Rust frontend — derive-macro codegen

• Type: feature — Track A
• Status: backlog
• Owner: —
• Opened: 2026-07-03
• Umbrella: [[feature-rust-frontend]] — sub-ticket 6/12. Depends on [[feature-rust-frontend-skeleton]].

What it does

#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)] synthesizes trait impl bodies mechanically from a struct/enum's field list — not macro_rules! (no token-tree matching/hygiene needed), just compiler-generated code walking fields:

• Clone/Copy — field-wise copy (bitwise for Copy, recursive .clone() calls for Clone).
• Debug — field-wise formatted-string builder (depends on [[feature-rust-misc-semantics]]'s format-string parser for the {:?} path other code uses to print it, but the derive itself just needs to emit a "TypeName { field: value, ... }" builder).
• Default — field-wise Default::default() construction.
• PartialEq/Eq — field-wise == chain.

Real-world confirmation: ~/nextlevel/engine uses these 10x across 6.1k LOC — common, not exotic.

Acceptance

• Each of the 5 derives above, on both plain structs and payload-enum variants (depends on [[feature-rust-match-enum-payload]] for the enum case), produces a correct impl callable the same way a hand-written one would be.
• Nested derive (a struct containing a field whose type also derives the same trait) composes correctly (recursive call, not inlined field access).

Log

• 2026-07-03 — split from [[feature-rust-frontend]] umbrella at ticket-craft time. No code written yet.