bug#48738: [PATCH] Optimized `describe-variable' predicate

[Lars Ingebrigtsen <larsi@gnus.org>]

Daniel Mendler <mail@daniel-mendler.de> writes:

> Do not switch to the original buffer.  Use `buffer-local-value`
> instead.  The buffer switching is more costly, which slows down
> continuously updating completion UIs like Icomplete or Vertico if
> there are many variables.

Thanks; applied to Emacs 28 with some changes -- I thought the
`ignore-errors' logic was kinda unclear, so I rewrote that bit using an
explicit `condition-case' checking for the error thrown in this case.

Adjusted patch included below; hopefully I didn't introduce any errors
here...

> Ideally there would be a `buffer-local-boundp` predicate we could use
> here.

Yes, that would be nice, I think.

And we probably have a lot of code in Emacs that uses
`with-current-buffer' instead of `buffer-local-value' that could be
similarly optimised (but I guess it mostly only makes sense to do so
when it's called in an intensive loop like here).

diff --git a/lisp/help-fns.el b/lisp/help-fns.el
index 666583db72..c8f078cb85 100644
--- a/lisp/help-fns.el
+++ b/lisp/help-fns.el
@@ -1024,12 +1024,15 @@ describe-variable
                 (format-prompt "Describe variable" (and (symbolp v) v))
                 #'help--symbol-completion-table
                 (lambda (vv)
-                  ;; In case the variable only exists in the buffer
-                  ;; the command we switch back to that buffer before
-                  ;; we examine the variable.
-                  (with-current-buffer orig-buffer
-                    (or (get vv 'variable-documentation)
-                        (and (boundp vv) (not (keywordp vv))))))
+                  (or (get vv 'variable-documentation)
+                      (and (not (keywordp vv))
+                           ;; Since the variable may only exist in the
+                           ;; original buffer, we have to look for it
+                           ;; there.
+                           (condition-case nil
+                               (buffer-local-value vv orig-buffer)
+                             (:success t)
+                             (void-variable nil)))))
                 t nil nil
                 (if (symbolp v) (symbol-name v))))
      (list (if (equal val "")


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