RFC: Better fee handling

proposals/0053-better-fee-handling.md

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+---
+Title: Fee handling generalization and simplification
+Number: 53
+Status: Draft
+Version: 5
+Authors:
+ - Francisco Aguirre
+Created: 2024-03-01
+Impact: Low
+Requires:
+Replaces:
+---
+
+## Summary
+
+XCM already handles execution fees in an effective and efficient manner using the `BuyExecution` instruction.
+However, other types of fees are not handled as effectively -- for example, delivery fees.
+Fees exist that can't be measured using `Weight` -- as execution fees can -- so a new method should be thought up for those cases.
+This RFC proposes making the fee handling system simpler and more general, by doing two things:
+- Adding a `fees` register
+- Deprecating `BuyExecution` and adding a new instruction `PayFees` with new semantics to ultimately replace it.
+
+This new instruction only takes the amount of fees that the XCVM can use from the holding register.
+The XCVM implementation is free to use these fees to pay for execution fees, transport fees, or any other type of fee that might be necessary.
+This moves the specifics of fees further away from the XCM standard, and more into the actual underlying XCVM implementation, which is a good thing.
+
+## Motivation
+
+Execution fees are handled correctly by XCM right now.
+However, the addition of extra fees, like for message delivery, result in awkward ways of integrating them into the XCVM implementation.
+The standard should have a way to correctly deal with these implementation specific fees, that might not exist in every system that uses XCM.
+
+## Specification
+
+The new instruction that will replace `BuyExecution` is a much simpler and general version: `PayFees`.
+This instruction takes one `Asset`, takes it from the holding register, and puts it into a new `fees` register.
+The XCVM implementation can now use this `Asset` to make sure every necessary fee is paid for, this includes execution fees, delivery fees, or any other fee.
+
+```rust
+PayFees { asset: Asset }
+```
+
+This new instruction will reserve **the entirety** of `asset` for fee payment.
+The asset can't be used for anything else during the entirety of the program.
+This is different from the current semantics of `BuyExecution`.
+
+If not all `Asset` in the `fees` register is used when the execution ends, then we trap them alongside any possible leftover assets from the holding register.
+
+### Examples
+
+Most XCM programs that pay for execution are written like so:
+
+```rust
+// Instruction that loads the holding register
+BuyExecution { asset, weight_limit }
+// ...rest
+```
+
+With this RFC, the structure would be the same, but using the new instruction, that has different semantics:
+
+```rust
+// Instruction that loads the holding register
+PayFees { asset }
+// ...rest
+```
+
+## Security considerations
+
+Overpaying for fees might have an increase in trapped assets.
+Depending on the method used for trapping assets, this might result in too much storage used.
+
+## Impact
+
+For systems with no fees other than execution, this change would be just a rename of the instruction.
+For systems with other types of fees, migration to this new structure would be advised, as they currently can't express the new fees.
+
+## Alternatives
+
+Alternatives include:
+1. Simply changing the semantics of `BuyExecution`
+2. Create a new instruction for each conceivable type of fees
+
+Alternative number 1, while possible, would result in a lot of existing programs breaking because of changed assumptions.
+The change must happen either in a new instruction, or in a new version, to ensure those programs are not broken.
+While the instruction could be called the same in a new version of the format, changing the name makes the changes clearer.
+
+Alternative number 2 is not only unfeasible, but also brings a lot of implementation-specific details into the format.
+
+The impact of not doing this is suffering from bugs born from failing to handle the different type of fees.