05 — Structural parse II: structs, enums, traits, impls — the full symbol table

[rahlk]

Part of #9. Phase 1 — level 1.

Learning goals

Rust's data-and-behavior split as an analysis subject (struct vs impl vs trait — you are now
modeling the thing TRPL taught you), collections at scale (TRPL Common Collections), interior
organization of a large module, two-pass algorithms over owned data.

Task

Extend the builder to the full node-kind set, one build_* method each:

• structs / enums-with-data / unions — fields with types + visibility, variant payloads,
  derives + attributes into decorators, generics + lifetimes + where-clauses.
• traits — required + default methods (default methods are callables WITH bodies — they get
  call sites), supertraits into base_classes.
• impl blocks — the crux. impl Point { ... } and impl Display for Point { ... } attach
  methods to the TYPE's entry; trait impls also record Trait into the type's base_classes
  and use the <Type as Trait>::method(...) signature form.
• modules (mod foo { ... } inline and file-based) shape the signature path; use imports
  into imports[] (you need them for issue 06's resolution map).
• module-level static/const into variables[]; macros per your issue-02 stance.

This requires the two-pass build. An impl block may live in a different file from its type
(the exact cross-file trap the skill calls out for Go — Rust has it via sibling-file impls):
pass 1 indexes every type declaration (crate, path) -> owning module; pass 2 attaches methods
through that index. Single-pass builders silently drop every sibling-file method.

Teacher's notes

• Grow the fixture NOW: a struct in shapes.rs, its impl in shapes_impl.rs, a trait with a
  default method, an enum with data variants, a pub and a private item.
• Ask of every new field: will a consumer branch on it? → typed field. Descriptive only? → tags.
  (references/schema-reference.md § expansion rubric.)

Gate (full symbol-table gate)

• Cross-file attachment test: the sibling-file impl's methods appear under the struct — the
  single highest-value test of this issue.
• Every Rust-specific field asserted by VALUE at least once (enum variant payload, trait default
  method's call sites, supertrait in base_classes, derive in decorators, pub vs private).
• Re-run reuses cache for unchanged files (mtime unchanged on second lazy run).