refactor(compiler): data-driven statement dispatch in LLVMCodeGenerator (#170)

[KrisSimon]

Migrated from GitLab MR !231 (open)
refactor/170-llvm-codegen-dispatch-table → main
Originally created: 2026-04-06
Author: Kris Simon
Labels: 0.9

Summary

Closes #170. `LLVMCodeGenerator.generateStatement()` used a 20-line `switch` on concrete `Statement` subtypes. Adding a new statement kind meant editing the switch, and the dispatch logic could not be tested in isolation without compiling a full program.

What changed

• `Sources/AROCompiler/LLVMC/LLVMCodeGenerator.swift` — introduce `StatementHandler = (LLVMCodeGenerator) -> (Statement, Int, BasicBlock) -> Void` and a static `[ObjectIdentifier: StatementHandler]` table keyed by each concrete `Statement` metatype. Each handler takes a generator instance and returns a closure that performs the checked downcast and forwards to the corresponding `generate...` instance method — no captured `self`, no reference cycles, no per-instance setup. `generateStatement` becomes a two-line lookup with a single fall-through error path.
• Expose an `internal static var supportedStatementTypeIdentifiers: Set` hook so the test suite can pin the expected coverage.
• `Tests/AROCompilerTests/LLVMCodeGeneratorTests.swift` — new `testStatementDispatchTableCoversSupportedTypes` regression test that locks in the set of supported `Statement` subtypes (`AROStatement`, `MatchStatement`, `ForEachLoop`, `RangeLoop`, `WhileLoop`, `BreakStatement`, `PublishStatement`, `RequireStatement`). Accidental drift — e.g. removing a handler, or adding a new `Statement` subtype to the AST without wiring it into code-gen — now fails loudly at test time instead of silently taking the error path at first use.

Why this shape

Using a static method-reference table (`(LLVMCodeGenerator) -> (Statement, ...) -> Void`) keeps the handlers in a stored `static let` with immutable shared state — annotated `nonisolated(unsafe)` because the closures are pure forwarders with no captured mutable state, which is sound under Swift 6 strict concurrency. This gives us data-driven dispatch without the overhead or reference-cycle hazards of per-instance closure tables captured in `init`.

Behavior preservation

Pure refactor: `PipelineStatement` and `ErrorStatement` continue to fall through to the default error branch exactly as before — the refactor deliberately does not change the set of supported statement kinds. Any follow-up that adds a new supported kind is a one-line table entry + a one-line test expectation.

Test plan

• `swift build`
• `swift test` — 1576/1576 pass
• `swift test --filter LLVMCodeGeneratorTests` — all 9 tests (including the new regression test) pass
• `swift run aro build Examples/HelloWorld` + run — OK
• `swift run aro build Examples/Iteration` + run — OK (exercises `ForEachLoop` codegen)