(error-data): Delete function

Remove `error-data` from the new error API: it is not really
compatible with a more abstract view of error descriptors, and
in practice it seems to be used only in two ways (both of them rare):

- To add some contextual info to an error.  We should maybe add
  a dedicated support for that, but it's not clear what shape it
  should take, ideally (there was a discussion about with Alan
  and myself in emacs-devel a few years ago).
- To do some other massaging whose correctness is dubious anyway.

* doc/lispref/control.texi (Handling Errors): Remove `error-data`.

* lisp/epa-file.el (epa-file--find-file-not-found-function): Don't use
`error-data`.
(epa-file--error-add-context): New function, extracted from
`epa-file-insert-file-contents`.
(epa-file-insert-file-contents): Use it instead of `error-data`.

* lisp/subr.el (error-data): Delete function.

doc/lispref/control.texi

@@ -2483,10 +2483,6 @@ to @code{condition-case} whose error condition you want to re-throw.
 This function returns the error symbol of the error descriptor @var{error}.
 @end defun
 
-@defun error-data error
-This function returns the data of the error descriptor @var{error}.
-@end defun
-
 @defun error-slot-value error pos
 This function returns the value in the field number @var{pos} of the error
 descriptor @var{error}.  The fields are numbered starting with 1.  E.g.,

lisp/epa-file.el

@@ -116,11 +116,15 @@ encryption is used."
 (defun epa-file--find-file-not-found-function ()
   (let ((error epa-file-error))
     (save-window-excursion
-      (kill-buffer))
+     (kill-buffer))
+    ;; FIXME: How do we know that slot 3 can hold only a message related
+    ;; to a wrong passphrase?
     (if (error-slot-value error 3)
         (user-error "Wrong passphrase: %s" (error-slot-value error 3))
+      ;; FIXME: Why does it make sense to add the data fields of ERROR,
+      ;; shifted by one position?
       (signal 'file-missing
-	      (cons "Opening input file" (error-data error))))))
+	      (cons "Opening input file" (cdr error))))))
 
 (defun epa--wrong-password-p (context)
   "Return whether a wrong password caused the error in CONTEXT."
@@ -136,6 +140,9 @@ encryption is used."
           error-string)
          (match-string 1 error-string))))
 
+(defun epa-file--error-add-context (err ctxt)
+  (setf (cdr error) (append (cdr error) (list ctx))))
+
 (defvar last-coding-system-used)
 (defun epa-file-insert-file-contents (file &optional visit beg end replace)
   (barf-if-buffer-read-only)
@@ -182,8 +189,7 @@ encryption is used."
                      ;; Don't display the *error* buffer if we just
                      ;; have a wrong password; let the later error
                      ;; handler notify the user.
-                     (setf (error-data error)
-                           (append (error-data error) (list wrong-password)))
+                     (epa-file--error-add-context error wrong-password)
 		   (epa-display-error context))
                  ;; When the .gpg file isn't an encrypted file (e.g.,
                  ;; it's a keyring.gpg file instead), then gpg will
@@ -201,8 +207,10 @@ encryption is used."
 		   (add-hook 'find-file-not-found-functions
 			     #'epa-file--find-file-not-found-function
 			     nil t)))
+	       ;; FIXME: Why does it make sense to add the data fields
+	       ;; of ERROR, shifted by one position?
 	       (signal (if exists 'file-error 'file-missing)
-		       (cons "Opening input file" (error-data error))))))
+		       (cons "Opening input file" (cdr error))))))
           (set-buffer buf) ;In case timer/filter changed/killed it (bug#16029)!
 	  (setq-local epa-file-encrypt-to
                       (mapcar #'car (epg-context-result-for

lisp/subr.el

@@ -586,10 +586,6 @@ Defaults to `error'."
  "Return the symbol which represents the type of ERROR.
 \n(fn ERROR)")
 
-(defalias 'error-data #'cdr
-  "Return the slots attached to ERROR, as a list.
-\n(fn ERROR)")
-
 (defun error-has-type-p (error condition)
   "Return non-nil if ERROR is of type CONDITION (or a subtype of it)."
   (unless (error--p error)