A way to require enum variant index being specified

[nazar-pc]

I think it would be valuable to make an enum variant index required in derived code.

The way to approach this might be to have a separate feature that would generate a compile-time warning when #[codec(index = something)] is missing.

This is important in blockchain environment where some data structures can evolve over time and refactoring can accidentally break things.

[gui1117]

A crate feature might not work, because features get merged. So if a crate A use scale-codec without this feature and B use A and scale-codec with the feature then A won't compile no?

Maybe a new optional attribute: codec::ensure_explicit_indices or codec::disallow_implicit_indices.

[nazar-pc]

It will if done the way I suggest: by generating compile-time warning. Dependencies will compile just fine with warnings or not, but anything in crate/workspace will refuse to compile if warnings are denied in the project.