read-print: 'read-with-comments' reads comments within gexps.

Fixes <https://issues.guix.gnu.org/62059>.
Reported by Maxim Cournoyer <maxim.cournoyer@gmail.com>.

* guix/read-print.scm (read-with-comments): Special-case #~, #$, and #+.
* tests/read-print.scm: Add two tests.

guix/read-print.scm

@@ -219,6 +219,27 @@ BLANK-LINE? is true, assume PORT is at the beginning of a new line."
               (list 'quote (loop #f return)))
              ((eq? chr #\`)
               (list 'quasiquote (loop #f return)))
+             ((eq? chr #\#)
+              (match (read-char port)
+                (#\~ (list 'gexp (loop #f return)))
+                (#\$ (list (match (peek-char port)
+                             (#\@
+                              (read-char port)    ;consume
+                              'ungexp-splicing)
+                             (_
+                              'ungexp))
+                           (loop #f return)))
+                (#\+ (list (match (peek-char port)
+                             (#\@
+                              (read-char port)    ;consume
+                              'ungexp-native-splicing)
+                             (_
+                              'ungexp-native))
+                           (loop #f return)))
+                (chr
+                 (unread-char chr port)
+                 (unread-char #\# port)
+                 (read port))))
              ((eq? chr #\,)
               (list (match (peek-char port)
                       (#\@

tests/read-print.scm

@@ -231,6 +231,21 @@ mnopqrstuvwxyz.\")"
   ;; Regular indentation for 'replace' here.
   (replace \"gmp\" gmp))")
 
+(test-pretty-print "\
+#~(modify-phases phases
+    (add-after 'whatever 'something-else
+      (lambda _
+        ;; This comment appears inside a gexp.
+        42)))")
+
+(test-pretty-print "\
+#~(list #$@(list coreutils ;yup
+                 grep) ;margin comment
+        #+sed
+
+        ;; Line comment.
+        #$grep)")
+
 (test-pretty-print "\
 (package
   ;; Here 'sha256', 'base32', and 'arguments' must be