As part of the ongoing overhaul of my Emacs setup I’ve been trying to make the most of the built-in functionality that recent Emacs releases keep quietly shipping. My in-buffer completion setup is based on corfu and cape these days, but it turns out I had overlooked a nice Emacs 30 addition in the same area - completion-preview-mode. It gives you inline completion suggestions - the “ghost text” UI that GitHub Copilot and friends made famous1 - except here it’s powered by plain old Emacs completions.

What It Does

As you type, completion-preview-mode shows a grayed-out preview of the top completion candidate right after point - inline, in the buffer itself. No popup, no minibuffer, no fuss. If the suggestion is what you want, press TAB to accept it. If not, just keep typing and the preview updates (or goes away).

Ghost text suggesting the rest of a symbol as you type

The part I like best - the candidates come from completion-at-point-functions, which means the preview is fed by the exact same backends as corfu and company. If you’ve already set up cape-dabbrev, cape-file or eglot, all of them power the ghost text automatically. Batteries very much included.

Enabling It

You can try it out in the current buffer with M-x completion-preview-mode. If you like what you see, you can enable it everywhere:

(global-completion-preview-mode 1)

Here’s the relevant bit of my config:

(use-package completion-preview
  :ensure nil ; built-in
  :config
  ;; cycle through the other candidates with M-n/M-p (those two
  ;; commands have no default bindings)
  (define-key completion-preview-active-mode-map (kbd "M-n") #'completion-preview-next-candidate)
  (define-key completion-preview-active-mode-map (kbd "M-p") #'completion-preview-prev-candidate)
  (global-completion-preview-mode +1))

Essential Keybindings

While a preview is visible you can make use of the following keybindings:

  • TAB (completion-preview-insert) - accept the suggested completion
  • M-i (completion-preview-complete) - insert only the longest common prefix of all the candidates (the preview underlines it, so you know in advance what you’ll get) and let you keep typing from there
  • M-n / M-p (completion-preview-next-candidate / completion-preview-prev-candidate) - cycle through the other candidates; those are not bound by default, but the docs themselves suggest M-n and M-p (see my config above)

Here’s M-i in action. I had typed my-pro, and pressing M-i filled in ject-find-, stopping exactly where the two candidates (my-project-find-file and my-project-find-dir) diverge:

M-i completes up to the longest common prefix of the candidates

And when you cycle with M-n, Emacs even tells you where you are in the candidate list in the echo area:

Cycling through the completion candidates with M-n

There’s basic mouse support as well - clicking the ghost text with mouse-1 inserts it, and scrolling the mouse wheel over it cycles through the candidates. (I doubt I’ll ever use that, but it’s kind of cute)

Fine-tuning

A few user options to adjust the behavior:2

;; show the preview only after typing at least 3 characters (the default)
(setopt completion-preview-minimum-symbol-length 3)

;; wait a bit before showing the preview (by default it shows up instantly)
(setopt completion-preview-idle-delay 0.2)

;; show a preview only when there's exactly one candidate
(setopt completion-preview-exact-match-only t)

There’s also completion-preview-commands - the list of commands after which the preview appears (things like self-insert-command). You’ll rarely need to touch it, but it’s good to know it’s there if some command you use doesn’t trigger the preview.

Do You Still Need Corfu?

When I first read about completion-preview-mode I assumed it was competing with corfu and company, but I’ve come to think that’s the wrong way to look at it. They draw candidates from the same source and simply present them differently - corfu gives you the full candidate list with annotations and documentation, while the preview gives you the single most likely candidate with zero visual ceremony. These days I run both: the ghost text handles the “obviously I meant that symbol” cases with a single TAB, and corfu’s popup kicks in when I actually need to browse.

Admittedly, two completion UIs at once won’t be everyone’s cup of tea. If you find it too busy, a nice middle ground is to enable completion-preview-mode only where a popup feels out of place - e.g. in eshell-mode or comint-mode buffers.

Closing Thoughts

Emacs 30 keeps chipping away at my list of third-party packages - which-key and EditorConfig support are now built-in, and completion-preview-mode covers a niche I didn’t even know I wanted covered. Not bad for a “boring” stable release!

Have you tried completion-preview-mode already? Are you a ghost text person or a popup person? (or both, like me) I’d love to hear your thoughts in the comments!

That’s all I have for you today. Keep completing!

  1. Unlike the AI assistants, the suggestions here come straight from your buffers and your completion-at-point-functions. No subscription needed and no hallucinations included. ↩

  2. If you’re wondering what’s this setopt thing - check out this article. ↩