lowerer: compile-time class-body declarations (enum / attribute / literal define_method) for Writebook + Lobsters #30

Motivation

Inspired by matz/spinel#1328 ("Add compile-time class-body declarations"), which adds a compile-time-resolvable path for class-body metaprogramming (attribute/attributes, literal-name define_method, literal .each method generation) while rejecting dynamic metaprogramming during analysis. That is exactly roundhouse's "transpile-time-resolvable Ruby" thesis.

This issue inventories the class-body declaration / metaprogramming gaps surfaced by auditing two anchor apps. Scope: this is the metaprogramming/DSL-expansion dimension only — it is not a complete port checklist for either app.

Today roundhouse recognizes only a fixed DSL set in src/ingest/model.rs (validates, scope, callbacks, has_many/has_one/belongs_to, plus marker/broadcasts_to special-cases in src/lower/model_to_library/). Everything else becomes ModelBodyItem::Unknown — re-emitted verbatim by the Ruby emitter, but producing nothing usable for the nominal targets (Rust/Go/Crystal/etc.).

What the anchor apps actually use

Writebook (~/git/writebook)

• enum ×5 — book.rb (enum :theme, %w[black blue green magenta orange violet white].index_by(&:itself), suffix: true, default: :blue), edit.rb, access.rb, leaf.rb, user/role.rb (enum :role, %i[member administrator], default: :member). Canonical literal-array → method generation (predicates, bang methods, scopes, mappings). Currently dropped to Unknown.
• attribute — current.rb:2 attribute :session, :user (ActiveSupport::CurrentAttributes form).
• literal define_method ×3 — app/models/concerns/positionable.rb:19-29, defined inside the parametric class-method macro positioned_within(parent, association:, filter:). Single call site, fully literal args: leaf.rb:6 positioned_within :book, association: :leaves, filter: :active. The generated methods are required by Leaf (previous/next/move_to_position depend on them), so emitting Unsupported here breaks Leaf.
• class_eval <<-CODE heredoc — lib/rails_ext/action_text_has_markdown.rb:7. Genuinely dynamic string-eval → should be Unsupported, never expanded.

Lobsters (~/git/ruby-bench/benchmarks/lobsters)

• attribute typed/virtual columns — replying_comment.rb:2 attribute :is_unread, :boolean, message.rb:14 attribute :mod_note, :boolean.
• No class-body define_method in app/lib. ([:error,:success,:notice].each do |f| exists only in a view — runtime loop, already fine.)

Work items

1. enum lowerer (highest leverage). Recognize enum :name, <literal> (%w[...].index_by(&:itself), %i[...], hash forms) + suffix:/prefix:/default: options. Expand into generated predicate/bang/scope methods + value↔int mapping, lowered to IR so all targets get real code instead of passthrough-Unknown. Covers 5 Writebook sites.

2. attribute / attributes lowerer. Typed virtual attributes (attribute :x, :boolean) → typed accessor + default; recognize the CurrentAttributes (Current) form. Covers both apps.

3. Compile-time define_method + concern-macro inlining (medium leverage, harder). Partial-evaluate an ActiveSupport::Concern class_methods macro at its literal call site, substituting literal args, then expand literal-name define_method bodies into real method defs (incl. collapsing send(:literal) → direct call). Required for Writebook Leaf. Single literal call site = best case for the bounded expander.

4. Reject dynamic metaprogramming via diagnostics (diagnostics: structured "unsupported construct" reporting from lowerers + emitters (degrade, don't panic) #28 machinery). class_eval/module_eval/string-eval/non-literal define_method → structured Unsupported diagnostic, never expanded.

Suggested order

1 (enum) → 2 (attribute) → 4 (reject dynamic) → 3 (define_method/concern inlining). Items 1–2 are targeted DSL lowerers that slot next to associations.rs/validations.rs; item 4 reuses existing diagnostics; item 3 is the only net-new machinery.

Refs: #21 (lobsters anchor), #28 (unsupported diagnostics).