one UMP and one HRMP at Acala. On AH, it will receive one DMP and one HRMP. how exactly is this case handled?

This new instruction is point-to-point (same as the rest). It can be used by higher layers that build cross chain xcm programs to move assets across multiple hops by calling it at every hop along the way. But the required/used path is outside the scope of concerns of the XCVM. It only executes this instr which explicitly specifies the dest (next hop).

Transferring assets that require different paths is still not supported within same XCM because of the async nature of cross chain messages..

I will include some of this information in the RFC for clarity.

[franciscoaguirre]

not sure if I missed something but what about the order of the asset deposit? e.g. transfer from Acala to AH, of 50 DOT and 10 USDT. it will result one UMP and one HRMP at Acala. On AH, it will receive one DMP and one HRMP. how exactly is this case handled?

If AH is used as a reserve then this would just be one HRMP and it'd be possible, no?

[vgeddes]

The design looks good. I've confirmed in https://hackmd.io/@vgeddes/rkeSGIAYR that InitiateAssetsTransfer unlocks new potential for Snowbridge, mainly in improving UX.

[acatangiu]

/rfc propose

[paritytech-rfc-bot]

Hey @acatangiu, here is a link you can use to create the referendum aiming to approve this RFC number 0100.

Instructions

1. Open the link.

2. Switch to the Submission tab.

3. Adjust the transaction if needed (for example, the proposal Origin).

4. Submit the Transaction

---

It is based on commit hash dc585343d23aa7685cce08f3d939829fb803703e.

The proposed remark text is: RFC_APPROVE(0100,86bf80319a20a97813c131ae3105074a228250077d6545f5f51812d61dbcab79).

[github-actions]

PR can be merged.

Write the following command to trigger the bot

/rfc process 0xfdf10d88746da6af4245b60405acbd49fbee059d109185b97c7b5fd8541b17c4

[acatangiu]

/rfc process 0xfdf10d88746da6af4245b60405acbd49fbee059d109185b97c7b5fd8541b17c4

[paritytech-rfc-bot]

The on-chain referendum has approved the RFC.