# Context

Estimating fees for XCM execution and sending has been an area with bad
UX.
The addition of the
[XcmPaymentApi](https://github.com/paritytech/polkadot-sdk/pull/3607)
exposed the necessary components to be able to estimate XCM fees
correctly, however, that was not the full story.
The `XcmPaymentApi` works for estimating fees only if you know the
specific XCM you want to execute or send.
This is necessary but most UIs want to estimate the fees for extrinsics,
they don't necessarily know the XCM program that's executed by them.

# Main addition

A new runtime API is introduced, the `XcmDryRunApi`, that given an
extrinsic, or an XCM program, returns its effects:
- Execution result
- Local XCM (in the case of an extrinsic)
- Forwarded XCMs
- List of events

This API can be used on its own for dry-running purposes, for
double-checking or testing, but it mainly shines when used in
conjunction with the `XcmPaymentApi`.
UIs can use these two APIs to estimate transfers.

# How it works

New tests are added to exemplify how to incorporate both APIs.
There's a mock test just to make sure everything works under
`xcm-fee-payment-runtime-api`.
There's a real-world test using Westend and AssetHubWestend under
`cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-westend/src/tests/xcm_fee_estimation.rs`.
Added both a test for a simple teleport between chains and a reserve
transfer asset between two parachains going through a reserve.

The steps to follow:
- Use `XcmDryRunApi::dry_run_extrinsic` to get local XCM program and
forwarded messages
- For each forwarded message
- Use `XcmPaymentApi::query_delivery_fee` LOCALLY to get the delivery
fees
- Use `XcmPaymentApi::query_xcm_weight` ON THE DESTINATION to get the
remote execution weight
- (optional) Use `XcmPaymentApi::query_acceptable_payment_assets` ON THE
DESTINATION to know on which assets the execution fees can be paid
- Use `XcmPaymentApi::query_weight_to_asset_fee` ON THE DESTINATION to
convert weight to the actual remote execution fees
- Use `XcmDryRunApi::dry_run_xcm` ON THE DESTINATION to know if a new
message will be forwarded, if so, continue

# Dear reviewer

The changes in this PR are grouped as follows, and in order of
importance:
- Addition of new runtime API
- Definition, mock and simple tests:
polkadot/xcm/xcm-fee-payment-runtime-api/*
- Implemented on Westend, Asset Hub Westend and Penpal, will implement
on every runtime in a following PR
- Addition of a new config item to the XCM executor for recording xcms
about to be executed
  - Definition: polkadot/xcm/xcm-executor/*
  - Implementation: polkadot/xcm/pallet-xcm/*
- had to update all runtime xcm_config.rs files with `type XcmRecorder =
XcmPallet;`
- Addition of a new trait for inspecting the messages in queues
  - Definition: polkadot/xcm/xcm-builder/src/routing.rs
  - Implemented it on all routers:
    - ChildParachainRouter: polkadot/runtime/common/src/xcm_sender.rs
- ParentAsUmp: cumulus/primitives/utility/src/lib.rs (piggybacked on
implementation in cumulus/pallets/parachain-system/src/lib.rs)
    - XcmpQueue: cumulus/pallets/xcmp-queue/src/lib.rs
    - Bridge: bridges/modules/xcm-bridge-hub-router/src/lib.rs
- More complicated and useful tests:
-
cumulus/parachains/integration-tests/emulated/tests/assets/asset-hub-westend/src/tests/xcm_fee_estimation.rs

## Next steps

With this PR, Westend, AssetHubWestend, Rococo and AssetHubRococo have
the new API.
UIs can test on these runtimes to create better experiences around
cross-chain operations.

Next:
- Add XcmDryRunApi to all system parachains
- Integrate xcm fee estimation in all emulated tests
- Get this on the fellowship runtimes

---------

Co-authored-by: Adrian Catangiu <adrian@parity.io>