Add fuse-map-with-for refactoring rule

[Copilot]

Implements a refactoring rule that fuses map expressions with for loops when the intermediate list is only used once, eliminating unnecessary list construction.

Example transformation:

;; Before
(define ys (map (λ (x) (g x)) xs))
(for* ([y (in-list ys)]
       [z (in-list (h y))])
  (displayln z))

;; After
(for* ([x (in-list xs)]
       [y (in-list (g x))]
       [z (in-list (h y))])
  (displayln z))

Implementation details:

• Multi-clause fusion: For for* loops with multiple clauses, inlines the map body as a nested clause
• Single-clause fusion: For simple for loops, converts map body to internal definitions
• Lambda handling: Supports single-body, multi-body, and let-refactorable lambdas. Multi-body lambdas have their side effects extracted before the final result binding
• Safety check: Only applies when the intermediate list has usage count of 1

Files:

• default-recommendations/loops/fuse-map-with-for.rkt - rule implementation with two variants
• default-recommendations/loops/fuse-map-with-for-test.rkt - test cases covering all lambda forms
• default-recommendations.rkt - added suite to default recommendations

Original prompt

---

This section details on the original issue you should resolve

<issue_title>New rule: fuse-map-with-for</issue_title>
<issue_description>### Rule summary

A map expression that produces a list which is immediately iterated over by a for* loop can be fused with the for* loop, if the intermediate list isn't used elsewhere. This avoids constructing the intermediate list and is usually more readable. This should only be applied when the map expression is given a short lambda.

Test case

#lang resyntax/test

test: "original code should be refactorable to new code"
--------------------
#lang racket
(define (f xs g h)
  (define ys (map (λ (x) (g x)) xs))
  (for* ([y (in-list ys)]
         [z (in-list (h y))])
    (displayln z)))
====================
#lang racket
(define (f xs g h)
  (for* ([x (in-list xs)]
         [y (in-list (g x))]
         [z (in-list (h y))])
    (displayln z)))
--------------------

No-change test case

#lang resyntax/test

no-change-test: "code not refactorable"
--------------------
#lang racket
(define (f xs g h)
  (define ys (map (λ (x) (g x)) xs))
  (for* ([y (in-list ys)]
         [z (in-list (h y))])
    (displayln z))
  (displayln ys))
--------------------

Additional context

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

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

• Fixes New rule: fuse-map-with-for #764

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