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  1. Nov 15, 2023
  2. Nov 13, 2023
    • Adrian Catangiu's avatar
      pallet-xcm: enhance `reserve_transfer_assets` to support remote reserves (#1672) · 18257373
      Adrian Catangiu authored
      
      
      ## Motivation
      
      `pallet-xcm` is the main user-facing interface for XCM functionality,
      including assets manipulation functions like `teleportAssets()` and
      `reserve_transfer_assets()` calls.
      
      While `teleportAsset()` works both ways, `reserve_transfer_assets()`
      works only for sending reserve-based assets to a remote destination and
      beneficiary when the reserve is the _local chain_.
      
      ## Solution
      
      This PR enhances `pallet_xcm::(limited_)reserve_withdraw_assets` to
      support transfers when reserves are other chains.
      This will allow complete, **bi-directional** reserve-based asset
      transfers user stories using `pallet-xcm`.
      
      Enables following scenarios:
      - transferring assets with local reserve (was previously supported iff
      asset used as fee also had local reserve - now it works in all cases),
      - transferring assets with reserve on destination,
      - transferring assets with reserve on remote/third-party chain (iff
      assets and fees have same remote reserve),
      - transferring assets with reserve different than the reserve of the
      asset to be used as fees - meaning can be used to transfer random asset
      with local/dest reserve while using DOT for fees on all involved chains,
      even if DOT local/dest reserve doesn't match asset reserve,
      - transferring assets with any type of local/dest reserve while using
      fees which can be teleported between involved chains.
      
      All of the above is done by pallet inner logic without the user having
      to specify which scenario/reserves/teleports/etc. The correct scenario
      and corresponding XCM programs are identified, and respectively, built
      automatically based on runtime configuration of trusted teleporters and
      trusted reserves.
      
      #### Current limitations:
      - while `fees` and "non-fee" `assets` CAN have different reserves (or
      fees CAN be teleported), the remaining "non-fee" `assets` CANNOT, among
      themselves, have different reserve locations (this is also implicitly
      enforced by `MAX_ASSETS_FOR_TRANSFER=2`, but this can be safely
      increased in the future).
      - `fees` and "non-fee" `assets` CANNOT have **different remote**
      reserves (this could also be supported in the future, but adds even more
      complexity while possibly not being worth it - we'll see what the future
      holds).
      
      Fixes https://github.com/paritytech/polkadot-sdk/issues/1584
      Fixes https://github.com/paritytech/polkadot-sdk/issues/2055
      
      ---------
      
      Co-authored-by: default avatarFrancisco Aguirre <[email protected]>
      Co-authored-by: default avatarBranislav Kontur <[email protected]>
      18257373