gexp: Correctly handle unquoting S-exp objects.

This fixes a false-positive in the linter:

  guix lint -c 'wrapper-inputs' libaio

* guix/gexp.scm (gexp->approximate-sexp): Allow the 'thing' in <gexp-input> to
  be a sexp, without approximation, by testing if it is a record.
* tests/gexp.scm ("unquoted sexp (not a gexp!)"): Test it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

guix/gexp.scm

@@ -4,7 +4,7 @@
 ;;; Copyright © 2018 Jan Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2019, 2020 Mathieu Othacehe <m.othacehe@gmail.com>
 ;;; Copyright © 2020 Maxim Cournoyer <maxim.cournoyer@gmail.com>
-;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
+;;; Copyright © 2021, 2022 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -174,12 +174,14 @@ As a result, the S-expression will be approximate if GEXP has references."
          (map (lambda (reference)
                 (match reference
                   (($ <gexp-input> thing output native)
-                   (if (gexp-like? thing)
-                       (gexp->approximate-sexp thing)
-                       ;; Simply returning 'thing' won't work in some
-                       ;; situations; see 'write-gexp' below.
-                       '(*approximate*)))
-                  (_ '(*approximate*))))
+                   (cond ((gexp-like? thing)
+                          (gexp->approximate-sexp thing))
+                         ((not (record? thing)) ; a S-exp
+                          thing)
+                         (#true
+                          ;; Simply returning 'thing' won't work in some
+                          ;; situations; see 'write-gexp' below.
+                          '(*approximate*))))))
               (gexp-references gexp))))
 
 (define (write-gexp gexp port)

tests/gexp.scm

@@ -1,6 +1,6 @@
 ;;; GNU Guix --- Functional package management for GNU
 ;;; Copyright © 2014-2022 Ludovic Courtès <ludo@gnu.org>
-;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
+;;; Copyright © 2021-2022 Maxime Devos <maximedevos@telenet.be>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -121,6 +121,19 @@
   (let ((inside (file-append coreutils "/bin/hello")))
     (gexp->approximate-sexp #~(display '#$inside))))
 
+;; See <https://issues.guix.gnu.org/54236>.
+(test-equal "unquoted sexp (not a gexp!)"
+  '(list #(foo) (foo) () "foo" foo #xf00)
+  (let ((inside/vector #(foo))
+        (inside/list '(foo))
+        (inside/empty '())
+        (inside/string "foo")
+        (inside/symbol 'foo)
+        (inside/number #xf00))
+    (gexp->approximate-sexp
+     #~(list #$inside/vector #$inside/list #$inside/empty #$inside/string
+             #$inside/symbol #$inside/number))))
+
 (test-equal "no refs"
   '(display "hello!")
   (let ((exp (gexp (display "hello!"))))