feat(tazjin/emacs): implement reliably-switch-buffer

Adds a completing-read function (defaulting to ivy for me, but it
doesn't matter) that offers a reliable alternative to standard
buffer-switching implementations.

In particular, this implementation retains a mapping of
buffer names to their buffer *objects*, so that the correct buffer is
selected even if some renaming took place during the selection.

I tried to account for a bunch of the common behaviours I could think
of:

* invisible buffers are ... invisible
* entering a buffer name manually creates that buffer, if there is no
  match
* ... unless that buffer is an invisible buffer, in which case it is
  selected and switched to
* the first element is always `(other-buffer (current-buffer))`,
  because of the ordering of #'buffer-list

Yet, the entire code of my implementation is less than the *setup*
code of ivy-switch-buffers, so it's possible I missed something. Well,
I'll find out ...

Change-Id: I08be0da0863d06c9a930e5efaf916719655db90e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9147
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

users/tazjin/emacs/config/bindings.el

@@ -1,3 +1,6 @@
+;; Switch buffers reliably in the face of spurious renames.
+(global-set-key (kbd "C-x b") #'reliably-switch-buffer)
+
 ;; Font size
 (define-key global-map (kbd "C-=") 'increase-default-text-scale) ;; '=' because there lies '+'
 (define-key global-map (kbd "C--") 'decrease-default-text-scale)

users/tazjin/emacs/config/functions.el

@@ -371,4 +371,23 @@ by looking for a `Cargo.toml' file."
                     (error "Not a .nix/.exp file!")))))
     (find-file other)))
 
+(defun reliably-switch-buffer ()
+  "Reliably and interactively switch buffers, without ending up in a
+situation where the buffer was renamed during selection and an
+empty new buffer is created.
+
+This is done by, in contrast to functions like
+`ivy-switch-buffer', retaining a list of the buffer objects and
+their associated names."
+
+  (interactive)
+  (let* ((buffers (seq-map (lambda (b) (cons (buffer-name b) b))
+                           (seq-filter (lambda (b) (not (string-prefix-p " " (buffer-name b))))
+                                       (buffer-list))))
+         (name (completing-read "Switch to buffer: " (seq-map #'car buffers)))
+         (selected (or (cdr (assoc name buffers))
+                       ;; Allow users to manually select invisible buffers ...
+                       (get-buffer name))))
+    (switch-to-buffer (or selected name) nil 't)))
+
 (provide 'functions)