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.

1 files changed, 15 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 9e45b1d80b2..f27cafb3d41 100644
--- a/etc/NEWS
+++ b/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,