lower x for x in ... to identity function in generator exprs (#34119)

Author: JeffBezanson

src/julia-syntax.scm

@@ -1612,22 +1612,28 @@
                       (else
                        `(,@(map (lambda (v) `(local ,v)) myvars)
                          (= (tuple ,@vars) ,argname))))))
-    (if (and (null? splat)
-             (length= expr 3) (eq? (car expr) 'call)
-             (eq? (caddr expr) argname)
-             (not (dotop-named? (cadr expr)))
-             (not (expr-contains-eq argname (cadr expr))))
-        (cadr expr)  ;; eta reduce `x->f(x)` => `f`
-        (let ((expr (cond ((and flat (pair? expr) (eq? (car expr) 'generator))
-                           (expand-generator expr #f (delete-duplicates (append outervars myvars))))
-                          ((and flat (pair? expr) (eq? (car expr) 'flatten))
-                           (expand-generator (cadr expr) #t (delete-duplicates (append outervars myvars))))
-                          ((pair? outervars)
-                           `(let (block ,@(map (lambda (v) `(= ,v ,v)) (filter (lambda (x) (not (underscore-symbol? x)))
-                                                                               outervars)))
-                              ,expr))
-                          (else expr))))
-          `(-> ,argname (block ,@splat ,expr))))))
+    (cond
+     ((eq? expr argname)
+      ;; use `identity` for x->x
+      `(top identity))
+     ((and (null? splat)
+           (length= expr 3) (eq? (car expr) 'call)
+           (eq? (caddr expr) argname)
+           (not (dotop-named? (cadr expr)))
+           (not (expr-contains-eq argname (cadr expr))))
+      ;; eta reduce `x->f(x)` => `f`
+      (cadr expr))
+     (else
+      (let ((expr (cond ((and flat (pair? expr) (eq? (car expr) 'generator))
+                         (expand-generator expr #f (delete-duplicates (append outervars myvars))))
+                        ((and flat (pair? expr) (eq? (car expr) 'flatten))
+                         (expand-generator (cadr expr) #t (delete-duplicates (append outervars myvars))))
+                        ((pair? outervars)
+                         `(let (block ,@(map (lambda (v) `(= ,v ,v)) (filter (lambda (x) (not (underscore-symbol? x)))
+                                                                             outervars)))
+                            ,expr))
+                        (else expr))))
+        `(-> ,argname (block ,@splat ,expr)))))))
 
 (define (expand-generator e flat outervars)
   (let* ((expr  (cadr e))