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

default-recommendations/syntax-shortcuts-test.rkt

@@ -53,3 +53,18 @@ test: "making a symbol with format from a keyword can be simplified to format-sy
 test: "making a symbol with format from a keyword syntax object can be simplified to format-symbol"
 - (string->symbol (format "make-~a" (keyword->string (syntax-e #'#:foo))))
 - (format-symbol "make-~a" #'#:foo)
+
+
+test: "flattening nested syntax templates with apply append can be simplified"
+--------------------
+(require racket/syntax)
+(define (f stx)
+  (with-syntax ([((a ...) ...) stx])
+    (apply append
+           (map syntax->list (syntax->list #'((a ...) ...))))))
+====================
+(require racket/syntax)
+(define (f stx)
+  (with-syntax ([((a ...) ...) stx])
+    (syntax->list (syntax (a ... ...)))))
+--------------------

default-recommendations/syntax-shortcuts.rkt

@@ -11,6 +11,7 @@
 
 (require racket/string
          racket/syntax
+         racket/list
          rebellion/private/static-name
          resyntax/base
          syntax/parse)
@@ -103,6 +104,28 @@
   (format-symbol template (~replacement arg.simplified #:original arg) ...))
 
 
+(define-refactoring-rule flatten-apply-append-syntax-template
+  #:description
+  "Flattening nested syntax templates with `apply append` and `map syntax->list` can be simplified\
+ by using a single `syntax->list` call on a flattened template."
+  #:literals (apply append map syntax->list syntax ...)
+
+  (apply append (map syntax->list (syntax->list (syntax ((inner ...) ...)))))
+
+  #:with flattened-template
+  (let* ([inner-attrs (attribute inner)]
+         ; When matching ((inner ...) ...), the inner attribute contains the elements from
+         ; within each nested list. If there's only one element, inner-attrs will be a single
+         ; syntax object, otherwise it's a list. We need a list for appending the ellipses.
+         [inner-list (if (list? inner-attrs) inner-attrs (list inner-attrs))]
+         [ellipsis-sym (datum->syntax #'here '...)])
+    ; Construct (inner-elements ... ...) by appending two ellipsis symbols
+    (datum->syntax #'here (append inner-list (make-list 2 ellipsis-sym))))
+
+  (syntax->list #'flattened-template))
+
+
 (define-refactoring-suite syntax-shortcuts
-  #:rules (format-string-to-format-symbol
+  #:rules (flatten-apply-append-syntax-template
+           format-string-to-format-symbol
            syntax-e-in-format-id-unnecessary))