Warn about unwind-protect without unwind forms

`unwind-protect` without unwind forms is not just pointless but often
indicates a mistake where the intended unwind part is misplaced, as in

    (unwind-protect (progn PROT-FORMS UNWIND-FORMS))   ; oops

or

    (unwind-protect PROT-FORM) UNWIND-FORMS            ; also oops

or entirely forgotten for that matter.  Warning about this makes
sense, and the warning can always be silenced by removing the
`unwind-protect` altogether if it shouldn't be there in the first
place.

* lisp/emacs-lisp/macroexp.el (macroexp--expand-all): Implement
warning.
* etc/NEWS: Announce.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-test--with-suppressed-warnings): Add test case.

etc/NEWS

@@ -418,6 +418,21 @@ was to catch all errors, add an explicit handler for 'error', or use
 This warning can be suppressed using 'with-suppressed-warnings' with
 the warning name 'suspicious'.
 
+---
+*** Warn about 'unwind-protect' without unwind forms.
+The compiler now warns when the 'unwind-protect' form is used without
+any unwind forms, as in
+
+    (unwind-protect (read buffer))
+
+because the behaviour is identical to that of the argument; there is
+no protection of any kind.  Perhaps the intended unwind forms have
+been misplaced or forgotten, or the use of 'unwind-protect' could be
+simplified away.
+
+This warning can be suppressed using 'with-suppressed-warnings' with
+the warning name 'suspicious'.
+
 +++
 ** New function 'file-user-uid'.
 This function is like 'user-uid', but is aware of file name handlers,

lisp/emacs-lisp/macroexp.el

@@ -383,6 +383,11 @@ Assumes the caller has bound `macroexpand-all-environment'."
               (format-message "missing `while' condition")
               `(signal 'wrong-number-of-arguments '(while 0))
               nil 'compile-only form))
+            (`(unwind-protect ,expr)
+             (macroexp-warn-and-return
+              (format-message "`unwind-protect' without unwind forms")
+              (macroexp--expand-all expr)
+              (list 'suspicious 'unwind-protect) t form))
             (`(setq ,(and var (pred symbolp)
                           (pred (not booleanp)) (pred (not keywordp)))
                     ,expr)

test/lisp/emacs-lisp/bytecomp-tests.el

@@ -1461,6 +1461,12 @@ literals (Bug#20852)."
    '((suspicious condition-case))
    "Warning: `condition-case' without handlers")
 
+  (test-suppression
+   '(defun zot (x)
+      (unwind-protect (print x)))
+   '((suspicious unwind-protect))
+   "Warning: `unwind-protect' without unwind forms")
+
   (test-suppression
    '(defun zot ()
       (let ((_ 1))