XER emits wrong CHOICE item name for SEQUENCE OF CHOICE

[k1ll3rb1z]

Hi,
I may have found a bug when decoding uper to xer, where choice item name is wrong.
A small patch is proposed as well.

Versions

• rasn: 0.28.13
• OS: Linux
• Rust: stable

Steps to Reproduce

1. Clone/run this minimal repro crate: rasn-xer-choice-repro
2. Run:

cd rasn-xer-choice-repro
cargo run

Actual Behavior

Output contains:

<Msg><messageData><elementIds><constrainedData><clearanceType><approach /></clearanceType><assignedTime /></constrainedData></elementIds></messageData></Msg>

The child of elementIds is emitted as constrainedData.

Expected Behavior

Output should contain:

<Msg><messageData><elementIds><uM270><clearanceType><approach /></clearanceType><assignedTime /></uM270></elementIds></messageData></Msg>

The child of elementIds should be the CHOICE alternative name uM270.

Why this looks like an encoder label bug

• Semantic decode/encode is consistent at UPER level (same payload round-trips).
• The mismatch is in XER element naming for CHOICE items inside SEQUENCE OF.

Suspected Area

Likely interaction in src/xer/enc.rs among:

• encode_sequence_of
• encode_choice
• field_tag_stack
• entering_list_item_type

Hypothesis: list-item CHOICE encoding can fall back to a parent/sibling identifier from field_tag_stack (constrainedData) instead of using the CHOICE variant identifier (uM270).

Suggested Fix Direction

1. Preserve CHOICE variant identifier context for list-item CHOICE emission.
2. Avoid parent stack fallback when encoding list-item CHOICE variants.
3. Add a regression test for SEQUENCE OF CHOICE with sibling optional fields.

Proposed patch

https://github.com/k1ll3rb1z/rasn-xer-choice-repro/blob/main/rasn_encode_choice_list_item_fix.patch

Attached Repro

• Repro project: rasn-xer-choice-repro
• Repro source: rasn-xer-choice-repro/src/main.rs

Thanks

Note: AI was used to find a patch for my project