XCM v6

[xlc]

closes #7095

Setup XCM v6 but with a more extensible approach. This should be 100% ABI with XCM v5 currently but need tests to assert that.

This is only a (not so) quick draft and looking for initial feedbacks.

Ideally, in future XCM versions, we only need to implement the new instructions without the need to duplicate all the existing ones.

TODOs:

• executor
• builder
• weight
• upgrade
• cleanup
• tests

---

Changes:

• Breakdown big Instruction enum into a struct for each instruction. Using macro to generate the Instruction enum.
• Breakdown big process_instruction method in XcmExecutor to have the logic belong to each instruction struct. Using macro to glue the code together.
• Having executor traits depends on latest XCM version, instead of needing to duplicate the traits for each version.

[xlc]

This is much more complicated than I initially anticipated. The struct Xcm is hardcoded everywhere so most of the new code are hardcoded to use Xcm v5. A big refactor is needed to fix those unwanted coupling.

[xlc]

most of the Xcm v5 code are written without any consideration of future compatibility, and now someone have to refactor all of it...

[acatangiu]

approach looks good!

@xlc if you don't have the bandwidth to do this to completion (likely), I'm sure we can find someone in the ecosystem to do it, especially if you provide a good plan/framework/starting-point - let me know

[xlc]

@acatangiu that will be great. I will spend a few more days to do the core refactoring and then someone can help fix integrations with different runtimes and remaining tests