Bug: incorrect index assignment with `#[codec(index = ...)]`

[breathx]

Hey! I recently encountered a bug with both the Encode and Decode derive macros, which causes incorrect discriminant values (indexes) to be assigned to enum variants.

Here’s a minimal reproducible example:

use parity_scale_codec::{Decode, Encode};

#[derive(Decode, Encode)]
enum MyEnum {
    #[codec(index = 1)]
    A, // will have index 1
    B, // will also get index 1, incorrectly
}

This results in a compile-time error:

error[E0080]: evaluation of constant value failed
 --> src/lib.rs:3:10
  |
3 | #[derive(Decode, Encode)]
  |          ^^^^^^ the evaluated program panicked at 'Found variants that have duplicate indexes. Both `A` and `B` have the index `1`. Use different indexes for each variant.', src/lib.rs:3:10
  |
  = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `::core::panic` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0080]: evaluation of constant value failed
 --> src/lib.rs:3:18
  |
3 | #[derive(Decode, Encode)]
  |                  ^^^^^^ the evaluated program panicked at 'Found variants that have duplicate indexes. Both `A` and `B` have the index `1`. Use different indexes for each variant.', src/lib.rs:3:18
  |
  = note: this error originates in the macro `$crate::panic::panic_2021` which comes from the expansion of the macro `::core::panic` (in Nightly builds, run with -Z macro-backtrace for more info)

If we change the index to 3, the code compiles, but B is still incorrectly assigned index 1, instead of the expected next available index (which should be 4 in this case).

[gui1117]

I agree the default index is not ideal and inconsistent with rust enum variant discriminant index.

But we can't change it to a different default, it would break a lot of codec in production.

The only way out would be to force defining an explicit index and in few years maybe reintroduce a better default.
Or a flag to set a different default strategy.

IMO, it doesn't worth the effort, instead we can just accept the current not idiomatic but safe and checked default, and people should define the index on every variant if they need to.

[breathx]

Yeah, this makes sense, but from my point of view, it’s already worth improving UX with minimal effort.

Here are some ideas:
1. Rename #[codec(index = ...)] to #[codec(override = ...)].
2. Forbid assigning a custom index to only some variants — it should be either all or none.