Expose local phase levels to rules

まず、Resyntax の現在の expansion observer hook とソース解析フローを調査し、訪問がどのように表現され、ルールがどのようにフェーズ情報を取得しているかに注目してください。この issue では、ルールにローカルなフェーズレベルを公開することが提案されています。完了条件には、フェーズ 0 の動作を維持したまま、begin-for-syntax の例のリファクタリングを成功させることを含めるべきです。

This rule:

(define-refactoring-rule redundant-or
  #:description "This `or` expression does nothing and can be replaced with its subexpression."
  #:literals (or)
  (or a1:id a2:id)
  #:when (free-identifier=? #'a1 #'a2)
  a1)

Fails to refactor this code:

(begin-for-syntax
  (define x 42)
  (or x x))

The reason for that is that free-identifier=? checks if two identifiers have the same binding at a specific phase level, and by default it uses (syntax-local-phase-level). This is reasonable for macros, which set (syntax-local-phase-level) to the level of the macro invocation. But refactoring rules aren't run during macro expansion, so (syntax-local-phase-level) always returns zero. This lets free-identifier=? in rules work fine on phase 0 code, but it fails on phase 1 code such as code inside begin-for-syntax.

Resyntax ought to expose the local phase level to rules. I'm not sure how to get that information out of the expander using the current expansion observer hook, but it's probably possible. One way to expose it to rules might be to add a (resyntax-local-phase-level) parameter and let rules use that to access the phase level. On the Resyntax source analysis side, there could be an expansion-visit struct / record type containing the syntax object that the expander visited and the phase level the visit occurred at.