in-value: fix inlined binding

The inlined version of `in-value` in a `for` form did not bind the
left-hand identifier at the right layer relative to other bindings in
`for/fold`, which caused the inlined `in-value` to behave differently
than a non-inlined `in-value`. Confusing about this `in-value`, in
turn, had led to incorrect documentation on `for/fold`.

It would be nice to clean up a little more of the evaluator and
availability of bindings in `for/fold`, but doing so runs a
significant risk of breaking existing code (unlike fixing `in-value`,
which behaved even worse and where the repair seems less likely to
break existing programs).

Closes #1659 (again)

pkgs/racket-doc/scribblings/reference/for.scrbl

@@ -374,19 +374,19 @@ terminates, if a @racket[result-expr] is provided then the result of the
 ]
 
 The binding and evaluation order of @racket[accum-id]s and
-@racket[init-expr]s does not follow the textual, left-to-right order
-relative to the @racket[for-clause]s . Instead, the sequence
-expressions in @racket[for-clause]s that determine the outermost
-iteration are evaluated first, the associated identifiers are bound,
-and then the @racket[init-expr]s are evaluated and the
-@racket[accum-id]s are bound. One consequence is that the
-@racket[accum-id]s are not bound in @racket[for-clause]s for the
-outermost initialization. Another consequence is that when a
-@racket[accum-id] is used as a @racket[for-clause] binding for the
-outermost iteration, the @racket[for-clause] binding is shadowed in
-the loop body (even though, syntactically, a @racket[for-clause] is
-closer to the body). A fresh variable for each @racket[accum-id] (at a
-fresh location) is bound to in each nested iteration created by a
+@racket[init-expr]s do not completely follow the textual,
+left-to-right order relative to the @racket[for-clause]s. Instead, the
+sequence expressions in @racket[for-clause]s that determine the
+outermost iteration are evaluated first, then the @racket[init-expr]s
+are evaluated and the @racket[accum-id]s are bound, and finally the
+outermost iteration's identifiers are bound. One consequence is that
+the @racket[accum-id]s are not bound in @racket[for-clause]s for the
+outermost initialization. At the same time, when a @racket[accum-id]
+is used as a @racket[for-clause] binding for the outermost iteration,
+the @racket[for-clause] binding shadows the @racket[accum-id] binding
+in the loop body (which is what you would expect syntactically).
+A fresh variable for each @racket[accum-id] (at a
+fresh location) is bound in each nested iteration that is created by a
 later group for @racket[for-clause]s (after a @racket[#:when] or
 @racket[#:unless], for example).
 

pkgs/racket-test-core/tests/racket/for.rktl

@@ -461,6 +461,25 @@
   (test 13 next)
   (test #f more?))
 
+(test 1 'in-value-bind-correctly (for/fold ([x #f])
+                                           ([x (in-value 1)])
+                                   x))
+(test 2 'in-value-bind-correctly (for/fold ([x #f])
+                                           ([x (values (in-value 2))])
+                                   x))
+
+(let ([x 'out]
+      [prints '()])
+  (for/fold ([x (begin
+                  (set! prints (cons (list 'top x) prints))
+                  'top)])
+            ([x (in-list (begin
+                           (set! prints (cons (list 'rhs x) prints))
+                           (list 1 2 3)))])
+    (set! prints (cons x prints))
+    x)
+  (test '(3 2 1 (top out) (rhs out)) values prints))
+
 ;; check ranges on `in-vector', especially as a value
 (test '() 'in-empty-vector (let ([v (in-vector '#())]) (for/list ([e v]) e)))
 (test '() 'in-empty-vector (let ([v (in-vector '#() 0)]) (for/list ([e v]) e)))

racket/collects/racket/private/for.rkt

@@ -2309,8 +2309,8 @@
     (lambda () #'in-value)
     (lambda (stx)
       (syntax-case stx ()
-        [[(id) (_ expr)]
-         #'[(id) (:do-in ([(id) expr]) #t () #t () #t #f ())]]
+        [[<(id) (_ expr)]
+         #'[(id) (:do-in ([(id*) expr]) #t () #t ([(id) id*]) #t #f ())]]
         [_ #f])))
 
   (define-sequence-syntax *in-producer