BN254 G2 msgpack serialization bug in barretenberg-rs #1669

Summary

Bn254G2Point in barretenberg-rs is missing serde_bytes annotations on its [Vec<u8>; 2] fields, causing msgpack serialization to produce arrays of integers instead of bin32 blobs. The C++ bbapi expects bin32 (via uint256_t::msgpack_unpack), so bn254_g2_mul calls fail with a deserialization abort.

Reported by Porco from Obsidion.

Details

Bn254G1Point works correctly because its Vec<u8> fields use #[serde(with = "serde_bytes")]:

pub struct Bn254G1Point {
    #[serde(with = "serde_bytes")]
    pub x: Vec<u8>,
    #[serde(with = "serde_bytes")]
    pub y: Vec<u8>,
}

Bn254G2Point is missing the equivalent annotation:

pub struct Bn254G2Point {
    pub x: [Vec<u8>; 2],  // no serde annotation
    pub y: [Vec<u8>; 2],  // no serde annotation
}

Without the annotation, rmp-serde serializes each Vec<u8> element as a msgpack array of integers rather than a bin32 blob.

Root cause

The Rust code generator (rust_codegen.ts) handles serde_bytes for bytes primitives and serde_array4_bytes for [Vec<u8>; 4] arrays, but has no handling for the field2 primitive type which maps to [Vec<u8>; 2].

Fix

1. Add a serde_array2_bytes module (same pattern as existing serde_array4_bytes)
2. Add needsSerdeArray2Bytes() detection for field2 primitive types in rust_codegen.ts
3. Annotate Bn254G2Point fields with #[serde(with = "serde_array2_bytes")]

Affected versions

Every published version from 4.1.0-rc.2 through 5.0.0-private.20260319. There are no tests for bn254_g2_mul which is why this was never caught.