bug#9035: 24.0.50; byte-compiler warnings with defstruct and lexical-binding

[Lawrence Mitchell]

In files with lexical-binding set to t, structure definitions
with :read-only slots lead to a byte-compiler warning about
the unused variable cl-x.

To reproduce, byte-compile a file consisting of:

;; -*- lexical-binding: t -*-
(eval-when-compile (require 'cl))
(defstruct foo (name nil :read-only t))

This is due to the setf-method that is produced when expanding
the defstruct form:
(define-setf-method ... (cl-x)
   (error (format "%s is a read-only slot" ...)))

The following patch fixes the problem by changing the setf-method
to:

(define-setf-method ... (cl-x)
  (progn (ignore cl-x)
         (error (format "%s is a read-only slot" ...))))

Commit message/changelog entry

Silence byte-compiler warning with :read-only defstruct slots

* emacs-lisp/cl-macs.el (defstruct): Ignore argument to setf method if
slot is read-only.

Patch:

diff --git a/lisp/emacs-lisp/cl-macs.el b/lisp/emacs-lisp/cl-macs.el
index 2813cc4..6181c6b 100644
--- a/lisp/emacs-lisp/cl-macs.el
+++ b/lisp/emacs-lisp/cl-macs.el
@@ -2389,8 +2389,10 @@ value, that slot cannot be set via `setf'.
              (push (cons accessor t) side-eff)
              (push (list 'define-setf-method accessor '(cl-x)
                             (if (cadr (memq :read-only (cddr desc)))
-                                (list 'error (format "%s is a read-only slot"
-                                                     accessor))
+                                 (list 'progn '(ignore cl-x)
+                                       (list 'error
+                                             (format "%s is a read-only slot"
+                                                     'accessor)))
                               ;; If cl is loaded only for compilation,
                               ;; the call to cl-struct-setf-expander would
                               ;; cause a warning because it may not be