Add flatten-apply-append-syntax-template refactoring rule

[Copilot]

Nested syntax template flattening operations can be simplified. The pattern (apply append (map syntax->list (syntax->list #'((a ...) ...)))) does the same work as (syntax->list #'(a ... ...)) but with unnecessary intermediate steps.

Changes

• New refactoring rule flatten-apply-append-syntax-template in syntax-shortcuts.rkt

  • Detects the apply append (map syntax->list ...) pattern on nested syntax templates
  • Replaces with direct syntax->list on flattened template
  • Uses datum->syntax to construct the flattened pattern, avoiding template ellipsis depth restrictions

• Test coverage in syntax-shortcuts-test.rkt validates the transformation

Example

;; Before
(define (f stx)
  (with-syntax ([((a ...) ...) stx])
    (apply append
           (map syntax->list (syntax->list #'((a ...) ...))))))

;; After
(define (f stx)
  (with-syntax ([((a ...) ...) stx])
    (syntax->list (syntax (a ... ...)))))

Note: Output uses (syntax ...) form rather than #' reader syntax, as Resyntax operates on expanded syntax forms.

Original prompt

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This section details on the original issue you should resolve

<issue_title>New rule: flatten-apply-append-syntax-template</issue_title>
<issue_description>### Rule summary

The code (apply append (map syntax->list (syntax->list #'((id ...) ...)))) is more directly expressed as (syntax->list #'(id ... ...)).

Test case

#lang resyntax/test

test: "original code should be refactorable to new code"
--------------------
#lang racket
(define (f stx)
  (with-syntax ([((a ...) ...) stx])
    (apply append
           (map syntax->list (syntax->list #'((a ...) ...))))))
====================
#lang racket
(define (f stx)
  (with-syntax ([((a ...) ...) stx])
    (syntax->list #'(a ... ...))))
--------------------

Additional context

Saw this here in racket/drracket#783.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes New rule: flatten-apply-append-syntax-template #769

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