Serde Code Review

Reviews serde serialization code for derive patterns, enum representations, custom implementations, and common serialization bugs. Use when reviewing Rust code that uses serde, serde_json, toml, or any serde-based serialization format. Covers attribute macros, field renaming, and format-specific pitfalls.

Review Workflow

1. Check Cargo.toml — Note serde features (derive, rc), format crates (serde_json, toml, bincode, etc.), and Rust edition (2024 has breaking changes affecting serde code)
2. Check derive usage — Verify Serialize and Deserialize are derived appropriately
3. Check enum representations — Enum tagging affects wire format compatibility and readability
4. Check field attributes — Renaming, defaults, skipping affect API contracts
5. Check edition 2024 compatibility — Reserved gen keyword, RPIT lifetime capture changes, never_type_fallback
6. Verify round-trip correctness — Serialized data must deserialize back to the same value

Gates (before reporting findings)

Run in order. Do not write a finding until the step that applies has passed.

1. Serde context on disk — Pass when: You have read the relevant Cargo.toml (crate or workspace root) and can state Rust edition, serde / serde_derive features if non-default (derive, rc), and which format crates apply (serde_json, toml, bincode, etc.) for the code under review. Then apply edition-specific checklist items (e.g. gen, RPIT/never_type_fallback) only when that file supports them.

2. Per-finding evidence — Pass when: Each issue cites [FILE:LINE] from the current tree for the struct/enum, Serialize/Deserialize impl, or attribute block in question (not from memory, docs-only, or another branch).

3. Category check vs protocol — Pass when: For the finding type (derive attrs, enum tagging, flatten, custom impl, sqlx + serde alignment), you ran the matching checks from the review-verification-protocol skill (e.g. full type definition + serde attrs before “wrong representation”; confirmed edition in Cargo.toml before edition-2024-only findings). Then add the finding.

4. Output shape — Pass when: The report lines match Output Format below (severity + description).

Output Format

Report findings as:

[FILE:LINE] ISSUE_TITLE
Severity: Critical | Major | Minor | Informational
Description of the issue and why it matters.

Quick Reference

Issue Type	Reference
Derive patterns, attribute macros, field configuration	references/derive-patterns.md
Custom Serialize/Deserialize, format-specific issues	references/custom-serialization.md

Review Checklist

Derive Usage

• #[derive(Serialize, Deserialize)] on types that cross serialization boundaries
• #[derive(Debug)] alongside serde derives (debugging serialization issues)
• Feature-gated derives when serde is optional: #[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
• Prefer #[expect(unused)] over #[allow(unused)] for serde-only fields (self-cleaning lint suppression, stable since 1.81)

Enum Representation

• Enum tagging is explicit (not relying on serde's default externally-tagged format when another is intended)
• Tag names are stable and won't collide with field names
• #[serde(rename_all = "...")] used consistently across the API

Field Configuration

• #[serde(skip_serializing_if = "Option::is_none")] for optional fields (clean JSON output)
• #[serde(default)] for fields that should have fallback values during deserialization
• #[serde(rename = "...")] when Rust field names differ from wire format
• #[serde(flatten)] used judiciously (can cause key collisions)
• No #[serde(deny_unknown_fields)] on types that need forward compatibility
• No fields or variants named gen — reserved keyword in edition 2024 (use r#gen or rename)

Database Integration (sqlx)

• #[derive(sqlx::Type)] enums use consistent representation with serde
• Enum variant casing matches between serde (rename_all) and sqlx (rename_all)

Edition 2024 Compatibility

• No fields or enum variants named gen (reserved keyword — use r#gen with #[serde(rename = "gen")] or choose a different name)
• Custom Serialize/Deserialize impls returning impl Trait account for RPIT lifetime capture changes (all in-scope lifetimes captured by default; use + use<'a> for precise control)
• Deserialization error paths handle never_type_fallback — ! falls back to ! instead of (), which affects match exhaustiveness on Result<T, !> patterns

Correctness

• Round-trip tests exist for complex types (serialize → deserialize → assert_eq)
• PartialEq derived for types with round-trip tests
• No lossy conversions (e.g., f64 → i64 in JSON numbers)
• Decimal used for money/precision-sensitive values, not f64

Severity Calibration

Critical

• Enum representation mismatch between serializer and deserializer (data loss)
• Missing #[serde(rename)] causing API-breaking field name changes
• #[serde(flatten)] causing silent key collisions
• Lossy numeric conversions (f64 precision loss for monetary values)

Major

• Inconsistent rename_all across related types (confusing API)
• Missing skip_serializing_if causing null/empty noise in output
• deny_unknown_fields on types consumed by evolving APIs (breaks forward compatibility)
• Missing round-trip tests for complex enum representations
• Field or variant named gen without r#gen escape (edition 2024 compile failure)

Minor

• Unnecessary #[serde(default)] on required fields
• Using string representation for enums when numeric would be more efficient
• Verbose custom implementations where derive + attributes suffice
• Using #[allow(unused)] instead of #[expect(unused)] for serde-only fields (prefer self-cleaning lint suppression)

Informational

• Suggestions to switch enum representation for cleaner wire format
• Suggestions to add #[non_exhaustive] alongside serde for forward compatibility

Valid Patterns (Do NOT Flag)

• Externally tagged enums — serde's default, valid for many use cases
• #[serde(untagged)] enums — Valid when discriminated by structure, not by tag
• serde_json::Value for dynamic data — Appropriate for truly schema-less fields
• #[serde(skip)] on computed fields — Correct for derived/cached values
• #[serde(with = "...")] for custom formats — Standard for dates, UUIDs, etc.
• r#gen with #[serde(rename = "gen")] — Correct edition 2024 workaround for gen fields in wire formats
• + use<'a> on custom serializer return types — Precise RPIT lifetime capture (edition 2024)

Before Submitting Findings

Complete Gates (before reporting findings) above; gate 3 incorporates the review-verification-protocol skill for serde-related issue types.