Better compilation of arguments to `ignore`

* lisp/emacs-lisp/bytecomp.el (byte-compile-form)
(byte-compile-ignore):
Instead of compiling each `ignore` argument for value which is then
immediately discarded, compile it for effect but suppress
ignore-return-value warnings by passing the special value
`for-effect-no-warn` as for-effect parameter.
Proposed by Stefan Monnier.

diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index 1b28fcd509396993b78525f19cd580511d0f68a2..c84c70971b3ed8c32d5dd72769f7e1dd1e2a57de 100644 (file)
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -3563,6 +3563,8 @@ lambda-expression."
                                ;;delq delete cl-delete
                                ;;nconc plist-put
                                )))
+                   ;; Don't warn for arguments to `ignore'.
+                   (not (eq byte-compile--for-effect 'for-effect-no-warn))
                    (byte-compile-warning-enabled-p
                     'ignored-return-value (car form)))
               (byte-compile-warn-x
@@ -4446,11 +4448,8 @@ This function is never called when `lexical-binding' is nil."
 
 (defun byte-compile-ignore (form)
   (dolist (arg (cdr form))
-    ;; Compile args for value (to avoid warnings about unused values),
-    ;; emit a discard after each, and trust the LAP peephole optimiser
-    ;; to annihilate useless ops.
-    (byte-compile-form arg)
-    (byte-compile-discard))
+    ;; Compile each argument for-effect but suppress unused-value warnings.
+    (byte-compile-form arg 'for-effect-no-warn))
   (byte-compile-form nil))
 
 ;; Return the list of items in CONDITION-PARAM that match PRED-LIST.