Peephole optimization for iterating over an empty set (no allocation).

collects/racket/set.rkt

@@ -309,6 +309,8 @@
   (lambda () #'in-set)
   (lambda (stx)
     (syntax-case stx (set)
+      ;; Set construction is costly, so specialize empty/singleton cases
+      [[(id) (_ (set))] #'[(id) (:do-in ([(id) #f]) #t () #f () #f #f ())]]
       [[(id) (_ (set expr))] #'[(id) (:do-in ([(id) expr]) #t () #t () #t #f ())]]
       [[(id) (_ st)]
        #`[(id)

collects/tests/racket/set.rktl

@@ -105,7 +105,11 @@
   (test '(1 2 3) sort (for/list ([v s]) v) <)
   (test '(1 2 3) sort (for/list ([v (in-set s)]) v) <)
   (test '(1 2 3) sort (let ([seq (in-set s)]) (for/list ([v seq]) v)) <)
+  ;; Optimized
   (test '(1) sort (for/list ([v (in-set (set 1))]) v) <)
+  (test #t values (let ([noset #t])
+                    (for ([v (in-set (set))]) (set! noset #f))
+                    noset))
         
 
   (void))